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neomorphism The diagenetic replacement (see DIAGENESIS) of a mineral by a different crystal form of the same mineral. In limestones neomorphism often results in the replacement of calcite by coarser calcite crystals, e.g. micrite may become neomorphically replaced by microspar (4–10 μm crystals), or pseudospar (10–50 μm crystals). Neomorphic spar is characterized by irregularly shaped crystals, often patchily developed, and passing gradationally into unaltered areas.