Tourmaline

Tourmaline, a complex mineral silicate of alumina, with magnesia, boric add, and smaller proportions of phosphoric acid, iron, manganese, calcium, potassium, sodium, lithium, fluorine, and water. Its composition may approximately be represented by the formula 3RO.SiO₂ + R₂O₃SiO₂. It crystallises in the Hexagonal system, generally in long three or six-sided prisms, hemimorphically terminated. It also occurs in needles, sometimes radiating. It has a sub-conchoidal fracture, a hardness of 6.5-7.5, a specific gravity of 3-3.3, and a vitreous lustre. It is very variable in being most commonly black and opaque (schorl); but sometimes transparent and rose-red (rubellite), blue (indicolite), green, yellow, brown, or colourless. Its crystals are sometimes banded or particoloured, the pink-and-green specimens from Paris, Maine, U.S.A., being among the most beautiful of American minerals. The transparent varieties when flawless are classed as precious stones, the blue being an excellent substitute for sapphire. Tourmaline is strongly doubly-refractive and pleochroic, the ordinary ray being so completely absorbed that a plate of one of the more translucent varieties, cut parallel to the chief axis of the crystal, acts as a polariser. Two such plates, suitably mounted, form the simple polariscope known as tourmaline tongs. When rubbed tourmaline becomes positively electric; and when heated or cooled the differently terminated ends of its hemimorphic crystals exhibit opposite alternations of electric polarity. Schorl is common in granite, gneiss, schists, and line limestones; with quartz it forms tourmaline rock; and with orthoclase the beautiful black-and-pink luxullianite (named from the village of Cornwall near which it occurs) of which the Duke of Wellington's sarcophagus is made.