Glossary: Petrography and petrology

Actinolite: see Tremolite

Aegirine – aegirine augite: NaFe³⁺(SiO₃)₂  Clinopyroxenes at the alkali end of the chemical spectrum. Distinct green-brown in thin section, and pleochroic in shades of green, blue green, yellow green, to yellow. Both minerals are monoclinic, but aegirine is biaxial (-) and aegirine augite biaxial (+). Both have typical (110) cleavage at 87^o and 93^o. Occur in alkali igneous rocks like alkali granites, syenites, monzonites, and feldspathoid-bearing rocks, as phenocrysts and groundmass laths.

Aggrading neomorphism  Neomorphism in which there is progressive change in crystal size.  Crystal aggradation can mimic pre-existing cement fabrics where crystal size increases from the cement boundary into pore spaces. It is common in carbonate mudrocks; the resulting textures commonly appear clotted, where remnant patches of micrite appear to float among clusters of coarser crystals. This kind of texture is also called structure grumeleuse after Lucien Cayeux. Clotted textures may also be pellet-like and distinguishing between the two requires careful examination in thin section.

Albite twins: Common twinning in plagioclases and potassium feldspars, presented as multiple, parallel lamellae that traverse the entire crystal section. The width of twin segments decreases and the number of lamellae increases in more calcic plagioclases.

Alizarin Red-S: This is a soluble organic acid that reacts with calcium. Distinguish between calcite (stains pink-red) and dolomite (no stain) can be easily done using this stain, on rock slabs or thin sections.

Allochem: Framework components of granular or rudaceous limestones that show some evidence of transport or movement; i.e. they have not formed in situ. Common examples are ooids, oncoids, pellets, fossils, and intraclasts.

Andalusite: Al₂SiO₅. Of the sillimanite group of minerals and polymorphic with kyanite and sillimanite. Orthorhombic, prismatic crystals. In thin section, commonly pleochroic in pale pinks, yellows and greens. Biaxial (-); two sets of good prismatic (110) cleavage at 90^o, sections parallel to ‘c’ have parallel extinction. Common in contact metamorphic and regional metamorphic schists, with biotite and garnet. A good provenance indicator in detrital form.

Anhedral: Refers to crystal form where the original crystal faces have been removed (by abrasion or dissolution) or are not recognizable because of crystal intergrowth. Neomorphic crystals are commonly anhedral.

Anhydrite: Ca SO₄. Colourless, orthorhombic, biaxial (+), low relief in thin section. Good cleavage at (100), (010), and (001); cleavage traces are orthogonal, cleavage fragments commonly show cubic terminations and indentations. Extinction parallel to cleavage traces. Twin lamellae intersect at about 90^o. Primarily a sedimentary chemical precipitate at surface temperatures, for example in sabkhas. A common component of evaporite deposits, with halite, gypsum, and sylvite. Also associate with calcite or dolomite.

Apatite: Ca₅(PO₄)₃F₂ (+/- Cl^– or OH^–) – also called fluorapatite. Hexagonal, usually needle of rod-like crystals with weak (0001) basal cleavage, uniaxial (-). In thin section and heavy mineral separates it mostly occurs as hexagonal cleavage fragments (isotropic) or as prisms in 1^st-order greys, with low birefringence. It is a common accessory in most igneous rocks, basic through acidic. Apatite is commonly used to determine radiometric fission track ages and when sedimentary rocks reached a burial temperature of 110^oC because at this temperature the tracks are annealed.

Aphanitic: Used to describe fine-grained volcanic and intrusive rocks where individual crystals cannot be observed without a microscope. Cf. Phaneritic.

Anisotropic minerals: Minerals in thin section reorient plain polarized light, resolving it into two vibration directions that will pass through the upper polarizer when nicols are crossed. One direction contains a fast light ray (also called the extraordinary ray), the other a slow ray (ordinary ray); the fast and slow rays are perpendicular to each other. Anisotropic minerals are further divided into uniaxial and biaxial based on the presence of one or two optic axes.

Aragonite: CaCO₃. A polymorph with calcite, and distinguished from the latter by its orthorhombic crystal form, usually fibrous needle or acicular habit, commonly as radial clusters in cement fill, biaxial (-) (calcite is uniaxial (-)). Aragonite lacks rhombohedral cleavage. It is a common mineral in geologically recent carbonate and evaporite deposits and invertebrate shells but tends to recrystallize to low magnesium calcite during burial. Also as a hydrothermal precipitate.

Arenite: Almost synonymous with sandstone, although its definition is a bit more precise: a rock composed mainly of sand-sized grains and having less than 15% matrix. Greater than 15% matrix and the rock is a wacke.

Argillite: The general name for highly indurated mudstone. They tend to have a greenish hue, in part because of chlorite cements, in addition to illite plus or minus calcite. In many argillites there is a subtle transition from burial diagenesis to incipient metamorphism with the alteration of illite clays to white micas, and the appearance of prehnite or pumpellyite.

Arkose: An arenite that has at least 25% feldspar in its grain framework. Between 5% and 25% feldspar is a subarkose.

Augite: Ca(Mg,Fe²⁺Al)(Si,Al)₂O₆ Part of the clinopyroxene solid solution series that includes diopside and hedenbergite. Pleochroism in shades of pale green, blue-greens, pink, and yellow green. Monoclinic. Characteristic (110) cleavage at 87^o and 93^o with inclined extinction. Biaxial (+). Single or multiple twinning along (001) is common. Occurs mainly in mafic and ultramafic igneous rocks, including basalts and andesites, peridotites. May replace hornblende and biotite. Also some gneisses and granulites.

Basic (igneous petrology): Volcanic and intrusive rocks poor in free silica (quartz), but enriched in feldspars and alkali earth (calcium, magnesium) ferromagnesian minerals like amphiboles, pyroxenes and olivines. Typical examples are basalt, gabbro, and diorite. Cf. felsic.

Biaxial minerals: Anisotropic minerals where plain polarized light entering at any angle, other than along two optic axes, is resolved into two planes of polarized light; these two planes each contain the fast and slow rays. The resulting colour depends on the different in the refractive indices of these two light paths – i.e., the birefringence. Minerals may be positively or negatively biaxial, depending on the orientation of fast and slow rays.

Bindstone: Consists of organically bound frameworks (not transported), such as encrusting algae or bryozoa, that bind some pre-existing substrate.

This term was introduced by Embry and Klovan (1971) as a modification of Dunham’s (1962) limestone classification scheme; see review and modification by Lockier and Junaibi (2016).

Bioclast: Any clast derived from vertebrate or invertebrate organisms. Bioclasts may represent an entire organism, such as a bivalve shell or foraminifera, or fragments thereof. They are common framework constituents of siliciclastics and carbonates.

Biomould: Also biomold. The impression of an organism left in a rock following dissolution of the original skeletal mineral – commonly calcite and aragonite. A cast of this impression is formed if the mould is filled with a new precipitate or sediment.

Biotite: A complex sheet silicate. Monoclinic, commonly prismatic, book-like. Perfect (001) cleavage. Strongly pleochroic in shades of brown and green. Maybe colour zoned. Biaxial (-) with strong birefringence. Extinction in prismatic sections is parallel. Sedimentary grains are usually cleavage fragments. Common in regional metamorphic schists, slates, and gneisses, and in most intrusive rocks. Also common in extrusive felsic rocks such as rhyolites, trachytes, and andesites A common detrital heavy mineral but because of its crystal habit it tends to behave as a lighter fraction during hydraulic sorting.

Birefringence: Plain polarized light that passes through a mineral is resolved into mutually perpendicular fast and slow rays that will each have different indices of refraction as (i.e., their refraction paths and velocities will be different). Birefringence is the maximum difference between these two index values. Under crossed nicols, the difference is manifested in the ‘intensity’ of interference colours.

Botryoidal cement: In limestones, this cement form is presented as radial clusters of fibrous or bladed calcite or aragonite that precipitate in more cavernous porosity. Common examples are found in reef frameworks, and fenestrae that form by mineral dissolution, gas bubbles, and crystal expansion (e.g. halite-gypsum crystal growth in sabkhas). Fenestrae are common in some cryptalgal laminates and mud mounds containing Stromatactis.

Boulder: In the gravel-conglomerate grain size category, it applies to clasts >256 mm, or >-8Φ on the Wentworth size scale.

Boundstone: A kind of fall-back term for limestone description where the mode of binding is not readily identifiable. This term replaces Embry and Klovan’s Bafflestone in which the mode of binding and identification of the organisms responsible was equivocal. This term is introduced by Lockier and Junaibi (2016).  in their review and modification of Dunham’s (1962) limestone classification.

Bowen reaction series:  A predictable order of mineral crystallization in a cooling magma, after the early 20^th Century geologist Norman Bowen. One of the first minerals to crystallize from magma is olivine (from about 1300^o to 1200^oC). Feldspar, the most common rock-forming mineral, begins to form below temperatures of about 1000^oC, and one of the last to appear, quartz at about 800^oC. Bowen’s discovery revolutionised the way we think about the evolution of igneous rocks.

Breccia: Consists predominantly of angular clasts larger than 2mm. Like conglomerates they are poorly sorted, clast-supported frameworks. The degree of clast angularity indicates little or no reworking.

Calcite: CaCO₃. Hexagonal-rhombohedral, polymorphous with aragonite. Uniaxial (-), excellent (1011) cleavage. Twinning common; in thin section twin lamellae tend to parallel the long diagonal of crystal rhombs. In thin section, it has high relief (RI 1.48-1.66), and high birefringence that creates a kind of twinkling appearance in thin section. Rare as a primary mineral in igneous rock (except in carbonatites), but does occur as an alteration product, and as vesicle filling; also common in hydrothermally altered rocks and geothermal precipitates. Common in many calcareous metamorphic rocks at amphibolite grade, and marble. Occurs in many guises in sedimentary rocks, as a primary constituent of skeletal and shelly fossils, as cements, and as matrix or framework replacement. Cement precipitates form euhedral rhombohedral or scalenohedral crystals; replacement precipitates tend to be anhedral. Magnesium replaces calcium up to 19% in some invertebrate shells. Distinguished from dolomite by one set of twin lamellae (not always obvious); dolomite commonly has turbid cores.

Carlsbad twins: Common twins in plagioclase and some potassium feldspars. It is an penetration twin with a plane that separates two crystal segments.

