Aphanites (adj. aphanitic; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye), however visible using magnification (in contrast to Cryptocrystallines, in which the crystals are not visible even under microscope). This geological texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. As a rule, the texture of these rocks is not the same as that of volcanic glass (e.g., obsidian), with volcanic glass being non-crystalline (amorphous), and having a glass-like appearance.

Aphanites are commonly porphyritic, having large crystals embedded in the fine groundmass, or matrix. The larger inclusions are called phenocrysts. They consist essentially of very small crystals of minerals such as plagioclase feldspar, with hornblende or augite, and may contain also biotite, quartz, and orthoclase.

Common rocks that can be aphanitic

• Andesite

• Basalt

• Basanite

• Dacite

• Komatiite

• Picrite basalt

• Phonolite

• Rhyolite

• Trachyte

See also

• Pegmatite

References