The main composition of sanidine is K[AlSi3O8], and it always contains some Na[AlSi3O8]. Because it forms under high-temperature conditions, it is also called high-temperature sanidine. It crystallizes in the monoclinic crystal system, and crystal twinning is common. It is light white or colourless with white streaks. Its diaphaneity is translucent with a vitreous lustre. Its specific gravity is 2.56–2.62, and its Mohs hardness is 6. It usually forms in extrusive rocks such as trachyte and rhyolite and sometimes in contact-metamorphosed rocks.