They are usually light in color and have specific gravities less than 3. The most common felsic rock is granite, but others include quartz, muscovite, orthoclase, and the sodium-rich plagioclasefeldspars. In terms of chemistry, felsic rocks are on the other side of the rock spectrum from the mafic rocks.
In modern usage, the term acid rock, although sometimes used as a synonym, refers to a high-silica-content (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 now, as the terms "acidic" and "basic rock" were based on an incorrect idea, dating from the 19th century, that silicic acidwas the chief form of silicon occurring in rocks.
The term "felsic" combines the words "feldspar" and "silica". The similarity of the term felsic to the German words Fels, meaning "rock", and felsig, meaning "rocky", is purely accidental, asfeldspar is a borrowing from German Feldspat, which derives from German Feld, meaning "field"