Cement: Precipitation of pore-filling minerals, such as quartz, calcite, aragonite, high-magnesium calcite, dolomite, clays, and gypsum, is an important process during sediment lithification. Crystal growth begins at grain boundaries, gradually filling the available pore space. Cementation can begin at the sea floor, particularly by aragonite and calcite, and continue during burial. Cementation gradually occludes effective porosity. Distinguishing cement and neomorphic textures, see Neomorphic textures in thin section

Chalcedony: A fibrous form of microcrystalline quartz, or chert. It commonly forms radial clusters. Under crossed polars, extinction patterns are sweeping or radial. Can occur as a replacement mineral for fossil and carbonate cements, as concretions. Is common as detrital lithic grains. Can occur with chert (more equant crystals) and other silicified deposits.

Chemical stability (of sedimentary grains): The ability to resist dissolution or chemical change during sediment transport and burial.  Quartz tends to be chemically stable, compared with feldspar that may react, particularly during burial diagenesis to form new minerals such as clays. Minerals like zircon are extremely stable and can survive several sedimentary cycles.

Chert: SiO₂. The general name given to micro- and cryptocrystalline quartz that precipitates in sediment, as biogenic products, in under volcanic and hydrothermal conditions. It may be bedded or laminated – a common occurrence in some carbonates associated with microbialites, or as siliceous oozes on the deep-sea floor and composed of diatoms or radiolarians. Sponges provide another source of fine particulate silica.  Nodular cherts tend to be diagenetic; classic examples are the nodules in Cretaceous chalks of southern England and Europe. Chalcedony is a fibrous form if microcrystalline quartz that under crossed polars shows sweeping or radial extinction. Recrystallization is common, in part because of the high surface area (and therefore reactivity) afforded by crystal size.

Chlorite: A group of minerals having low birefringence, in varying shades of green (in PPL), and crystal habit that is also variable, from fibrous, spherulitic or vermiform (worm-like). May be pleochroic in shades of green and yellow. It is commonly associated with low grade metamorphism and hydrothermal alteration. In greywackes and other mud rocks it is a common replacement for clay matrix, micas, and  ferromagnesian minerals. Mostly biaxial (+) but sedimentary chlorite tends to be microcrystalline, and interference figures difficult to obtain.

Clast-supported framework: This term applies to granular rocks where clasts are mostly in contact with one another. It usually refers to lithologies containing clasts that are sand sized and larger; it does not apply to mudstones or siltstones because it is difficult or impossible to distinguish framework from matrix. This textural property applies to siliciclastics and carbonates. Cf. matrix-supported framework.

Clay: This term has two meanings: (1) as a layered or sheet-like silicate mineral such as kaolinite and illite, and (2) as sediment with grain size less than 4 microns. See also Mud which consists of a clay-silt mix.

Cleavage (crystallography): A plane of weakness within a crystal that will break with relative ease. It is a function of weak bonds between certain planes of atoms within a crystal lattice; the pattern of weakness repeats regularly through a crystal. Some minerals have poor or no cleavage (e.g. quartz, olivine); others have good cleavage along several lattice planes (e.g. calcite, feldspar). Cleavage can be a defining characteristic of a mineral, particularly in thin section.

Clinopyroxene: The monoclinic members of the pyroxene group of minerals. Most common are diopside, augite, pigeonite and aegirine that form solid solution series. Characterized by (110) cleavage in two sets almost at right angles in 4 and 8 sided basal sections, and parallel cleavage in prismatic sections. In the latter extinction is inclined.

Clinozoisite: Ca₂Al₃ (SiO₄)₃ OH. Compositionally the same as zoisite, but monoclinic crystals. Part of the epidote group of minerals. Colourless, maybe pleochroic in shades of yellow-green, pink and red. Forms as bladed crystals or radial aggregates of bladed crystals. Not usually fibrous, like zoisite. Biaxial (+), excellent (001) cleavage. Inclined extinction, except along elongate sections of blades. Mostly found as an alteration product in contact metamorphism and hydrothermally altered rocks.

Cobble: In the gravel-conglomerate grain size category, it occupies the range 64-256 mm, or -6Φ to -8Φ on the Wentworth size scale.

Coccoliths: Marine phytoplankton that secrete calcium carbonate skeletons; they are one of the main constituents in natural chalk. Coccospheres are algal cells surrounded by coccoliths arranged into spheres tubes and cup-shaped bodies, up to 100 microns in diameter. They are  one of the culprits responsible for marine algal blooms.

Codiacean algae: A group of green algae that precipitate aragonite needles 2-3µm long. Two common species are Halimeda and Penicillus that, across carbonate platforms and reefs, produce large volumes of aragonite mud. Cf. coralline algae.

Cohesionless grains: Grains (usually sand or silt) that do not stick together. This property is necessary for most sandy bedforms to form. Cohesion in finer grained particles prevents the formation of sediment bedload and saltation load movement.

Conchoidal fracture: A curved to slightly concave, sometimes corrugated surface that develops from brittle failure in non-crystalline minerals (e.g., opal, volcanic glass) and in crystals lacking cleavage – common examples occur on quartz and garnet fracture surfaces.

Concordia plot: (Geochronology). Devised by George Wetherill in 1956, For the Pb/U decay system (commonly used for zircon geochronology), the curve plots age against the three Pb/U ratios for the two U-Pb decay systems, assuming an ideal closed system (i.e. no loss of any isotope during a crystal’s lifetime). The measured isotope ratios for any crystal or batch of crystals are compared with this ideal curve; if the age from each decay system is the same and they lie on the curve, i.e. they are concordant, then that is the true age of the sample.

Discordance ages arise from Pb loss, inherited Pb from the time of crystallization, from the analysis of several crystals of different ages in a single sample, or from single zoned crystals where ages from the inner to outer most zones become progressively younger. A straight line discord will plot below the concordia and intersect it at two points – the oldest will be close to the age of crystallization.

Conglomerate: Sedimentary rock where the framework consists of clasts coarser than 2 mm (granule). Clasts show variable degrees of rounding and shape. Sorting tends to be poor. The term gravel is used for modern sediments. They typically represent high energy conditions like those found in braided rivers, alluvial fans, and gravel beaches. Cf. breccia, pebbly mudstone.

Cool-water limestone: Predominantly bioclastic limestones typically made up of bryozoa, various molluscs, brachiopods, calcareous algae, barnacles, and echinoderms. Isopachous, micritic, and pore-filling cements are mostly calcite; aragonite cement is uncommon.

Coquina: A limestone made up of shells, shell fragments and other bioclasts, with a degree of sorting that indicates relatively high depositional energy. Where the fragments are mostly sand-sized, the Dunham limestone classification equivalent is grainstone.

Coralline algae: Calcite and high magnesium calcite precipitating red algae, that build upon substrates such as bioclasts and rock surfaces and other algae. All begin life as encrusters, but grow to different forms such as articulated branches, or nodular clusters around shells or pebbles (e.g. Lithothamnion). They are an important contributor to cool-water bioclastic limestones.

Cordierite: (Mg,Fe²⁺)₂ Al₄Si₅O₁₈. Part of the Cordierite group that includes synthetic minerals. Orthorhombic, pseudohexagonal, with poor (010) cleavage. Biaxial (-), 2V changes according to the degree of hydration. Colourless, weakly pleochroic from pale yellow to blue hues. Twinning commonly as sector and lamellar twins that are a diagnostic property. Mostly found in relatively high temperature metamorphic rocks – schist, gneiss, amphibolite, plus as contact metamorphic phases. It alters easily to mica-clay mixes and is not common in detrital form.

Crossed polars (nicols: The upper polarizer, between the objective lenses and the Bertrand lens, filters out all remaining frequencies present in plain polarized light (PPL). The nicols can be moved in and out of the light path. If you look through the oculars when the polarizer is in the light path (i.e., crossed nicols) then no light will reach the oculars – all will be black. However, if a thin section is placed between the lower and upper polarizers, most minerals will reorient the PPL such that some of this light will pass through the upper polarizer; this light will contain slow (ordinary) and fast (extraordinary) vibration directions that will arrive at the eye pieces at slightly different times. The resulting interference produces the kaleidoscope of colour among all the minerals present. Minerals that permit the passage of light are called anisotropic; those that do not are isotropic.

Cryptocrystalline: Materials like chert and chalcedony where visual resolution of individual quartz crystals requires a very high powered microscope, or Scanning Electron Microscope.

Crystal forms: These are the familiar geometric arrangements of crystal faces such as pinacoids, prisms and pyramids. Forms are sets of crystal faces that share properties of symmetry. A form can be a single face (pedions) or many faces. Closed forms enclose space – the most common type is a pyramid that is closed at its termination. Open forms do not close space – prisms are defined by three or more faces that are parallel (they do not meet). There are 48 possible forms in the 7 crystal systems. Thus, for prisms, there are variations such as trigonal (3 faces), tetragonal (4 faces) and hexagonal prisms (6 faces), along with ditrigonal, ditetragonal, and dihexagonal forms. Pinacoids are 2-face forms. Form notation is a variation on the Miller Indices – a general form is designated {hkl} or {hkil} (curly brackets).

Crystal overgrowths: A term usually reserved for diagenetic textures in rock cements, where a new crystal overlies, or overgrows the detrital mineral. The overgrowth may be the same composition and in crystallographic and optical continuity with its parent grain (this is a syntaxial overgrowth) as is common in quartz arenites, or different composition and unrelated crystallographically (epitaxial overgrowths).

Crystal symmetry: Symmetry describes the shape of an object and can be represented both mathematically and visually. In crystallography, the two most useful forms of symmetry are:

1. Axes of rotation (crystallographic axes) where a particular crystal face will be repeated during rotation through 360^o. The number of repetitions for a 360^o rotation can be 2, 3, 4, or 6, that are referred to as two-fold, three-fold, four-fold, and six-fold (axial) symmetry respectively.
2. Planes of symmetry where two parts of a crystal are mirror images. For an analogy, think of this concept in terms of the common bilateral symmetry in many living organisms, such as people, and many classes of mollusc. Note that planes of symmetry are NOT the same as twin planes.

Crystal systems: There are 6 crystal systems based on combination of the elements of symmetry; a seventh system – trigonal – is usually considered a subclass of the hexagonal system. The defining criteria are axial lengths, the angles between axes, and axial symmetry (the number of repetitions about an axis). The systems are: cubic (isometric – the most symmetrical), tetragonal, hexagonal, orthorhombic, monoclinic, triclinic (the least symmetrical).

