Aphanitic

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[1] (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 the same as that of volcanic glass (e.g., obsidian), with volcanic glass being non-crystalline (amorphous), and having a glass-like appearance.[2]

The smooth texture of this basaltic volcanic bomb is aphanitic.
IUGS 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. Original source: *Le Maitre, R.W. (ed.); 1989: A classification of igneous rocks and glossary of terms, Blackwell Science, Oxford.
An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope

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.[1]

Common rocks that can be aphaniticEdit

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


This article uses material from the Wikipedia article
 Metasyntactic variable, which is released under the 
Creative Commons
Attribution-ShareAlike 3.0 Unported License.