Conditions and criteria for "grain growth" in metals are not followed in diagenesis of most sedimentary carbonates; thus the concept should not supersede "recrystallization" as first documented by H. C. Sorby for both petrographers and metallographers. An inclusive term, "neomorphism," is herein proposed to embrace the following isocompositional and replacive processes: inversion, recrystallization (calcite calcite), and stain-recrystallization. Neomorphism and its daughter processes may be aggrading or degrading; the former may be prophyroid (a few crystals growing to replace a passive groundmass) or coalescive (nearly all crystals are consuming or being consumed). In these processes driving forces and physical conditions (porosity, solutions, etc.) vary considerably.

Three phases of neomorphism are discussed. All micritic limestones have undergone porphyroid neomorphism (?), probably from 2µ needles or plates to 2µ subequant polyhedral blocks of calcite (electron microscope work of R. Shoji), involving digestion of the vast majority of original mud particles of similar length but much more slender than the polyhedra.

In some limestones, neomorphism bursts through the "micrite curtain" to form microspar. Normal micrite measures 1½-2µ; a saddle exists at 3-4µ, before another peak frequency at 5-6µ (microsparite). This volumetrically very important type of neomorphism is probably specifically produced by coalescive recrystallization; it results in uniformly-sized grains, usually of simple loafish form, and is most frequent in limestones with shale interbeds, probably not in brackish environments as has been claimed.

In freakish limestones, diagenesis may go still further, producing pseudospar or fibers largely by porphyroid neomorphism. These may mimic closely the appearance of normal porefill calcite; criteria of grain shape, orientation, uniformity and boundaries are equivocal. The only firmly diagnostic criteria are those based on grosser fabric relations such as transection of allochems, occupation of large areas unsupported by allochems, or presence of undigested inclusions.

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