Crystal zoning: Zoning commonly displays as concentrically arranged crystal growths, where the composition changes outwards from the crystal interior. The zones maintain the same crystallographic and optic axis. The changes in composition involve substitution of certain cations, for example in calcite Fe²⁺ and Mn²⁺ substitute for Ca²⁺, and in plagioclase sodium may substitute for calcium such that the inner core is a calcium anorthite and the outer zone is a sodium albite. Zoning indicates changing fluid or magma crystallization conditions. Zoned crystals may also be twinned.

Crystallographic axes: Three or four axes about which a crystal can be rotated through 360^o.  The axes intersect at a single point (the centre of symmetry). They are labelled according to their lengths. If axes are the same length, then they are referred to as a₁, a₂, a₃ etc. If they have different lengths, they are labelled a, b, and c. Thus, in the cubic (isometric) crystal system they are labelled a₁, a₂, a₃, and in the tetragonal system a₁, a₂, c. The hexagonal system is the only one with four axes. Angles between axes are labelled α, β, γ.

Decay system: (isotopic) Radioactive decay is a spontaneous process. The decay from a parent isotope to its stable daughter product is called a decay system. Decay systems can incorporate a number of unstable intermediate daughter products that decay at different rates (i.e. have different half lives). For example ²³⁸U  →    ²⁰⁶Pb  occurs via 14 decay products. In Zircon geochronology, three decay systems are used: ; ²³⁸U→²⁰⁶Pb (the half-life of 4.47 Ga is almost the same as the age of the Earth), ²³⁵U→²⁰⁷Pb (half-life of 0.70 Ga), and ²³²Th→²⁰⁸Pb (half-life 14.01 Ga).

Diagenesis:  The sum of physical and chemical processes in sediment, beginning soon after deposition at or immediately below the sediment-water interface, and continuing at depth in concert with increased burial temperatures, lithostatic and hydrostatic pressures, and changing fluid composition.

Diatoms: A diverse group of single-celled photosynthesizing algae that secrete silica along cell walls. They are an important part of the phytoplankton food web. They occur in marine and non-marine waters. When the alga dies the silica plates gradually settle on the lake and sea floor. Diatomaceous earth will form if the silica plates are sufficiently concentrated over long periods without disturbance by currents.

Diopside: CaMgSi₂O₆ – Ca(Mg,Fe²⁺)Si₂O₆. Part of the clinopyroxene solid solution series that includes augite and hedenbergite. Pleochroism increases with Fe – blue-greens through yellow greens. Monoclinic. Characteristic (110) cleavage at 87^o and 93^o with inclined extinction. Biaxial (+). Occurs mainly in carbonate-bearing metamorphic rocks, and contact metamorphic rocks, and mafic igneous rocks. Very similar to augite except the latter is more common in mafic and ultramafic igneous rocks.

Dolomite: CaMg(CO₃)₂. In highly ordered dolomite the unit cell lattice contains Ca and Mg layers that alternate separately with carbonate layers – it is sometimes referred to as a double salt. The atomic arrangement differs from magnesium calcite where the Mg ions are more or less randomly replace Ca in the unit cell lattice. Many sedimentary dolomites are calcium-rich. Hexagonal (Trigonal) rhombohedral crystals, uniaxial (-) with high relief, high birefringence that imparts a characteristic twinkling in thin section. Unlike calcite, twinning is not common but where it does occur twin lamellae parallel both long and short diagonals in rhombohedra. Most commonly a sedimentary mineral, replacing calcite, high magnesium calcite, and aragonite, but it can also form primary cements during sediment burial. Also occurs as a contact metamorphic and hydrothermal alteration phase. cf. calcite, protodolomite.

Drusy cement: Cements consisting of calcite rhomb mosaics that line and fill pores, intraskeletal chambers, and more cavernous porosity. The size of calcite rhombs commonly increases towards the center of void spaces. Intercrystalline boundaries tend to be planar. They are common in meteoric and burial environments where they may overlie earlier fibrous or bladed cements.

Effective porosity: The component of porosity that permits significant flow. Microporosity (intergranular, intercrystalline) is commonly excluded from this porosity value.

Endolithic algae: Eukaryotic algae that live in micropores of skeletons and shelly material, and in pore throats of granular sediment. They are capable of dissolving calcium carbonate and promoting early diagenesis or weakening organic structures that leads to their fragmentation. They also play a role in micritisation of bioclasts. The term also applies to endolithic fungi and bacteria.

Enstatite: (Mg,Fe²⁺)SiO₃.  An orthorhombic, colourless to pale green orthopyroxene that is part of a solid solution series with bronzite and hypersthene, written in the form En₁₀₀ – En_88.5 where En is the proportion of Enstatite. Basal sections are 4- or 8-sided, with characteristic (110) cleavage in two directions sets that intersect almost at right angles (93^o and 87^o). Longitudinal sections have parallel cleavage – extinction is also parallel to cleavage traces. Biaxial (+). Most common in ultramafic, upper mantle rocks like peridotite and serpentinite, less common in gabbros.

Epiclastic: Sedimentary clasts formed from pre-existing rocks; this applies to most siliciclastic rocks and to many redeposited volcaniclastic sediments.

Epidote: Ca₂ (Al, Fe³⁺)₃ (SiO₄)₃ OH. Colourless to pale yellow, pleochroic in shades of pale green, yellow and brown. Monoclinic, as fibrous aggregates of blades. Excellent (001) cleavage. Biaxial (-). Parallel extinction in blade sections. Commonly found in altered and metamorphic rocks of all kinds, including clay-rich sediments such as greywackes where it forms an important part of the matrix. Distinguished from clinozoisite by its common fibrous habit and (-) sign and parallel extinction.

Epitaxial overgrowth: Cement overgrowths that are not in optical continuity with the substrate grain, and have a different mineral composition. Cf. syntaxial overgrowth.

Euhedral: Refers to crystal forms where the original crystal faces are present. For example, quartz overgrowths that present crystal terminations.

Extinction: As the microscope stage is rotated, under crossed nicols, either the fast or slow vibration direction of light exiting an anisotropic mineral will be blocked by the upper polarizer – at this point no light is transmitted through polarizer and it appears black – the transmitted light has been extinguished. This alignment occurs four times during one 360^o rotation (because the fast and slow rays are perpendicular), and therefore each extinction event is 90^o apart.

Extinction angle: The angle at which extinction occurs relative to crystal habit or prominent cleavage, may vary between 0^o and 89^o. The angle can be easily measured using the grid on the rotating stage. Extinction angles can be used to help identify minerals.

• Parallel extinction: Extinction parallel to the crystal elongation direction or cleavage, that also parallels the crystallographic c axis. A common example is muscovite
• Inclined extinction: The most common type, between 0^o and 89^o.
• Symmetrical extinction: In minerals that have two prominent cleavage planes (such as calcite) – if the extinction angles measured from each cleavage are the same, then extinction is symmetrical.

Fabric: A textural property of rocks that emphasizes clast or crystal orientation or alignment that, in sedimentology can be related to the hydraulics of sediment transport and deposition, in volcanic rocks to lava flow, and in deformed or metamorphic rocks to stress fields.

Fascicular optic calcite: Void filling cement that consists of radially fibrous calcite clusters, where the crystal optic axes diverge towards the centre of the void. in concert with diverging crystals. Cf. Radiaxial fibrous cement.

Fayalite: Fe₂SiO₄. The iron (Fe²⁺) mineral in the olivine group. Commonly pale yellow-green, but also colourless, orthorhombic, usually poor cleavage with breakage along curved fractures. Biaxial (-) with parallel extinction. Not easily distinguished from forsterite. Pure fayalite is much less common than forsterite, with which it forms a solid solution. Found primarily in mafic and ultramafic rocks, high temperature slags, and rapidly quenched melts. Rare in detrital rocks.

Feldspar: The most abundant mineral in Earth’s crust. It is present in nearly every kind of igneous rock; it is also a common hydrothermal product. In sedimentary rocks it occurs as a common detrital component, and as an authigenic phase (usually albite). Feldspar is a sheet silicate comprising two main groups: the Alkali feldspar group – most common are potassium-bearing feldspars (K-spar) that forms a solid solution series with Albite; and the Plagioclase group that form a solid solution series from Albite (sodium end member) to Anorthite (Calcium end member). All feldspars have good cleavage. There are several types of twinning, for example the common perthite and Gridiron twinning of K-spars, and albite, carlsbad, pericline twins in plagioclases.

Feldspar laths: Microscopic, needle-like crystals of feldspar, usually plagioclase, in a glassy groundmass. Alignment of laths provides an indication of flow. These textures commonly develop in rapidly cooled basalt lavas.

Felsic: A contraction of ‘feldspar and silica’, that describes in very general terms igneous and metamorphic rocks that are rich in silica and minerals containing lighter alkali metals like sodium and potassium. For igneous rocks this includes rhyolites, dacites, and granites. Cf. Basic igneous rocks.

Floatstone: More than 10% allochems coarser than 2 mm (upper limit of coarse sand), supported by grains finer than 2 mm. Dunham’s (1962) limestone classification scheme reviewed and modified by Lockier and Junaibib (2016).

Fluid inclusions: Microscopic bubbles trapped in a crystal as it precipitates, contain samples of the fluid from which the minerals were originally derived. The fluid occurs as a single phase – liquid or vapour, or two phases with both liquid and gas. Most inclusions are less than 100µm (0.1mm) long.  The fluids within may be fresh water or brines, or hydrocarbons. Samples are usually viewed in thin sections, gradually heated until the liquid and vapour homogenize into a single phase. The homogenisation temperature is generally considered to be the temperature at which the mineral crystal formed.

Fluorite: CaF₂. Isometric, colourless to purple, usually isotropic, with perfect octohedral (111) cleavage – most detrital grains are triangular cleavage fragments. Refractive index is 1.43-1.45 and hence has negative relief in thin section. A common accessory in many rock types.

Form (textural): A textural term that describes the overall shape of grains such as elongate (rod-like), spheroidal, bladed, platy, and equant (equidimensional).

Forsterite: Mg₂SiO₄. The magnesian form of olivine. Colourless to pale yellow-green, orthorhombic, usually poor cleavage with breakage along curved fractures, but there may be some (010) cleavage. Biaxial, but the sign changes according to the proportion of forsterite/fayalite: thus Fo₁₀₀ – Fo₈₈ is biaxial (+), Fo₈₈ – Fo₅₉ is (-), with parallel extinction. Found primarily in mafic and ultramafic rocks; an important mineral in the upper mantle. Rare in detrital rocks except 1^st cycle basic volcaniclastic sediments.

Fracture porosity: The pore space permitting fluid flow through rock fractures and joints. Fracture and joint networks are oriented according to ancient stress fields, hence the porosity will also be focused at these orientations. It tends to occur in hard rock. In crystalline or volcanic rock (the latter includes columnar joints) it is the only effective porosity.

