Felsic
Felsic is a term used in Geology to refer to Silicate minerals, magmas, and rocks which are enriched in the lighter elements, such as silica, 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, Biotite, Muscovite, Hornblende, Orthoclase, and the sodium rich Plagioclase feldspars. The most common felsic rock is Granite.
On the opposite side of rock spectrum are the Iron and Magnesium rich Mafic and Ultramafic minerals and rocks.
The term acid rock, although sometimes used as a synonym, in current usage refers to a high silica (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 the incorrect idea dating from the 1800s that Silicic acid was the chief form of silicon in rocks.
Felsite is a petrologic field term used to refer to very fine grained light colored volcanic rocks that may be later reclassified after more detailed microscopic or chemical analysis.
| 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 | None |
| Amygdaloidal | None |
| Many small vesicles | Pumice |
| Glassy | Obsidian |
Reference
- Le Maitre, L.E., ed., (2002) Igneous Rocks: A Classification and Glossary of Terms 2nd edition, Cambridge.