Neomorphism

Neomorphism is the umbrella term covering the combined diagenetic processes of inversion and recrystallization in carbonate rocks, as coined by Folk (1965). ^[1] The term is useful because inversion and recrystallization so commonly occur together in natural carbonate diagenesis that separating them is often impractical, and the single word captures the fact that the original carbonate texture has been reorganised without necessarily specifying which mechanism was dominant.

Inversion and Calcitization

Inversion refers to the transformation of one mineral into its polymorph — most commonly the transformation of aragonite to calcite, since both have the formula CaCO₃ but different crystal structures. ^[1] Strictly speaking, true inversion occurs only in the solid (dry) state; when the transformation takes place in the presence of water — as it usually does during diagenesis — it actually proceeds by dissolution of the less stable aragonite and nearly simultaneous precipitation of more stable calcite in its place, a process most geologists call calcitization. ^[1] The distinction matters because a dry solid-state inversion would preserve the crystal morphology of the precursor mineral, while calcitization — the dissolution-reprecipitation pathway — tends to modify or destroy the original texture at the same time. ^[1] During diagenesis, most aragonite is eventually calcitized as it passes through successively reactive fluid environments. ^[1]

Recrystallization

Recrystallization refers to a change in the size or shape of crystals with little or no change in chemical composition or mineralogy — the mineral remains the same, but its crystal dimensions are reorganised. ^[1] Calcitization and recrystallization commonly occur together: as aragonite dissolves and calcite precipitates, the new calcite crystals grow to a different size than the precursor mineral, so the product is both compositionally transformed (inversion) and texturally reorganised (recrystallization). ^[1]

Where Neomorphism Operates

Neomorphism can occur in all three diagenetic realms — marine, meteoric, and subsurface — but is most important in the meteoric and subsurface environments, where reactive fluids are most chemically aggressive or where temperatures and pressures are high enough to drive re-equilibration. ^[1] It affects both carbonate grains and micrite, and it commonly increases crystal size — meaning that neomorphism typically drives a change from finer- to coarser-grained carbonate. ^[1]

When neomorphism is pervasive, the entire rock may recrystallize: a fine-grained (micritic) limestone can be converted into a coarse-grained sparry rock, with all trace of the original depositional texture obliterated. ^[1] On a smaller scale, recrystallization produces large clear calcite crystals that closely resemble sparry calcite cement precipitated in pore spaces, and distinguishing between neomorphic spar and sparry calcite cement is one of the most difficult problems in the microscopic study of carbonate rocks. ^[1] The distinction matters because cement records the history of pore fluids, while neomorphic spar records the transformation of existing grains — confusing the two leads to incorrect interpretations of the diagenetic history of a carbonate rock.

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