Neomorphic textures in thin section

Pervasive neomorphism of this oolite sand has increased calcite crystal size overall, and in the process has overprinted the primary concentric layering. Plain polarized light.

The term neomorphism is reserved for recrystallization of a mineral and its polymorphs to new crystal phases having the same composition. In calcium carbonates this involves recrystallization of calcite, high-magnesium calcite, and aragonite to calcite (usually low-magnesium calcite). Dolomites are also prone to neomorphism (dolomite to dolomite replacement); note that the replacement of calcite or aragonite by dolomite is not a neomorphic process. Neomorphism commonly results in an increase in crystal size and the destruction or overprinting of original fabrics.

The actual process of neomorphism is still a bit of mystery. Initiation of recrystallization begins with dissolution at the crystal-fluid interface. Subsequent precipitation must involve changes in the local thermodynamic conditions (such as increased saturation, or a change in the activity of carbonate species). If crystal size continues to increase (via syntaxial growth) then the transfer of dissolved mass to the crystal interface must continue.

Neomorphic versus cement textures

Cements grow by precipitating into the available inter- or intragranular pore space. Thus, crystal faces are free to grow and therefore they tend to be planar, with sharp crystal edges and terminations. In contrast, the locus of neomorphic replacement of pre-existing crystal masses (e.g., cements, bioclasts) and subsequent precipitation will tend to be focused along crystal discontinuities such as cleavage and twin planes, as well as crystal edges and terminations, in part because they are regions of higher surface free energy. Thus, neomorphic crystal boundaries are commonly irregular and characterised by embayments, re-entrants, or segmented.

Neomorphic replacement of carbonate cements, bioclasts, microbial laminates, and mudrocks commonly proceeds fastest in fine-grained deposits such as micrite – again this may be a function of higher surface free energies relative to much larger crystals and coarse-grained clasts.

Recrystallization may also show a progressive change in crystal size – referred to as aggrading neomorphism. Crystal aggradation can mimic pre-existing cement fabrics where crystal size increases from the cement boundary into pore spaces. Aggrading neomorphism is common in carbonate mudrocks; the resulting textures 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; the distinction between original peloidal textures and these neomorphic fabrics requires careful attention. Neomorphic fabrics frequently cut across cement and clast boundaries.

Some criteria that distinguish neomorphic from cement fabrics are listed below; the information is gleaned from R.L. Folk (1968), Robin G.C. Bathurst Chapter 12 (1976), J.W. Morse and F.T. Mackenzie 1990, and Erik Flugel. (2010, Chapter 7).