Felsic
In
Felsic rocks are usually light in color and have
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)
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
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.
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
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 |
Vitreous (Glassy) |
Obsidian or porcellanite |
See also
- QAPF diagram
- List of minerals
- List of rock types
- Bowen's reaction series
- Archean felsic volcanic rocks
Notes
- ISBN 978-0393196566
- ^ Harper, Douglas. "feldspar". Online Etymology Dictionary. Retrieved 2008-02-08.
- ^ 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