aphanite

Aphanite (from the Greek αφανης, invisible) is a name given to certain typically dark-coloured igneous rocks which are so fine-grained that their component mineral crystals are not detected by the unaided eye. This texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments.

Aphanites are commonly porphyritic, having large crystals embedded in the fine groundmass or matrix. The large inclusions are called phenocrysts.

They consist essentially of plagioclase feldspar, with hornblende or augite, and may contain also biotite, quartz and a limited amount of orthoclase. Although a few authorities still recognize the aphanites as a distinct class, most systematic petrologists have discarded it, and regard these rocks as merely textural varieties of other species. The term is used as a field name pending laboratory identification. Hence, although typically an igneous term, any rock (sedimentary, igneous, or metamorphic) which is compact, crystalline and too fine-grained for its constituents to be identifiable in hand sample, is frequently said to be aphanitic, without implying exactly to which of the principal rock groups it really belongs.

Common aphanites

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

References

• This article incorporates text from the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.