Silicate of potassium and aluminium, belonging to the feldspar group; it is the phase of this compound that is stable at low temperature.

Crystal system Triclinic.
Appearance Individual crystals are squat, vaguely prismatic. with a tine network of striations consisting of whitish. slightly more opaque veins corresponding to twinning planes. The color can be milky, white, pink, or pale To blue-green; it has very poor transparency. Compact aggregates of crystals are also common.

Physical properties It has a hardness of 6. The density is 2.56-2.58 g/cma. The refractive indices are nu 1.522. ny 1.530. It has two directions of cleavage which are almost at right angles to each other: hence its name, which means "small angle."

Genesis It is found in metamorphic rocks, intrusive magmatic rocks. and pegmatites, which provide the largest crystals.

Occurrence It is widely distributed in the form of minute crystals in rocks. Fine crystals occur in Brazil, the United States (Colorado and Virginia), Madagascar, Norway, The Soviet Union (Urals), India. and Italy (in the granite ot Maddalena island).