Framboid: Small, spherical aggregates of microscopic crystals, commonly measured in diameters of microns to 10s of microns. One of the more common occurrences is pyrite. They can form as primary precipitates in aqueous environments, or as diagenetic products. From the French framboise for strawberry.

Framestone: A limestone composed of in situ frameworks build by organisms (i.e. not transported). Common examples include corals, stromatoporoids, and oysters. The matrix between framework components should be described separately. This term was introduced by Embry and Klovan (1971) as a modification of Dunham’s (1962) limestone classification scheme; see review and modification by Lockier and Junaibi (2016).

Garnet: A group of isometric minerals, commonly found in igneous and metamorphic rocks, in the latter garnet amphibolites and garnet-biotite schists. Colours range from colourless to dark red and brown depending on composition. Cleavage is lacking; detrital fragments are commonly irregular shapes with curved or conchoidal surfaces. Metamorphic varieties may have inclusions of biotite-muscovite or quartz. Refractive indices range from 1.72-1.89. They are invariably isotropic and have very high relief in thin section.

Geopetal: Textures and fabrics that allow the interpretation of stratigraphic top, or ‘way-up’. This definition would include normal grain size grading in a turbidite. However, there is a tendency these days to restrict the meaning to structures where cements or sediments partially fill a void, such that the top of the fill represents a depositional or precipitation surface. Examples include fossils that have preserved chambers, the interstices between pillow lavas, and cavernous porosity in reef frameworks or caves.

Glauconite: A distinctive green to dark green-brown, K-Ca-Na sedimentary mica that usually forms as clay-like crystal masses, as cement fill in fossil chambers (foraminifera, gastropods, corals) or as replacement in fecal pellets. Strongly pleochroic in shades of green and brown although individual crystals are usually too small to identify and identification of pleochroism can be difficult. Monoclinic, biaxial (-).

Glaucophane: Na₂ (MgFe²⁺)₃ Al₂Si₈O₂₂ (OH)₂. Forms a series with actinolite and tremolite. Characteristic blue-violet with strong pleochroism in shades of yellow-green and blue-green, violet to blue or red violet, dark blue to blue or brown-green. Monoclinic, with crystals prismatic to fibrous, cross-sections 6- and 4-sided with typical amphibole (110) cleavage at 56^o and 124^o. Biaxial (-). Found primarily in regional metamorphic rocks.

Goethite: HFeO₂. Crystalline forms are orthorhombic, biaxial (-). Orange-brown in thin section. Most common as earthy or concretionary masses, but also oolitic (with hematite) and fibrous. It is more translucent than hematite, and limonite is isotropic.

Grain size:      There are several ways to measure the size of clasts: grain diameters (maximum, intermediate, minimum), surface area, or by using the assumption that grains approximate spheres. Sieve sizes use the minimum diameter where a grain will pass through a particular mesh. The most popular scale is the Wentworth scale, based on a geometric progression of sizes, centred on silt and sand populations. An important modification of the Wentworth model is the Phi scale (Φ), calculated as Φ = -Log₂ of the grain size in millimetres.

Grain size distribution: A measure of the size distribution of particles in granular sediments. Measurement of the range of grain sizes in a sample provides data for detailed descriptions, calculation of population statistics such as mean, mode, and sorting, and may even provide clues to deciphering depositional hydraulics.

Grainstone: The cousin to siliciclastic sandstone, where sand-sized carbonate particles (<2 mm) form a grain-supported framework, relatively free of or carbonate mud (micrite).Dunham’s (1962) limestone classification scheme reviewed and modified by Lockier and Junaibi (2016).

Granule: The alternative name for very fine pebbles, with size range from 2-4 mm, or -1Φ to -2Φ. It is the finest grade in the gravel-conglomerate category of the Wentworth scale.

Gravel: The unconsolidated equivalent of conglomerate, comprising clasts coarser than 2 mm. The general class is divided into granules (2-4 mm; -1 to -2 phi); pebbles (4-64 mm; -2 to -6 phi); cobbles (64 – 256 mm; -6 to -8 phi); and boulders (larger than 256 mm).

Greywacke: An indurated muddy sandstone having 15-75% muddy matrix. Typically they are poorly sorted, and contain grains having variable roundness-angularity. It can be difficult distinguishing between original matrix (detrital) and diagenetic cements, particularly with the recrystallization of clays, the transformation of illite to white mica, and the addition of minerals like chlorite that imparts typical grey-greenish hues. Likewise, distinguishing between lithic fragments and diagenetically altered matrix can be difficult.

Gridiron twinning: A common diagnostic twin in potassium feldspars twins that belongs exclusively to microcline. It is presented as a cross-hatching of thin albite and perthite twin lamellae. Also called tartan twins.

Gypsum: CaSO₄.H₂O. Monoclinic, colourless, biaxial (+), good (010) cleavage bnut poor (100) and (111) cleavage, which means cleavage fragments do not generally show cubic patterns – cf. anhydrite. Slightly lower relief than anhydrite with RI from 1.52 – 1.53. A primary constituent of sedimentary evaporites in marine basins and terrestrial salt lakes-salars, along with other common minerals – anhydrite, halite, and sylvite.

Halite: NaCl. Isometric, colourless, usually cubic, and in thin section isotropic; refractive index is 1.54 (close to that of Canada balsam) hence low relief. Because of its solubility, thin sections need to be made using oil. In sedimentary rocks, can occur as a cement or as framework crystals – the latter are common in massive evaporite deposits. It has slightly higher relief than gypsum, and lower relief than anhydrite.

Hardness (mineralogy): A measure of a mineral resistance to scratching. The common Moh Scale has hardness gradations from 1 – the softest (e.g. talc), to 10 (diamond). The scale does not include mineral propensity for breakage (along cleavage planes). Common indicator minerals are (increasing hardness):

1 Talc          2 Gypsum        3 Calcite          4 Fluorite          5 Apatite          6 Orthoclase

7 Quartz          8 Topaz          9 Corundum          10 Diamond.

 

Heavy minerals: A broad group of minerals having specific gravities 2.9 and greater (quart and feldspar are 2.6), that make up <1% of most arenites; exceptions are iron sands (high proportions of magnetite and ilmenite) and some volcanic sands with abundant pyroxenes and olivines. Common groups include tourmalines, zircon, sphene, amphiboles, pyroxenes, micas, iron oxides, and other ferromagnesian minerals like olivine. They are important for assessing sediment provenance.  Zircon is important because it is both mechanically and chemically stable, and it contains measurable Uranium and lead isotopes making it useful for dating and tracking potential source rocks (provenance).

Hedenbergite: Ca(Mg,Fe²⁺)Si₂O₆ – CaFe²⁺ Si₂O₆ –. Part of the clinopyroxene solid solution series that includes augite and diopside. Mg 50%-0%. Pleochroic in shades of purple-pink, blue-green through yellow green. Monoclinic. Characteristic (110) cleavage at 87^o and 93^o with inclined extinction. Biaxial (+). Most common in contact metamorphic calc-silicates.

Hematite: Fe₂O₃. Fe is in the oxidized III state. An important iron-bearing mineral, occurs in some hydrothermally altered rocks and veins, less common in igneous rocks. It is one of the foundation minerals of Proterozoic and Archean banded iron formations, as stratiform deposits commonly associated with chert, and mobile clasts like ooids and oncoids. It can also occur in some stromatolitic structures. Hematite forms hexagonal crystals, is uniaxial positive but this is difficult to determine because of its opaqueness. Has a distinct reddish hue in thin flakes and along the edges of broken fragments. Distinguished from magnetite by crystal form and colour.

Hexagonal crystal system: Crystals have four axes: three equal axes labelled a1, a2, and a3, at 120^o, in a plane at right angles to the c axis, such that a1 = a2 = a3 ≠ c. There is one 6-fold axis of symmetry. Common forms are prisms, pyramids, and bipyramids. Common mineral representatives include apatite, quartz. cf. Trigonal system.

Hornblende: A monoclinic amphibole having considerable chemical composition depending on Mg/Fe²⁺, Al/Fe³⁺, Al/Si, and Ca/Na ratios. Distinctive green to green-brown, and markedly pleochroic in shades of green-yellow;-blue green, and yellow-brown – brown – red brown. Usually prismatic with 6-sided cross-sections that have typical (110) cleavage sets at 56^o and 124^o (cf. the almost orthogonal cleavage in pyroxenes). Biaxial (-).  Common in a variety of igneous and metamorphic rocks. Hornblende that replaces pyroxene is sometimes called uralite.

Hypersthene: (Fe²⁺,Mg)SiO₃, En₈₀ – En₅₀.  An orthorhombic, colourless to pink or pale green orthopyroxene that is part of a solid solution series with bronzite and hypersthene. Basal sections are 4- or 8-sided, with characteristic (110) cleavage in two directions sets that intersect almost at right angles (93^o and 87^o). Longitudinal sections have parallel cleavage – extinction is also parallel to cleavage traces. Biaxial (-). Pleochroic in hues of pink, yellow, pale green, blue-green and red-brown. Most common in mafic rocks like gabbro, basalt, andesite, less so in intrusive rocks and amphibolites. Distinguished from enstatite by (-) sign and from clinopyroxenes by straight extinction.

Iddingsite: A distinctive red-brown microcrystalline alteration product of olivine, occurring as crystal rims and as patches along fracture planes. It is an Fe-Mg silicate, but usually contains clay minerals and hematite. In thin section it is generally not possible to obtain an interference figure (because of crystal size), and interference colours are masked by the mineral colour.

Ilmenite: FeTiO₃. A common heavy mineral in sedimentary rocks and in many black sands. It is an opaque, black, hexagonal-rhombohedral ,metallic oxide, that is a common accessory in intermediate and basic igneous rocks. It has weak magnetic susceptibility compared to magnetite (that it superficially resembles and in thin section is difficult to distinguish).

Induration: Refers to the degree of sedimentary rock hardening during compaction and chemical diagenesis, as temperatures, pressures, and fluid compositions evolve during burial.

Intergranular: Literally, between grains. The term is commonly used to describe matrix, cements, and porosity, and sedimentary fabrics such as preferred orientation.

Interference figures: These appear as curved isogyres or crosses when a mineral is viewed under crossed nicols at high magnification with the Bertrand lens inserted. There are two basic types:

1. Uniaxial crosses that do not break up or rotate as the stage is rotated.
2. Biaxial isogyres or crosses that rotate and move with the stage; crosses will break into two curved isogyres.

