What is "aphanite"

Aphanite \Aph"a*nite\, n. [Gr. ? invisible; 'a priv. + ? to appear.] (Min.) A very compact, dark-colored ?ock, consisting of hornblende, or pyroxene, and feldspar, but neither of them in perceptible grains.

n. (context geology English) Certain dark igneous rocks having grain so fine that the individual crystals cannot be seen with the naked eye.

n. fine-grained homogeneous rock (such as basalt) containing minerals undetectable by the naked eye

Aphanite, or aphanitic as an adjective (from the Greek αφανης, "invisible"), is a name given to certain igneous rocks that are so fine-grained that their component mineral crystals are not detectable by the unaided eye (as opposed to phaneritic igneous rocks, where the minerals are visible to the unaided eye). This geological texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. As a rule, the texture of these rocks is not quite 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 large inclusions are called phenocrysts.

They consist essentially of very fine-grained minerals, such as plagioclase feldspar, with hornblende or augite, and may contain also biotite, quartz, and orthoclase.