microcristaline

Microcristaline, or microcrystalline, describes a rock or mineral texture in which the constituent crystals are too small to be seen with the unaided eye. In rocks with this texture, grains typically measure well below 1 millimeter across. This contrasts with macrocrystalline textures, where crystals are visible, and with glassy or amorphous textures, where crystals are absent.

Common examples include microcrystalline quartz, often referred to as cryptocrystalline quartz, which encompasses chalcedony varieties such as agate, onyx, carnelian, and the rock types chert and flint. Another group comprises microcrystalline calcite found in certain limestones and marbles where the calcite grains are extremely fine. These textures are particularly common in siliceous and carbonate-rich settings.

Formation occurs mainly through diagenetic and hydrothermal processes that precipitate silica or carbonate minerals from solution, followed by compaction and subtle recrystallization that reduces crystal size. Microcrystalline textures can form in sedimentary environments (for example siliceous nodules in cherts or siliceous sediments) and in volcaniclastic or hydrothermal contexts, where rapid precipitation favors fine-grained crystals.

Identification and use: microcrystalline quartz tends to be harder (Mohs hardness around 7) and can be translucent to opaque with a waxy to dull luster; chalcedony varieties often display a waxy luster and a broad color range. Microcrystalline calcite has a lower hardness (about 3) and a dull to pearly luster. In thin section, microcrystalline quartz appears as fine-grained quartz cement or aggregates, while microcrystalline calcite shows a fine calcite cement.

In geology and material sciences, microcrystalline textures indicate rapid growth or diagenetic alteration and help distinguish rock types such as cherts and microcrystalline limestones. The term is sometimes used interchangeably with cryptocrystalline in certain contexts.