Ingeology,felsic is amodifier describingigneous rocks that are relatively rich in elements that formfeldspar andquartz.[1] It is contrasted withmafic rocks, which are richer inmagnesium andiron. Felsic refers tosilicate minerals,magma, androcks which are enriched in the lighter elements such assilicon,oxygen,aluminium,sodium, andpotassium. Molten felsic magma and lava is moreviscous than molten mafic magma and lava. Felsic magmas and lavas have lower temperatures of melting and solidification than mafic magmas and lavas.
Felsic rocks are usually light in color and havespecific gravities less than 3. The most common felsic rock isgranite. Common felsic minerals include quartz,muscovite,orthoclase, and the sodium-richplagioclase feldspars (albite-rich).
In modern usage, the termacid rock, although sometimes used as a synonym, normally now refers specifically to a high-silica-content (greater than 63% SiO2 by weight)volcanic rock, such asrhyolite. Older, broader usage is now considered archaic.[citation needed] That usage, with the contrasting term "basic rock" (MgO, FeO,mafic), was based on an ancient concept, dating from the 19th century, that "silicic acid" (H4SiO4 or Si(OH)4) was the chief form of silicon occurring in siliceous rocks. Although this intuition makes sense from anacid-base perspective inaquatic chemistry considering water-rock interactions and silica dissolution, siliceous rocks are not formed by this protonated monomeric species, but by a tridimensional network of SiO44– tetrahedra connected to each other. Once released in water and hydrolyzed, these silica entities can indeed formsilicic acid in aqueous solution.
The term "felsic" is a derivation of the words "feldspar" and "silica".[2] The similarity of the resulting termfelsic to the Germanfelsig, "rocky" (fromFels, "rock"), is accidental.Feldspar is from the GermanFeldspat, acompound of the GermanFeld, meaning field, plusspat[h], meaning mineral.[3]
In order for a rock (rather than amineral) to be classified as felsic, it generally needs to contain more than 75% felsic minerals (namely quartz,orthoclase andplagioclase). Rocks with greater than 90% felsic minerals can also be calledleucocratic,[4] from the Greek words for white and dominance.
Felsite is apetrologic field term used to refer to very fine-grained oraphanitic, light-coloredvolcanic rocks which might be later reclassified after a more detailed microscopic or chemical analysis.
In some cases, felsic volcanic rocks may containphenocrysts ofmafic minerals, usuallyhornblende,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 theTAS classification of Le Maitre (1975). However, this only applies to volcanic rocks. If the rock is analyzed and found to be felsic but ismetamorphic and has no definite volcanicprotolith, it may be sufficient to simply call it a 'felsic schist'. There are examples known of highlysheared granites which can be mistaken for rhyolites.
Forphaneritic felsic rocks, theQAPF diagram should be used, and a name given according to thegranite nomenclature. Often the species of mafic minerals is included in the name, for instance, hornblende-bearing granite,pyroxenetonalite oraugite megacrysticmonzonite, because the term "granite" already assumes content with feldspar and quartz.
The rock texture thus determines the basic name of a felsic rock.
| Rock texture | Name of felsic rock |
| Pegmatitic | Granitepegmatite |
| Coarse-grained (phaneritic) | Granite |
| Coarse-grained andporphyritic | Porphyriticgranite |
| Fine-grained (aphanitic) | Rhyolite |
| Fine-grained and porphyritic | Porphyriticrhyolite |
| Pyroclastic | Rhyolitictuff orbreccia |
| Vesicular | Pumice |
| Amygdaloidal | None |
| Vitreous (Glassy) | Obsidian or porcellanite |