micrite

Micrite is a fine-grained carbonate rock texture consisting of microcrystalline calcite crystals, generally smaller than about 4 micrometers. The resulting uniform, compact matrix is commonly referred to as lime mud, and rocks with this texture are described as micritic limestone or calcilutite in lithologic terms. In thin section, micrite appears as a continuous groundmass with few or no visible grains; it can be the dominant component of the rock or form the matrix surrounding larger skeletal grains.

Origin and formation vary. Micrite can form by direct authigenic precipitation of calcite in low-energy, quiet-water environments, such as deep lagoons, shelves, or basinal settings. It also develops through micritization, a diagenetic process in which existing grains and fossils are dissolved and reprecipitated as microcrystalline calcite, creating a micritic envelope or cement. Diagenetic processes, including compaction and the growth of calcite cement, can modify micritic textures, sometimes transforming parts of the rock into sparite.

Classification and occurrence. In carbonate sedimentology, micrite describes a limestone matrix texture and is closely associated with lime mudstones and calcilutites. It is common in both ancient and modern carbonate successions and can be a major component of mudstones and wackestones. Micrite can preserve fine-scale chemical and biological information from the depositional or early diagenetic environment, making it an important indicator in paleoenvironmental reconstructions.