Intragranular: Literally within grains. It is most commonly used to describe textures, cements, and porosity in the chambers and whorls of bioclasts such as corals, bryozoa, gastropods and various microfossils.

Isometric crystal system: Also called the cubic system. Crystals have three equal axes all at right angles. The basic cube, the simplest and most symmetrical crystal form, has 8 faces; an octahedron 8 faces, a dodecahedron 12 faces. Simple cubes have 2-, 3- and 4-fold axes of rotational symmetry. Common minerals include garnet, fluorite, pyrite, halite.

Isopachous cement: Cements that rim clast surfaces and are of approximately equal thickness throughout. Common types include aragonite and calcite as fibrous, bladed or drusy crystals, prismatic quartz, and clays like kaolinite and illite. They are commonly overlain by later pore-filling cements. Isopachous cementation implies fluid saturated pore spaces.

Isotropic minerals: Unlike anisotropic minerals, this group cannot reorient plain polarized light which means that no light will pass through the upper polarizer; they will appear black through all rotations of the stage. All isometric minerals (cubic system) are isotropic (e.g., garnet, fluorite, halite, spinels). Note that an anisotropic mineral oriented at right angles to its optic axis will appear isotropic (because there is no resolution of fast and slow rays along the optic axis, i.e., no birefringence).

Jadeite: An alkali clinopyroxene – NaAlSi₂O₆, found in high pressure metamorphic rocks, usually associated with subduction zones, commonly with serpentinites. It is part of the solid solution series that includes omphacite and Na-bearing diopside. It is colourless to pale green, and unlike many other pyroxenes, is not pleochroic. Monoclinic, with typical pyroxene cleavage at 87^o and 93^o. Commonly in granular and fibrous masses. Biaxial (+).

Kaolinite: A triclinic clay mineral with the general formula Al₂Si₂O₅(OH)₄, presented as flakes a few microns wide, or aggregates of flakes into mica-like books. A common weathering product of feldspars, a common diagenetic product as a cement or replacement mineral.

Kyanite: Al₂SiO₅. Of the sillimanite group of minerals and the high pressure polymorph of andalusite and sillimanite. Triclinic, usually as bladed crystals; perfect (100) and less perfect (010) cleavage; twinned along (100). Common in blue hues, but colourless in thin section. Biaxial (-) with 2V 82^o. A common component of regional, high pressure – low temperature schists (like those formed in subduction zones). A good provenance indicator as a detrital component.

Laths (igneous petrography): Very small needle-like crystals commonly set in very fine or glassy groundmass. Aphanitic volcanic rocks may have abundant feldspar laths; alignment of crystals during lava flow is common.

Limonite: Fe(OH).nH₂O. An amorphous, soft, yellow-orange to red-brown hydrated iron oxide that usually occurs as the weathering product of iron-bearing minerals, as cements in some coarse-grained sediments, and ferricretes. It is commonly an indicator of ancient water tables in coarse-grained aquifers where it forms crusts and nodules. It is also responsible for the iron staining in Liesegang rings. Tends to be opaque in thin section. Commonly mixed with clays.

Lithic (petrology): Also called rock fragments. A general term for clastic material that cannot be pigeonholed into the other main framework components like quartz and feldspar. It usually applies to fine grained lithologies such as mudstone and siltstone, or aphanitic, glassy volcanic rock (with or without small feldspar laths). Chert fragments present something of a problem in sandstone classifications because they consist of micro- and cryptocrystalline quartz; they are sometimes placed in the total quartz category, at other times in the lithics category.

Lithification: The combination of compaction and cementation that produces hard, hammer-ringing rock from loose, uncompacted sediment. Lithification depends on a complex association of physical and chemical processes. Cementation can occur at very shallow depths in the case of carbonates, or at different stages of burial depending on temperature, and rock – fluid chemistry. Compaction begins soon after deposition and continues at depth.

Magnesium calcite: Also called magnesian calcite. In the calcite crystal lattice, magnesium can occupy the position of calcium, up to about 20 mole percent. Two varieties predominate in carbonate sediments and limestones: Low magnesium calcites (LMC) with <4 mole % Mg), and high magnesium calcites (HMC) with 11-19 mole % Mg). HMC commonly recrystallize to LMC during burial diagenesis.

Magnetite: Fe₃O₄. A common opaque, black, accessory spinel in igneous, metamorphic and hydrothermally rocks, and as a heavy mineral in sedimentary rocks. Euhedral habits range from cube to octahedra. Detrital forms tend to be rounded. High magnetic susceptibility (cf. ilmenite) – can be easily separated using a magnet. An important component of black sands.

Matrix: The fine-grained material among coarser framework components like sand grains or pebbles. Matrix is a depositional product, that accumulates at the same time as the coarser grained sediment. Its fine-grained quality commonly renders it diagenetically more reactive than the associated framework grains. Note the term groundmass is reserved for the finely crystalline to glassy component of volcanic rocks.

Matrix-supported framework: Applies to mud-dominated lithologies where sand-size and larger clasts appear to float in the mud-silt matrix. A classic example includes pebbly mudstones. Cf. Clast-supported framework.

Mean grain size:  The most common statistic used in describing grain size distributions, is the average size in a population of grains; it is one measure of central tendency (see also Median, Mode, and Skewness). The mean of a population should always be quoted with its standard deviation. The Folk-Ward (1957) calculation uses Phi values at the 16^th, 50^th and 84^th percentiles:  Mϕ = [ϕ₁₆ + ϕ₅₀ + ϕ₈₄] /3

Mechanical stability (of detrital grains): The ability to resist abrasion and breakage during sedimentary transport. It depends on grain-mineral hardness, and in the case of crystalline minerals the presence of cleavage or twinning. Thus quartz is mechanically more resistant than feldspar, even though their hardness is similar; both are more resistant than calcite that is softer and has good cleavage.  Cf. Chemical stability.

Median grain size:  One of the three common measures of central tendency, the median is taken as the Phi value at the 50^th percentile. See also Mean, Mode and Skewness.

Meniscus cement: Cements that accumulate at clast contacts in vadose, or unsaturated conditions, where precipitation (commonly calcite or aragonite) takes place in the fluid meniscus. The meniscus itself is caused by surface tension forces at the boundaries of grains in contact.

Microcrystalline: From a practical point of view, crystals too small to see with a hand lens, requiring a microscope to see individual microcrystals. Common examples are chert, flint, and chalcedony composed of microcrystalline quartz. See also Cryptocrystalline.

Micron: A micro-metre, or millionth of a metre, usually written µm.

Microporosity: Porosity that is 1-2 µm contributes to the total pore volume of a rock or sediment, but in terms of advective fluid flow it is inefficient. Transfer of dissolved mass probably takes place by diffusion.  Common examples are present in pore throats of granular rock, between clay particles in mudrocks, and between pore-filling cements.

Micrite: A contraction of microcrystalline carbonate mud, refers to aragonite or high-magnesium calcite crystals less than 4 microns, that form as cements (usually isopachous cements), or the product of substrate alteration by boring algae, fungi, and larger critters like boring Clionid sponges – a process call micritization. Micrite is highly reactive and susceptible to recrystallization during meteoric and burial diagenesis because the original mineralogy is metastable, and because of the high surface area afforded by the micron-sized crystals.

Miller Indices: William Hallowes Miller (1801-1880) was a Welsh mineralogist who took the role of Professor of Mineralogy at Cambridge University, replacing William Whewell, a well-known polymath, philosopher, and social commentator. Miller’s primary contribution to mineralogy was published in A Treatise on Crystallography (1834). In this he outlined a method to simply the numerical system of crystal nomenclature first proposed by Christian Weiss; Millers systematic approach stands today. Miller, and Miller-Bravais indices are the standard where every face on a crystal is given a unique description. The general notation for any crystal face is (hkl) for three-axis crystal systems, and (hkil) for 4-axis systems.

Miller-Bravais indices: The system of notation applied to crystals with 4 axes – namely trigonal and hexagonal crystal systems. The general designation is (hkil).  The method for calculating the notation of each face is the same as that for three-axis systems (Miller Indices).

Modal grain size:  The mode of a grain population is the Phi percentile for the most abundant grain size class on a frequency distribution curve. See also Median, Mode and Skewness.

Monoclinic crystal system: Two equal axes in the ab plane –  a = b ≠ c, where the angle b^c and a^b are 90^o, but the angle a^c ≠ 90^o. There is a 2-fold axis of rotational symmetry. The low degree of crystal symmetry is next only to the triclinic system. Common examples include gypsum, muscovite and orthoclase.

Monocrystalline quartz: Applies to grains of quartz that consist of a single crystal. It is used in petrographic descriptions of arenite and coarser lithologies to distinguish this variety of quartz from polycrystalline quartz. See also strained quartz, volcanic quartz.

Mud: A mix of silt- and clay-sized particles. On the Wentworth scale it includes all sizes smaller than 0.0625 mm, or 4 phi. Grain size analysis of unconsolidated mud samples is usually by pipette, or Laser Size Analyser.

Mud-supported: Synonymous with matrix-supported, where large framework clasts appear to float in a mud matrix.

Muscovite: K₂Al₄ (Si₆Al₂)O₂₀(OH)₄. Monoclinic, commonly prismatic, book-like. Perfect (001) cleavage. Colourless to faintly pleochroic in shades of blue, yellow-green and blue green. Biaxial (-) with strong birefringence. Extinction in prismatic sections is parallel. Muscovite has a distinctive birdseye, or twinkle appearance at extinction. Sedimentary grains are usually cleavage fragments. Present in some granites, but mostly found in regional metamorphic schists, slates, and gneisses. A common detrital heavy mineral but because of its crystal habit it tends to behave as a lighter fraction during hydraulic sorting.

Neomorphism: Defined by R. Folk in 1965 as the transformation between one mineral and itself or a polymorph. In other words, neomorphism is a product of recrystallisation where the bulk composition does not change, only the size and/or shape of crystals. It is common in carbonate lithologies and involves recrystallisation of both framework clasts and cements. As such it tends to cross-cut pre-existing textures and fabrics; relict textures may be preserved. Aggrading neomorphism is common in micrites where crystals increase in size in a more-or-less radial fashion. See Neomorphic textures in thin section

Olivine: An important group of ferromagnesian minerals in mafic and ultramafic igneous rocks, from basalts to dunites (almost pure olivine), and in stoney meteorites (chondrites and achondrites). It is one of the main mineral phases in upper mantle rocks. The two main types are forsterite and fayalite (see the entries for these). The general formula is (Fe₂,Mg2)SiO4. But within each Fe and Mg mineral type there is a range of compositions depending on the ratio of Fe and Mg. Melting points change according to composition. It is one of the first minerals to crystallize from magmatic melts and tends to be unstable above pressures equivalent to about 400 km depth. At shallower conditions it tends to alter – a common alteration product is red-brown, microcrystalline iddingsite. It is generally rare as a detrital mineral, except in first cycle volcaniclastic sands.

