Micritic

Micritic refers to a texture or composition of carbonate rocks in which the primary constituent is microcrystalline calcite or dolomite, with crystal sizes typically less than about 4 micrometers. Rocks that are predominantly micritic are called micritic limestones or micrites. In hand specimen and thin section, micrite appears as a fine, uniform, dull matrix that may enclose microfossils, peloids, or rare larger grains; when cemented within a rock, sparry calcite cement (sparite) may overgrow the micritic matrix.

Origins of micrite are varied. It can form as carbonate mud in very low-energy environments, by direct precipitation of calcite or dolomite from seawater, or by micritization, where microbial activity or diagenetic alteration coats grains with microcrystalline carbonate. Biochemical reprecipitation by algae and bacteria often contributes to micrite-type textures. The term is frequently used in contrast to sparite, the coarse crystalline calcite cement, and to peloidal or peloidal-micritic textures.

Micritic textures are common in many carbonate rocks worldwide, including chalk, micritic limestones, and some dolostones. In classification schemes such as Dunham and Folk, micrite constitutes the matrix or mud component; rocks can be described as mudstone, wackestone, or packstone depending on grain support and preservation of fossils within the micritic matrix.

Analytical identification relies on petrographic microscopy and, where possible, geochemical tests. The presence of micrite implies a calm depositional setting and a diagenetic history that favors stable, fine-grained carbonate precipitation.