Silicate of potassium and aluminium, belonging to the feld spar group. The mineral got its name because it has two directions of cleavage, which are orthogonal (at right angles]. It has the same composition as microclina. but is stable at slightly higher temperatures. It is widely distributed in many lithologies.
Crystal system Monoclinic.
Appearance It occurs as prismatic. sometimes flat-sided crystals. but in rocks it is usually anhedral. It may be pertectly transparent and yellow or almost colorless, but is more often semiopaque and white to grayish white. yellowish white, pink. or reddish.
Physical properties It has a hardness ct 6. The density is about 2.56 g/cm“. The refractive indices are approximately nu 1.520, ﬂy 1.52?. It has two directions ol cleavage at right angles to one another.
Genesis It is common in intrusive. magmatic rocks. and metamorphic rocks. but the largest crystals are found pegrﬂatites.
Occurrence It is widely distributed. Important sources are Italy, Switzerland. Czechoslovakia. and Spain. The transparent variety comes from Madagaﬁr.