Oncoids: Oncoids are rounded, spherical to oblate, concentrically laminar growths of algae around a nucleus (shells, mud intraclasts, broken lumps of algal crust). They grow at the sediment surface in supratidal (including sabkhas), intertidal and shallow subtidal environments that are agitated by currents and waves, and perhaps frequented by storm surges. Oncoid laminae grow asymmetrically while the oncoid is at rest, and discordantly when growth has been interrupted by clast jostling and rolling.

Oncolite: The rock name for limestone consisting of oncoids.

Ooids: Ooids are spherical to subspherical grains, typically 0.5mm diameter, characterized by concentrically layered, micron-sized calcite or aragonite crystals. They are common constituents of carbonate platforms and ramps where they form extensive bars and sandwaves. There is usually a nucleus of skeletal debris or pelloids. In saline lakes the ooids may consist of radial aragonite. Ancient ooids also show radial crystal fabrics. They precipitate from seawater, but there is mounting evidence that this is mediated by microbes. They are also known from Precambrian iron formations.

Oolite:  A limestone made up predominantly of ooids. It is a rock name. There will usually be evidence for bedload transport of ooids, such as crossbeds and scours, or preserved ripple forms.

Optic axes: Axes along which plain polarized light is NOT split into fast and slow rays. In other words, there is NO birefringence parallel to an optic axis. Minerals having a single optic axis are uniaxial; those having two are biaxial.

Orthoquartzite: Synonymous with quartz arenite – a sandstone with 95% and more quartz grains.

Orthopyroxene: The orthorhombic members of the pyroxene group of minerals. Most common are enstatite, hypersthene, and bronzite that form a solid solution series. Characterized by (110) cleavage in two sets almost at right angles in 4 and 8 sided basal sections, and parallel cleavage in prismatic sections. In the latter extinction is parallel (cf. clinopyroxene).

Orthorhombic crystal system: Analogous to an isometric cube that is stretched along the c-axis and one other axis. a ≠ b ≠ c; interaxial angles all 90^o. There is 2-fold rotational symmetry on each of the faces. Common examples include sulphur, olivine, aragonite.

Packstone: A clast-supported limestone framework with >90% of clasts finer than 2 mm (upper limit of coarse sand). Dunham’s (1962) limestone classification scheme reviewed and modified by Lockier and Junaibi (2016).

Paragenetic sequence: (paragenesis) In sedimentary petrology, the sequence of mineral components precipitated (and dissolved) during diagenesis. Sequential changes in mineral composition and/or crystallographic form reflect evolving fluid compositions, fluid flow, burial temperatures, and compaction. It is analogous to cement stratigraphy. Also see sandstones in thin section.

Pebble: In the gravel-conglomerate grain size category, it occupies the range 2-64 mm, or -2Φ to -6Φ on the Wentworth size scale.

Pelloids: Any spheroidal, sand sized grain that is an aggregate of micro- to cryptocrystalline carbonate. There is little or no internal structure. If they are known to be fecal then they are called pellets. Otherwise, the term pelloid should be used.

Pedion crystal forms: A crystal form with only one face. This only occurs in rare triclinic minerals that have no symmetry.

Pendant cement: Stalactite-like cements that accumulate on the low point of grains during gravity drainage of interstitial fluid. They are common in carbonates subjected to vadose zone diagenesis.

Pericline twins: A common twin in plagioclases and potassium feldspars. Twins are commonly multiple, parallel laminae, similar to albite twins but oriented at different angles, commonly at 90^o to albite twins.

Permeability: A measure of the ease with which fluids flow through porous sediment and rock. In groundwater studies it is expressed as hydraulic conductivity that has dimensions of distance/time. The hydrocarbon industry uses a dimensionless number for intrinsic permeability, the Darcy, that depends only on the porous medium. The unit reduces mathematically to units of area (ft², m²). It is basically a measure of pore size.

Pericline twins are similar to albite twins but are oriented at different angles, commonly at 90^o to albite twins.

Perthites: One of the key identifiers of potassium feldspars under a polarizing microscope is perthitic texture, which is a mix of two different exsolution feldspar phases – albite and orthoclase. Exsolution occurs during crystallization from the melt.

Petrography: Is the description of sediments and rocks; their colour, hardness, texture, mineral and chemical composition, and fabric. Petrography provides a focal point petrologists.

Petrology: Is the study of the origin of sediments and rocks, the physical, chemical, and organic conditions and processes that lead to their formation.

Phenocryst: Conspicuous, relatively large crystals in igneous rock, particularly in volcanics where they can easily be distinguished from groundmass. A rock that has lots of phenocrysts is described as porphyritic.

Phi scale  A linear grain size scale based on the logarithm (to base 2) of the size in millimetres, devised by W.C. Krumbein; Φ = -Log₂.  Negative values of Phi apply to all grain sizes greater than one millimetre that, on the Wentworth scale, subdivides coarse from very coarse sand. Phi values smaller than 1 mm are positive. Phi values have proven to be useful for calculating statistical parameters such as mean and standard deviation (sorting).

Pigeonite: A clinopyroxene at the Mg-Fe end of the series – Ca(Mg,Fe²⁺,Al)(Si,Al)₂O₆. Colourless to faintly pleochroic in shades of pink, brownish to green-pink, to pale red brown. Monoclinic, with typical (110) cleavage at 87^o and 93^o. May be intergrown with hypersthene or augite. Biaxial (+), commonly twinned. Occurs mostly as fine laths in basalts, diabases.

Pinacoid crystal forms: An open crystal form in which two faces are parallel

Pisoid: Concentrically layered ovoid to markedly elongate carbonate bodies that superficially resemble oncoids. However, unlike oncoids and ooids, pisoids form in meteoric vadose conditions, commonly associated with calcretes (paleosols). Common textures include: close-fitted and intergrown pisoids (hence their shapes are irregularly elongate), evidence of multiple stages of precipitation and dissolution (commonly as cross-cutting fabrics), and gravitationally-induced pendant cements.

Pisolite: A limestone made up predominantly of pisoids. It is a rock name. Commonly form in vadose zone soils or caliches.

Plane polarized light:  (PPL) The light transmitted through a polarizer, located below the microscope condenser (the condenser focuses this light through an opening in the rotary stage). The polarizer filters out all light frequencies other than those that vibrate in a single plane. The polarizers here are oriented E-W (Note the upper polarizer is oriented N-S).  Minerals in thin section examined under PPL show important identifying characteristics such as crystal shape, cleavage, breakage patterns, relief, and pleochroism. The light path for PPL is shown diagrammatically below.

Pleochroism: Under plain polarized light, some minerals (in thin section) will change colour as the microscope stage is rotated. This is caused by absorption of certain light frequencies as the crystal is rotated (under polarized light). The colour changes, and the angles of change relative to crystal axes, are important identifying criteria. For example, biotite (a brown mica) presents colour changes from almost colourless to shades of brown and green.

Poikiloblast: Crystals that have skeletal, sieve- or net-like form where the gaps in the parent mineral are filled by inclusions of another mineral. Poikiloblasts usually grow around their inclusions and therefore are later mineral phase. The arrangement of inclusions (e.g., rotation) may indicate earlier stage of deformation. A common form in metamorphic rocks; e.g. staurolite, andalusite, garnet, biotite.

Poikilotopic cement: Growth of large crystals (commonly calcite, dolomite) that enclose several/many framework grains.  In thin section, the enclosing crystals have uniform extinction (under crossed nicols) whereas the individual framework grains will usually have disparate extinctions.

Polarizer (microscope): In an optical microscope, the polarizer changes plain white light (that contains many frequencies through the electromagnetic spectrum), into light that vibrates in one plane – by blocking all other vibration directions. This is plain polarized light.

Polycrystalline quartz: Used to description of sand-size and larger quartz grains (usually in thin sections) that consist of two or more, usually more, subcrystals. Subcrystal boundaries are generally irregular – even embayed. Under crossed polars each subcrystal will move into extinction at different stages of thin section rotation. The mechanical breakdown of polycrystalline grains produces very fine sand to silt-sized particles. This category of quartz is common in arenites; the most common source is metamorphic rocks.

Polymorphic crystals: Minerals having the same composition but different crystal form. Calcium carbonate has two common polymorphs: calcite (hexagonal crystal form) and aragonite (orthorhombic). Likewise, quartz has three common polymorphs that exist under different temperature and pressure regimes: common quartz (hexagonal), tridymite (orthorhombic), and cristobolite (tetragonal).

Pore throat: The narrow passages between grains in contact, that connect the larger intergranular pores. Pore throat sizes are variable, depending in part on the packing arrangement of grains and grain shapes, and range from submillimetre to a few microns. Their size and distribution are a primary control on the characteristics of fluid flow. Pore throats can be blocked and their efficacy reduced by cements, particularly clays.

Porosity – intergranular:  the void space between framework clasts within a rock or sediment. It is presented as the ratio of total void space versus total sample volume and is therefore dimensionless. Pore spaces below the watertable are always occupied by fluid – aqueous, or hydrocarbon. The porosity of a clean sand is commonly 30-35% but can be reduced to less than 1% by compaction and cementation. Mud porosity can be as high as 70% at deposition, but this too rapidly decreases during compaction.

Porphyritic: An igneous rock composed of a large proportion of phenocrysts.

Potassium feldspar: (K-spar, or alkali feldspar) A complex group of feldspars where the silicate and aluminium lattice structures are bound by potassium (in the plagioclases they are bound by calcium and sodium). Varieties include sanidine, orthoclase, microcline, anorthoclase, and adularia. In igneous melts there they form a solid solution series, which means that compositions and crystallography vary considerably from one type to another. The most common twin types are carlsbad, pericline and albite that often combine to form a characteristic gridiron or cross-hatched pattern. Some K-feldspars are untwinned.

Potassium ferricyanide: A cheap and easy to use water-soluble salt to determine the degree of ferroan iron (Fe²⁺) in calcite and dolomite. The intensity of blue stains in ferroan calcite increases with increasing Fe content; ferroan dolomite stains in green hues.

