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 TextureName of Felsic Rock
PegmatiticGranite pegmatite
Coarse grained (phaneritic)Granite
Coarse grained and porphyriticPorphyritic granite
Fine grained (aphanitic)Rhyolite
Fine grained and porphyriticPorphyritic rhyolite
PyroclasticRhyolitic tuff or breccia
GlassyObsidian or porcellanite


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

ee also

*QAPF diagram
*List of minerals
*List of rock types

Wikimedia Foundation. 2010.

Игры ⚽ Нужен реферат?

Look at other dictionaries:

  • felsic — 1912, from FELDSPAR (Cf. feldspar) + SILICA (Cf. silica) + ic …   Etymology dictionary

  • felsic rock — Igneous rock dominated by the light coloured, silicon and aluminum rich minerals feldspar and quartz. The presence of these minerals gives felsic rock its characteristic light gray colour. The presence of small amounts of dark minerals rich in… …   Universalium

  • felsic and mafic rocks — ▪ igneous rock       division of igneous rocks (mafic rock) on the basis of their silica content. Chemical analyses of the most abundant components in rocks usually are presented as oxides (oxide) of the elements; igneous rocks typically consist… …   Universalium

  • felsic — /fel sik/, adj. Geol. (of rocks) consisting chiefly of feldspars, feldspathoids, quartz, and other light colored minerals. Cf. mafic. [1910 15; FEL(DSPAR) + S(ILICA) + IC] * * * …   Universalium

  • felsic — adjective Enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Ant: mafic See Also: felsite …   Wiktionary

  • felsic — ALASKA VOLCANO OBSERVATORY GLOSSARY An igneous rock having abundant light colored minerals …   Glossary of volcanic terms

  • felsic — [ fɛlsɪk] adjective Geology relating to or denoting a group of light coloured minerals including feldspar, quartz, and muscovite. Often contrasted with mafic. Origin early 20th cent.: from feldspar + a contr. of silica …   English new terms dictionary

  • felsic — fel·sic …   English syllables

  • felsic — /ˈfɛlsɪk/ (say felsik) adjective 1. of or relating to light coloured rocks containing an abundance of such minerals as felspar, felspathoid and silica. 2. of or relating to the minerals themselves such as quartz, felspars, felspathoids and… …  

  • felsic — ˈfelsik adjective Etymology: feldspar + silica + ic : consisting of or chiefly consisting of feldspar or feldspathoid quartz …   Useful english dictionary

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”