STAUROLITE— ANDALUSITE 415 



vein in granite ; a few of these are of gem-quality. Stones suitable for cutting are some- 

 times to be found also in the river sands of Brazil ; and the mineral is found in association 

 with the diamond and with other minerals at Diamantina. The best crystals are said, 

 however, to come from some unknown localities in India, a country in which kyanite is 

 more extensively used than in Europe. There is no doubt but that kyanite occurs at many 

 places in India, but it has been suggested that the stones worn in that country have all 

 come from Europe. 



STAUROLITE. 



Staurolite consists essentially of silicate of magnesium and aluminium with a small 

 amount of iron ; it occurs always in well-developed crystals belonging to the rhombic system. 

 These are elongated prisms usually with six sides ; frequently two crystals are twinned 

 together, the angle between the prism-edges being approximately 60^ or 90°. Staurolite is 

 usually of a dark reddish-brown colour very similar to that of some garnets ; some specimens, 

 however, are more like dark yellowish-brown topaz. It is usually cut in the same form as 

 is garnet, but may be distinguished from this stone by the fact that it is doubly refracting. 

 It is rarely transparent enough for cutting as a gem. Its hardness is 7J and the specific 

 gravity 3'7 to 3"8. The crystals usually occur embedded in mica-schist, argillaceous schist, 

 or other similar rocks. They are found in large numbers together with kyanite in the 

 white mica-schist of Monte Campione, near Faido, in Canton Tessin, Switzerland ; a few are 

 sufficiently pure and transparent for cutting as gems. The mineral occurs also with 

 euclase and other precious stones in the gold-washings on the Sanarka river in the district 

 of the Orenburg Cossacks ; and a few grains of it are met with among the diamond-bearing 

 sands of Salobro in Brazil. Staurolite of a rich garnet-red colour, perfectly transparent and 

 fit for cutting as gems, has been found recently (1898) with ruby and rhodolite in Mason's 

 Branch, Macon County, North Carolina, U.S.A. The stone is used to a very limited extent 

 and is worth very little. 



ANDALUSITE. 



Andalusite is identical in chemical composition with kyanite, having the formula 

 AlgOg.SiOg. It differs, however, in crystalline form, the crystals be- 

 longing, not to the triclinic but to the rhombic system. They take 

 the form of elongated, almost rectangular columns, which are usually 

 terminated by basal planes perpendicular to the prism-edges. They are 

 rarely modified by other faces, and are therefore very simple in form 

 (Fig. 76). There is a distinct cleavage parallel to the faces of the prism. 



Such crystals are found in large numbers at many localities, especially 

 in gneiss and other crystalline schistose rocks. Though often of considerable 

 size they are useless as gems, for they are nearly always opaque and of a Fig. 76. Crystal- 

 dingy grey, green, or red colour. For cutting as gem-stones the transparent J^"^? ^°^"^ °^ ^^^^' 

 pebbles found with white and blue topaz in the gem-gravels of the Minas 

 Novas district in Brazil are exclusively used. These are usually green, but specimens of a 



V^ 



