Division of igneous rocks on the basis of their silicate mineral content, these minerals usually being the most abundant in such rocks. Rocks are described as felsic, intermediate, mafic, and ultramafic, in order of decreasing silica content, and, in general, the gradation from felsic to mafic corresponds to an increase in colour (i.e., light to dark). Due to the erroneous belief that silica present in rock magmas occurred in the form of silicic acid, high- and low-silica rocks were once known as acid and basic rocks, respectively.

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Felsic is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. The term combines the words "feldspar" and "silica." Felsic minerals are usually light in color and have specific gravities less than 3. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars. The most common felsic rock is granite. On the opposite side of the rock spectrum are the iron and magnesium-rich mafic and ultramafic minerals and rocks.

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").

Classification of felsic rocks

In order for a rock to be classified as felsic it generally needs to contain >75% felsic minerals; namely quartz, orthoclase and plagioclase. Rocks with greater than 90% felsic minerals can also be called leucocratic, meaning 'light-coloured'.

Felsite is a petrologic field term used to refer to very fine grained or aphanitic, light colored volcanic rocks that may be later reclassified after a more detailed microscopic or chemical analysis.

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
Pegmatitic	Granite pegmatite
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
Vesicular	Pumice
Amygdaloidal	None
Glassy	Obsidian or porcellanite

References

• Le Maitre, L.E., ed. 2002. Igneous Rocks: A Classification and Glossary of Terms 2nd edition, Cambridge.

See also

• QAPF diagram
• List of minerals
• List of rock types
• Granite
• Mafic