In modern usage, the term acid rock, although sometimes used as a synonym, refers to a high silica content (greater than 63% SiO2 by weight) volcanic rock such as rhyolite. The term was used more broadly in older geologic literature. It is considered archaic as the terms acidic and basic rock were based on an incorrect idea dating from the 1800s that silicic acid was the chief form of silicon occurring in rocks.
The term felsic is related to but not derived from the German adjective felsig ("rocky").
In some cases, felsic volcanic rocks may contain phenocrysts of mafic minerals, usually hornblende, pyroxene or a feldspar mineral, and may need to be named after their phenocryst mineral, such as 'hornblende-bearing felsite'.
The chemical name of a felsic rock is given according to the TAS classification of Le Maitre (1975). However, this only applies to volcanic rocks. If the rock is analyzed and found to be felsic but is metamorphic and has no definite volcanic protolith it may be sufficient to simply call it a 'felsic schist'. There are examples known of highly sheared granites which can be mistaken for rhyolites.
For phaneritic felsic rocks, the QAPF diagram should be used, and a name given according to the granite nomenclature. Often the species of mafic minerals is included in the name, for instance hornblende-bearing granite, pyroxene tonalite or augite megacrystic monzonite as the term granite already assumes feldspar and quartz.
The rock texture thus determines the basic name of a felsic rock.
|Rock Texture||Name of Felsic Rock|
|Coarse grained (phaneritic)||Granite|
|Coarse grained and porphyritic||Porphyritic granite|
|Fine grained (aphanitic)||Rhyolite|
|Fine grained and porphyritic||Porphyritic rhyolite|
|Pyroclastic||Rhyolitic tuff or breccia|
|Glassy||Obsidian or porcellanite|