aphanitic

Aphanitic is a texture term used in geology to describe igneous rocks whose crystals are too fine to be seen with the naked eye. This texture results from rapid cooling of molten rock at or near the Earth’s surface, which prevents crystals from growing large and yields a fine-grained groundmass.

In practice, aphanitic textures are typical of extrusive or volcanic rocks that form when lava erupts or rapidly erupts and cools. The resulting crystals are usually less than about 1 millimeter in size; some grains may be visible only under magnification. If no crystals form at all, the rock is described as glassy rather than aphanitic. Rocks can also be microcrystalline, where crystals are visible only under a microscope.

Common examples of aphanitic rocks include basalt, andesite, rhyolite, and dacite, covering a range of chemical compositions from mafic to felsic. Rocks of this texture may be entirely aphanitic or porphyritic, containing a few larger phenocrysts embedded in a fine-grained or aphanitic groundmass. In such cases the rock is described as porphyritic-aphanitic. The term is distinct from phaneritic, which refers to rocks with visible crystals due to slower cooling underground.

Etymology: aphanitic comes from Greek a-, not, and phanēs, visible, meaning not visible to the eye. The concept is widely used in petrology to describe the cooling history and texture of igneous rocks, independent of their mineral composition.