Prism crystal forms: Prisms are common open forms having 3 (trigonal), 4 (rhombic and tetragonal), 6 (ditrigonal and hexagonal), 8 (ditetragonal), or 12 (dihexagonal) parallel faces.

Protodolomite: The term was originally applied by Graaf and Goldsmith (1956) to imperfectly ordered dolomite with excess calcium, later modified to include ordered lattices. Whether this is a useful definition is still debated. It commonly refers to geologically Recent, disordered dolomites, many of which are found in sabkha-like environments and saline lagoons. Production of protodolomite and dolomite in low, Earth-surface temperature lab experiments is difficult – common products in these experiments are high magnesium calcite and brucite (Mg(OH)₂), and magnesite (MgCO₃).

Provenance: Provenance studies the origins of detrital clasts in sedimentary rocks. It determines where and when they came from; i.e. provenance attempts to determine the composition of the original source rock, the tectonic environment from which it was delivered, and how far the source rock or sediment has traveled. Potentially it permits the reconstruction of ancient plate locations and plate motions.

Pyramid crystal forms: A common open form that has 3 (trigonal), 4 (tetragonal, rhombic), 6 (ditrigonal, hexagonal), 8 (ditetragonal) or 12 (dihexagonal) faces that meet at a crystal termination (a point), or would meet if they were extended.

Pyrite: FeS₂. A common accessory, opaque mineral in many metamorphic and igneous rocks, in hydrothermally altered sedimentary rocks, as an authigenic phase in anoxic sediments (e.g., black shales), and as a diagenetic product when reduced iron is available (Fe is in a reduced state). Forms isometric crystals with cubic habit, singly or in clusters. Oxidizes to limonite (FeO(OH) · nH₂O) and goethite (FeO(OH)) and hence is less common as a heavy mineral in sedimentary rocks.

QFL plots: Ternary plots of quartz, feldspar and lithics, and variations on this theme, to describe sediment composition (principally in sandstones and conglomerates), and to track stratigraphic and basinal changes in composition with reference to the provenance of epiclastic sediment.

Quartz: One of most common detrital constituents of sedimentary rocks, and second only to feldspar to all rocks in general. Its chemical formula is SiO₂. Crystallographic form is hexagonal rhombohedral. It is uniaxial positive and non-pleochroic. Strained quartz will show undulatory extinction. It occurs in felsic igneous and metamorphic rocks, hydrothermal deposits, fracture fills, and as diagenetic products (e.g. cements). It forms chert and chalcedony in its micro- and cryptocrystalline form.

Quartz arenite: A sandstone with 95% and more quartz grains. Also called an orthoquartzite. The term quartzite tends to be reserved for metamorphic rocks.

Quartzite: An older term synonymous with quartz arenite and orthoquartzite. The term is more appropriately applied to metamorphosed equivalents of these rock types.

Radiaxial fibrous cement: Radiaxial fibrous calcite and its companion fascicular optic calcite are common cavity-filling cements in ancient limestones. Both consist of radially fibrous calcite clusters but differ in their crystallographic orientation. Cleavage and twin planes in radiaxial crystals are concave toward the pore interior, in concert with optic axes that converge toward pore spaces (i.e. opposite the divergence in the actual crystals). In some cases, the fabric may represent calcite-dolomite replacement of initially fibrous aragonite. In others the fabric may precipitate directly.

Recrystallisation:  In sedimentary rocks this involves the transformation or replacement of a mineral with itself, and usually entails changes in crystal size and shape (but not bulk composition): as in micritic calcite to sparry calcite, or aragonite to its polymorph calcite. The term was originally coined for the process of annealing in metals, which is a dry process. Recrystallisation in sedimentary rocks is always a wet process that involves dissolution of a mineral at grain boundaries, followed by precipitation. It tends to cross-cut original textures, destroying them in the process. See also Neomorphic textures in thin section

Relief: (crystallographic) The refractive index (RI) of a mineral is the ratio of the velocity of light in air to that in the mineral. Indices differ amongst minerals such that:

• Isotropic minerals have a single RI
• Uniaxial minerals have two RI along different axes, and
• Biaxial minerals have 3 RI.

The different RIs for any mineral are manifested as relief in plain polarized light, where one mineral may appear to stand above or below its neighbours. The orientation of uniaxial and biaxial minerals will also determine their relief.

Rhodolith: Pebbles and shells encrusted with calcareous coralline red algae. They are important contributors to cool-water carbonate sediments and limestones A common example is the alga Lithothamnion.

Rock fragments: See lithics.

Roundness: The textural quality that describes the degree of rounding, or angularity of clasts. It DOES NOT refer to the overall shape of the clast. Thus, a rod-shaped clast can be as well rounded as a spheroidal clast. Although roundness can be quantified, it is usually identified by comparing the clasts under investigation with grain silhouettes, as in a handy-dandy grain size comparator. The Powers scale used today identifies a range from very angular to very rounded.

Rudstone: A clast-supported framework where more than 10% allochems coarser than 2 mm (upper limit of coarse sand), supported by grains coarser than 2 mm. In other words, everything is coarser than 2 mm. Approximately equivalent to one of Folks poorly sorted, sorted, or rounded sparites, where the qualification might include bio, ooid, pellet and so on. Dunham’s (1962) limestone classification scheme reviewed and modified by Lockier and Junaibi (2016).

Rutile: TiO₂. Red brown to yellow brown, tetragonal, uniaxial (+) with very high birefringence. Most common in metamorphic rocks, less so in igneous rocks. A common heavy mineral in sedimentary rocks in euhedral to rounded prismatic or acicular crystals.

Sand: According to the Wentworth scale, it includes all grains from 0.0625 mm to 2 mm (4 to -2 phi).

Secondary porosity: Porosity that is created during burial diagenesis by the dissolution of chemically reactive grains such as carbonates and feldspars. Secondary porosity can enhance the overall porosity of a rock, particularly if primary intergranular pore volumes have been occluded by cements. Secondary pores may be larger than those formed during deposition, where entire grains are dissolved. Partial dissolution along twin or cleavage planes in minerals like feldspar, will result in irregular grain boundaries.

Sericite: A flaky white mica and common alteration product of feldspar. In thin section it usually presents as fine ragged crystals (rather than the more uniform muscovite flakes), concentrated along feldspar cleavage planes, or distributed across the entire crystal. It has high birefringence and appears to sparkle against the dull background of altered grains and matrix.

Siderite: (Fe,Mg)CO₃ – FeCO₃. An iron carbonate commonly found in mudrocks, as concretions in paludal deposits, and as ooids. In hues of grey to orange brown. Also as a hydrothermal alteration mineral. Hexagonal, uniaxial (-), very high relief and birefringence. Alters to limonite and goethite.

Sieve analysis: A method for measuring grain size distribution in unconsolidated granular sediment, that uses a series of stacked sieves, each sieve containing a standard mesh aperture, coarsest at the top of the stack. Each sieve will retain sediment that is coarser than the mesh size; grains with a minimum diameter less than that mesh diameter will pass through to the next sieve below.

Sieve diameter: The minimum diameter of a particle-grain that will pass through a particular sieve aperture. Spherical grains have equal diameters at all orientations.More elongate or platy grains have maximum and minimum diameters. Sieve aperture sizes are standardised, commonly over a range from -6Φ (pebbles – 4-64 mm) to 4Φ (very fine sand – 0.0625 mm).

Sieve texture: (Mineralogy). See poikiloblast.

Siliciclastic:  Sediments composed predominantly of detrital, silica-based minerals; the most common components are quartz, feldspar, and lithic fragments. Heavy minerals such as magnetite, zircon, and tourmaline are important constituents, usually in trace amounts. This broad category includes all grain sizes. It does not include clastic carbonates.

Sillimanite: Al₂SiO₅. Of the sillimanite group of minerals and the polymorph of kyanite and andalusite. In contrast to Kyanite, it is found in high pressure – high temperature metamorphic rocks. Orthorhombic, usually as narrow prisms, needles, and fibrous arrays. Biaxial (+), colourless in thin section, but commonly pleochroic in shades of pale yellow, green and blue. Prismatic crystals show (010) cleavage. 2V is much lower than kyanite at 30^o. A good provenance indicator in detrital form.

Silt: According to the Wentworth scale, it includes all grains from0.004 mm to 0.062 mm, or 8Φ to 4Φ.

Skewness (grain size):  Skewness describes the asymmetry of a frequency distribution. It is a dimensionless number. For grain size statistics, the Folk and Ward Phi-based formula is most commonly used:

Sk = [(ϕ₁₆ + ϕ₈₄ – 2 ϕ₅₀) /2(ϕ₈₄ – ϕ₁₆)]  +  [(ϕ₅ + ϕ₉₅ – 2 ϕ₅₀) / 2(ϕ₉₅ – ϕ₅)]

Solid solution series: Minerals that share the same basic chemical formula but have different proportions of key elements in their crystal lattice such that crystal form may vary. In sedimentary rocks the most important examples are the alkali (K-Na end-members) and plagioclase (Na-Ca end-members) feldspar groups. Olivine also forms a series with fayalite and forsterite end-members. A mineral’s position in a series reflects the composition and temperature of, for example, the original igneous melts (in the case of feldspar and olivine.

Sorting (grain size):  Sorting is a measure of standard deviation – the spread of measurements about the mean of a population. The importance of sorting lies in its relationship to the hydraulics of deposition, particularly in relation to reworking and winnowing of certain size classes. For example, in a high energy shallow marine environment, silt and clay sized material will be removed, leaving grains having a relatively narrow size range. The Folk and Ward formula commonly used is:

σϕ = (ϕ₈₄ – ϕ₁₆ /4) + (ϕ₉₅ – ϕ₅ / 6.6)

Sphene: CaTi SiO₅. Monoclinic, high refractive indices, pleochroic from colourless to green-yellow, pink, orange, red brown. Detrital forms commonly brown hues. Biaxial (+), rhombic cross-sections common, show symmetrical extinction; moderate (110) cleavage. Common accessory mineral in intrusive igneous rocks, some extrusive rocks, and a variety of metamorphic rocks. A common heavy mineral in detrital sediments.

Sphericity: In describing the shape of sedimentary grains, reference is made to a standard sphere. Sphericity can be purely descriptive or quantified by measuring the lengths of the three axes and comparing them to an ideal sphere. The term spheroid is synonymous with ellipsoid.

