Felsite (also called felstone) -micro- or cryptocrystalline rock formation of feldspathic-quartz (similar to composition of granite). It is a very fine grained mass of acidic effusive rocks: potassic feldspar and crystalline silica, sometimes with content of plagioclase, pyroxene and black mica.
Felstone contains 68-75% SiO2, it depends on the degree of quartz and vary from dacite (68% SiO2) to rhyolite (72-75%).
The density is 2200—2400 kg/m³, fusing temperature is 1470—1500 °C, specific electrical resistance is104 O·m. Acid resistance of vitriol oil is 98, 5—99, 3 %. The absorption of CaO is 80-120 mg / g of solution.
In the chemical industry felsites are used as acid-proof material. In the construction industry floured felsites are used as a hardening additive (20-30% by weight) for cement.
Felsite also used as a decorative stone, but using of felsites is limited as in the difficulty of mechanical tooling. For example, in Yerevan Armenian felsite was used in the design of the Presidential Palace, the architectural ensemble at the Independence Square, "Moscow" cinema, etc.