Aphanite

{{short description|Igneous rock composed of very small crystals invisible to the naked eye}}

File:VolcanicBombMojaveDesert.JPGic volcanic bomb is aphanitic.]]

File:Classification extrusive rocks EN.svg classification of aphanitic extrusive igneous rocks according to their relative alkali (Na₂O + K₂O) and silica (SiO₂) weight contents. Blue area is roughly where alkaline rocks plot; yellow area where subalkaline rocks plot.{{cite book |editor-last=Le Maitre |editor-first=R. W. |year=1989 |title=A classification of igneous rocks and glossary of terms |publisher=Blackwell Science |location=Oxford}}]]

File:LvMS-Lvv.jpg, as seen through a petrographic microscope]]

Aphanites (adj. aphanitic; {{etymology|grc|αφανης (aphanḗs)|invisible}}) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye{{EB1911|inline=y|wstitle=Aphanite|volume=2|page=163}} (in contrast to phanerites, in which the crystals 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 the same as that of volcanic glass (e.g., obsidian), with volcanic glass being non-crystalline (amorphous), and having a glass-like appearance.{{cite book |author=Bates and Jackson |year=1984 |title=Dictionary of Geological Terms |edition=3rd |publisher=American Geological Institute}}

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.