Felsic

The word "felsic" is a term used in geology to refer to silicate minerals, magma, and igneous rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium.

A felsic volcanic rock fragment, as seen in a petrographic microscope. Scale box is in millimeters

Close-up of granite from Yosemite National Park

A specimen of Rhyolite

They are usually light in colour and have specific gravities less than 3. The most common felsic rock is granite, but others include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars. In terms of chemistry, felsic rocks are on the other side of the rock spectrum from the mafic rocks.

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, meaning 'light-coloured'.

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	Obsidian or porcellanite

References

• Le Maitre L.E. (ed) 2002. Igneous rocks: a classification and glossary of terms'. 2nd ed, Cambridge University Press.