Felsic

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In

silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity
than mafic magma/lava, and have low temperatures to keep the felsic minerals molten.

Felsic rocks are usually light in color and have

specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite
-rich).

Terminology

In modern usage, the term acid rock, although sometimes used as a synonym, normally now refers specifically to a high-silica-content (greater than 63% SiO2 by weight)

aquatic 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 form silicic acid
in aqueous solution.

The term "felsic" combines the words "feldspar" and "silica". The similarity of the resulting term felsic to the German felsig, "rocky" (from Fels, "rock"), is purely[citation needed] accidental. Feldspar is from the German Feldspat, a compound of the German Feld, meaning field, plus spat[h], meaning mineral.[2]

Classification of felsic rocks

volcanic lithic fragment, as seen in a petrographic microscope
. Scale box is in millimeters.

In order for a rock to be classified as felsic, it generally needs to contain more than 75% felsic minerals (namely quartz, orthoclase and plagioclase). Rocks with greater than 90% felsic minerals can also be called leucocratic,[3] from the Greek words for white and dominance.

volcanic
rocks which might 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
, because the term "granite" already assumes content with feldspar and quartz.

The rock texture thus determines the basic name of a felsic rock.

Close-up of granite from Yosemite National Park.
A specimen of rhyolite.
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
Vitreous (Glassy)
Obsidian or porcellanite

See also

Notes

  1. ^ Harper, Douglas. "feldspar". Online Etymology Dictionary. Retrieved 2008-02-08.
  2. ^ Aurora Geosciences Ltd. "Technical report Mackenzie mountains iron-copper property, Northwest Territories, Canada" (PDF). Archived (PDF) from the original on 2017-10-10.

References

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