STAUROLITE  , a

mineral consisting of basic aluminium and ferrous iron silicate with the formula HFeA15Si2O13 . The material is, however, usually very impure, the crystals enclosing sometimes as much as 30 or 40% of quartz and other minerals as well as carbonaceous matter . Crystals are orthorhombic and have the form of six-sided prisms . Interpenetrating cruciform twinned crystals are very common and characteristic; they were early known as pierres de croix or lapis crucifer, and the name staurolite, given by J . C . Delametherie in 1792, has the same meaning (Greek, aeavpos, a cross, and XWBos, a stone) . In fig. r the twin-plane is (032) and the two prisms intercross Twinned Crystals of Staurolite . at an angle of 91° 22'; in fig . 2 the twin-plane is (232) and the prisms intercross at nearly 6o° . The mineral is translucent to opaque and dark reddish-brown in colour; it thus has a certain resemblance to garnet, and on this account has been called grenatite . Waterworn pebbles of material sufficiently trans-parent for cutting as gem-stones are occasionally found in the diamantiferous sands of Brazil . The hardness is 72 and the specific gravity 3.75 .

Staurolite is a characteristic mineral of crystalline

schists, and it is also a product of contact-metamorphism . Large twinned crystals with rough surfaces are found in mica-schist in Brittany and at several places in the United States, e.g. in Fannin county, Georgia . Untwinned crystals, translucent and of a rich brown colour (grenatite), are abundant in the silvery white paragonite-schist of Monte Campione, St Gothard . (L . J .