Glauconite in thin section

Glauconite filling of rotaliid foram chambers — Thin section of glauconite filling rotaliid foram chambers

Glauconite – an important stratigraphic and paleoenvironmental indicator

Glauconite is one of the most recognizable minerals in the entire sediment suite, primarily because of its green hues, its common peloidal grain morphology, and microcrystalline structure. Green-ness does apply to a few detrital minerals like amphiboles, pyroxenes and chlorite, but they can be distinguished by their crystal habit and prominent cleavage; sedimentary glauconite is always microcrystalline.

Glauconite is a sheet silicate, distinctive in its high concentrations of Fe²⁺, Fe³⁺, and K. It occurs as different types depending on the relative proportions of Fe and K. Well-ordered (mature) glauconite is high in K (8% to 9%, derived from sea water) and its crystal structure is closest to that of micas. Precipitates with lower K contents tend to have more clay-like structure, particularly smectite (a mixed crystallographic bag of expandable clays). Total Fe is commonly >15%, and the amount of Fe³⁺ is usually > Fe²⁺ (B. Velde, 2014). Glauconites with low K and high Fe tend to have brown-yellow hues. Most glauconites begin precipitation as K-poor smectites that, over about 10⁵ to 10⁶ years, become increasingly K-rich and crystallographically ordered, or mica-like (Odin and Matter, 1981). The increase in crystallographic ordering can be measured using X-ray diffraction.

Glauconite is a marine, authigenic mineral that precipitates at very shallow depths beneath the sediment-water interface. This requires very low sedimentation rates, normal salinity and pH of the ambient seawater, and suboxic conditions where both Fe³⁺ and Fe²⁺ are thermodynamically stable. The oxidation of organic matter in peloids (particularly fecal pellets) and invertebrates may help push the oxidation state of Fe towards the 2+ state. Note that soluble Fe²⁺ will oxidize rapidly to relatively insoluble Fe³⁺ if exposed to normal, oxygenated, shelf sea water, forming compounds like limonite and goethite. Modern glauconite commonly accumulates at depths <300-500m on siliciclastic and carbonate platforms, shelves, and continental slopes. Ancient accumulations are frequently encountered as greensands.

The most common granular form of glauconite is sand- and silt-sized peloids. Other grain morphologies include vermiform (worm-like), tabular, mammillated, and composite types (e.g. Triplehorn, 1966). Glauconite can also replace or infill the chambers of bioclasts like gastropods, forams, bryozoa, corals, and the perforations in echinoid plates and spines. In hand specimen, they usually exhibit as dark green to black, sub-millimetre, ovoid pellets.

Accumulations of glauconite in the rock record are of great value because:

They indicate a unique set of paleoenvironmental conditions.
They are excellent indicators of stratigraphic condensation during transgression (look for a maximum flood surface at the top of the condensed section).
They may coexist with phosphate nodules and phosphatized fossils (that also indicate condensed stratigraphy).
Although they form authigenically, glauconite can be reworked locally. Glauconite can also be moved to deeper waters by sediment gravity flows. It is soft, friable, and has a hardness of 2 on the Moh scale, matching that of gypsum. Thus, it rarely survives beyond the first cycle of sedimentation.

Glauconite and glaucony

The name glaucony was introduced by Odin and Letolle (1980) for all forms of glauconite characterized by smectite clay crystal structure; the term glauconite was reserved for the mica-structured mineral end-member. However, the name ‘glauconite’ is firmly entrenched in decades of literature and for better or worse, I have continued with this literary laziness.

Glauconite (glaucony) in thin section

Chatham Rise

We begin with some glauconite-rich deposits from Chatham Rise, a ridge that is part of the submarine extension of Zelandia continent. The deposits form part of a highly condensed stratigraphic package dating back to the Oligocene and have been interpreted as palimpsest (relict) remnants of glacially lowered sea levels. Glauconite peloids that presently carpet the sea floor atop Chatham Rise have been K-Ar dated at 5-6 million years old (Nelson et al., 2021).

Modern sea floor, Three Kings Islands, northern New Zealand

The shallow, modern sea floor around Three Kings Islands is accumulating bioclastic sediment, dominated by bryozoa, forams, echinoids, barnacles, and molluscs. In this mix are glauconite pellets and glauconite infilled bioclasts.