Aphanite

The smooth texture of this basaltic volcanic bomb is aphanitic.

An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope

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.^[1]

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.

Common rocks that can be aphanitic

• Andesite
• Basalt
• Basanite
• Dacite
• Felsite
• Phonolite
• Rhyolite
• Trachyte

References

1. ↑ Bates and Jackson, 1984, Dictionary of Geological Terms, 3rd ed., Prepared by the American Geological Institute

•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Aphanite". Encyclopædia Britannica (11th ed.). Cambridge University Press.