KYANITE, a rock-forming mineral of the composition Al2SiO5, crystallizing in the triclinic system, and named in allu sion to its most common colour (Gr. KVILPEOS, blue). The min eral has the same percentage chemical composition as andalusite (q.v.) and sillimanite (q.v.), but differs from these in crystal form and physical properties. It is the densest of the three miner als of the composition G=3.6. (Andalusite G=3.1, sillimanite G=3.25.) The habit of kyanite crystals is usually that of blades or elongated tables with prominent (ioo) faces. Pure kyanite is colourless, but the mineral is usually a pale sky blue, though greenish and yellowish tints occur. It has a perfect (ioo) and a good (ow) cleavage, with usually a parting parallel to the base, which is a glide plane along which secondary twinning is developed. The mineral is distinguished by having a variable hardness on different faces, and in different directions on the same face. On (ioo) the hardness is 7 parallel to b (the edge 'co :ooi) and 4.5 parallel to c (the edgeioo :oio) ; the name disthene (Gr. Sts and ceivos, "double strength") is given in allusion to this property. When heated to 1,2o0° C, kyanite slowly dissociates
into mullite (q.v.) and silica, the alteration taking place on the surface of the crystal and advancing inwards. This decomposition is accompanied by an absorption of heat. Kyanite is a stress min eral, almost wholly confined to crystalline schists, particularly those of argillaceous composition. Its common associates are almandine garnet, staurolite and muscovite. It is almost unknown as a product of pure thermal metamorphism. In metamorphosed igneous rocks it is found in some eclogites (q.v.), such as those of the Fichtelgebirge and in the acid granulites of Saxony and Austria. The finest examples of kyanite occur in a paragonite schist of Pizzo Forno, near St. Gotthard, in Switzerland, where blue crystals occur united in regular position with brown stauro lite. In the crystalline schists large crystals are not infrequently associated with quartz in lenses and veins in kyanite-bearing schists. (C. E. T.)