Stalactite:  Tubes, straws. and threads of calcite that hang from the ceiling in the drip zone of caves. Groundwater, initially undersaturated with respect to calcite can, with sufficient transfer of atmospheric CO₂, become supersaturated, promoting precipitation. Pillars or columns form when stalactites meet stalagmites, their cave-floor counterpart. They are a type of speleothem, a group of cave precipitation structures that includes cave wall linings (drapery), flowstone, and cave pearls. Stalactites and stalagmites can also form from dripping lava.

Stalagmite: Commonly conical shaped mounds of calcite that grow from cave floors as a result of the steady drip of seepage groundwater. They are the cousin of stalactites.

Staurolite: Fe²⁺₂ Al₉Si₄O₂₂ (O,OH)₂. Orthorhombic, poor cleavage such that broken fragments are random sections. Euhedral in metamorphic rocks, commonly anhedral in detrital sediments. Biaxial (+), pleochroic in shades of colourless, pale yellow to orange or red brown. Commonly found as poikiloblasts with inclusions of quartz, garnet, biotite – a texture that helps to distinguish from tourmaline.

Strained quartz: Used in petrographic descriptions for quartz grains that under crossed polars exhibits sweeping extinction. It results from crystal lattice dislocations during deformation.

Structure grumeleuse: A term introduced by Lucien Cayeux in 1935, refers to clotted limestone textures where isolated, diffuse patches of micrite are surrounded by coarser neomorphic spar; the overall texture appears clotted. At times it can be difficult to distinguish between this recrystallisation texture and primary peloidal limestones. See also Neomorphic textures in thin section

Stylolite:  Saw-tooth like, discordant seams that signify pressure solution of rock components (framework clasts and cements). They are most common in carbonates but can form in siliciclastic rocks. They represent differential compressive stresses at grain-to-grain contacts, the dissolution and mass transfer of carbonate by diffusion and fluid flow. Stylolites commonly parallel bedding (from normal compressive stress) but also form oblique to bedding.

Syntaxial overgrowths: Crystals that have grown on, and are in optical continuity with a substrate (of the same composition) are referred to as syntaxial overgrowths or cements; i.e. they share the same optic axis (the term epitaxial is also used in this sense by some, although the International Mineralogical Association insists that in epitaxy the overgrowing phase is mineralogically different to the substrate). Well known examples involve quartz overgrowth cements in quartz arenites; in many cases the original grain outline is distinguished by a rim of inclusions. In limestones, echinoderm plates commonly have syntaxial calcite overgrowths. In polarized light and crossed nicols, both the overgrowth and original grain move into extinction together.

Terrigenous: A very general term for detrital sediment derived from sources on land. Sediment production is primarily by weathering and erosion. Terrigenous sediment is transported to the sea mainly by rivers. Air-borne dust may also find its way into the marine environment. Most siliciclastic sediment deposited in marine environments is ultimately land-derived. See also provenance.

Tetragonal crystal system: Analogous to a isometric-cubic system but stretched along the c-axis. Axes a and b are equal, but not equal to c. All axes at right angles. Mostly 2- and 4-fold symmetry. Common examples include zircon, rutile.

Textural maturity: Describes the grain size range of granular sediments in terms of its sorting. For example, a well-sorted sand is regarded as texturally mature by virtue of the limited range of grain sizes present. It also implies a degree of hydraulic reworking, where fine-grained sediment has been winnowed. Compare a well-washed beach sand (mature) with a turbidite sandstone where there is virtually no reworking (immature).

Texture: In rock descriptions, texture refers to the physical qualities of detrital clasts and crystals: their size, shape, roundness, surface structures, and fabric. See also grain size distributions, sorting, textural maturity.

Thin section: A slice of rock glued to a glass slide and ground to about 30 micron thickness. It is used for petrographic analysis using a polarizing microscope. The rock wafer is either covered by a very thin glass cover slip, or left uncovered to be used with other petrographic analytical methods such as mineral staining. The thickness corresponds to quartz with pale grey interference colours.

Tourmaline: A complex group of aluminium-boron silicates; there are three main minerals – Schorl  (Na-Fe²⁺-B), Dravite (Na-Mg—B-Al), and Elbaite (Na-Al-Fe²⁺-Mn-Li-B). There is a range of crystal colours. All three types are hexagonal, uniaxial (-), commonly forming needle and elongate prisms; they also have parallel extinction. Schorl is strongly pleochroic in shades of grey, green, and blue. It is a common heavy mineral in detrital sediments and easily identified by its crystal habit and pleochroism.

Tremolite – Actinolite: Monoclinic amphiboles that are completely gradationl chemically and optically. Tremolite composition is Ca₂Mg₅Si₈O₂₂ (OH)₂, and Actinolite with Fe²⁺ replacing Mg. Colourless to pale green. Actinolite may be pleochroic in shades of yellow-green – brown and blue green. Crystals bladed, fibrous, or in radial clusters; cross-sections are 4-sided or diamond-shape, with two good (110) cleavage sets at 56^o and 124^o – cf. the pyroxene cleavage which is almost orthogonal. Both are biaxial (-). Mostly found in contact and regional metamorphic rocks.

Triclinic crystal system: Has the lowest degree of symmetry of all crystal systems. Three axes, a ≠ b ≠ c; All interaxial angles are unequal and ≠ 90^o.  There are no axes of rotational symmetry. Common examples include the plagioclase feldspars, potassium feldspars like andesine and anorthoclase, kyanite.

Trigonal crystal system: Crystals have four axes: three equal axes labelled a1, a2, and a3, at 120^o, in a plane at right angles to the c axis, such that a1 = a2 = a3 ≠ c. The fundamental difference with hexagonal crystals is the 3-fold axis of symmetry. Common forms are prisms, pyramids, and bipyramids. Common mineral representatives include tourmaline, dolomite, calcite, illmenite.

Tufa: A natural, surface precipitate of calcium carbonate in alkaline lakes, rivers, springs and geothermal hot pools, promoted by degassing of CO₂ as the waters exit to the surface. Degassing of CO₂ results in an increase in pH, and concomitant increase in the stability of CO₃^2- and HCO₃^– aqueous species and the degree of calcite saturation. It is also possible that microbial activity also promotes precipitation. Tufas tend to be highly porous; they can encase dead critters and vegetation. Travertines are a denser form of surface calcite precipitation. Extensive deposits are typically terraced.

Twinning (crystallography): A symmetrical intergrowth of two separate crystals of the same mineral, that share the same mineral lattice. In thin section under crossed nicols, each individual will go into extinction at different rotations of the microscope stage. The kind of twinning, and its optical properties are important defining characteristics for mineral identification.

Uniaxial minerals: Anisotropic minerals where plain polarized light entering at any angle other than along a single optic axis, is resolved into two planes of polarized light; these two planes contain the fast and slow rays. The resulting colour depends on the different in the refractive indices of these two light paths – i.e., the birefringence. Note that mineral sections where the optic axis is vertical will appear isotropic under crossed polars. However, when the Bertrand lens is inserted, you will see a dark cross that is also centered. The N-S axis of the interference cross parallels the fast (extraordinary) ray, and the E-W axis the slow (ordinary) ray. Minerals may be positively or negatively uniaxial, depending on the orientation of fast and slow rays.

Unit cell: At the atomic scale, the arrangement of atoms that represents the fundamental structure of a mineral in crystal form. The crystals we see consist of a three-dimensional array of stacked unit cells. This means that the overall shape of the crystal mimics its unit cell.

Unroofing (tectonics): Describes the uplift and erosion of rocks that in stratigraphic successions records the order that successive rock types or layers were exposed.  For example, the stratigraphic expression of unroofing of a magmatic arc will be an initial pulse of volcanic and volcaniclastic sediment, overlain successively by exposure and erosion of progressively deeper intrusive igneous rocks.

Ultrabasic (igneous petrology): Igneous rocks that are poor in feldspar and have abundant, mafic, ferromagnesian minerals like olivine, pyroxene, and amphibole.

Wacke: A sandstone with 15-75% matrix; less than 15% matrix and it is an arenite. The carbonate version is called a wackestone. The most frequently cited example in siliciclastic rocks is greywacke. The induration of greywackes renders the distinction between matrix and diagenetic minerals difficult, particularly distinguishing between detrital and diagenetic clays, with the added overprint of micas (particularly muscovite), chlorite, and variable amounts of calcite.

Wackestone: A mud-supported framework with >10% coarser than mud grade, but less than 10% coarser than 2 mm. The Folk equivalent is a micrite qualified with the dominant allochem, such as bioclasts or ooids. Dunham’s (1962) limestone classification scheme reviewed and modified by Lockier and Junaibi (2016).

Weiss’ crystal parameters: Christian Weiss (1780 – 1856), a German mineralogist devised a system of crystal classification based on the relative lengths of crystal face intersections with  crystal axes. Each face could be labeled in the fashion 1a, 1/2b, ∞c that in this example indicated unit intersection length on axis ‘a’, a fractional length on ‘b’, and no intersection (i.e., parallel) with ‘c’. His method was later modified (and simplified) by William Miller of Miller Indices fame.

Wentworth grain size scale: Also called the Udden-Wentworth scale (1922). A scale based on the geometric progression of grain size, centred on the sand size-range: Thus, very coarse sand is 1-2 mm; coarse sand 0.5-1.0 mm; medium sand 0.25 – 0.5 mm; fine sand 0.125 – 0.25 mm, and so on. Krumbein’s Phi scale is a Log₂ conversion of these size ranges.

Zircon: ZrSiO₄. A common accessory in igneous and metamorphic rocks, and an important heavy mineral in detrital sedimentary rocks. It can survive several depositional cycles. It is one of the most common minerals used for radiometric dating. Also occurs as inclusions in other minerals, and itself may contain inclusions. Tetragonal crystals, uniaxial (-), high relief, strong birefringence, mostly colourless in thin section. Coloured crystals may be pleochroic. Elongate crystals parallel prism faces and show parallel extinction. Poor prismatic and pyramidal cleavage.

Zoisite: Ca₂Al₃ (SiO₄)₃ OH. Part of the epidote group. Orthorhombic, as prisms and fibrous or bladed clusters. Excellent (010) cleavage. Biaxial (+), colourless to pleochroic in shades of pale pink and yellow. Parallel extinction in prismatic cleavage fragments. Found in metamorphic and hydrothermally altered rocks, and as an alteration product of plagioclase in igneous rocks. Is common as part of the matrix in greywackes and low rank metasediments. Very similar to clinozoisite (monoclinic, cleavage fragments are not prismatic).

Zoning (in crystals):  Zoning commonly displays as concentrically arranged crystal growths, where the composition changes outwards from the crystal interior. Zones have the same crystallographic and optic exes as the parent crystal. Zoned crystals may also be twinned.