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298 SILICATES. FIG. 315. FIG. 316. o' FIG. 317. t \ b Combinations. lm..c.a, ermb..aw, crzmb..k.v.i.o, ckrztovimeba. Twins. Twin-face a. The faces a, n, m, e, b are sometimes striated parallel to their intersections with each other. The faces are frequently uneven and rounded. Cleavage, m, very perfect; a, b indistinct. Fracture imperfect conchoidal. Slightly translucent...opaque. Lustre vitreous, in the light coloured varieties inclining to pearly. Colourless, white, usually green of various shades, brown, yellow, grey, black. Streak greyish-white...brown. Brittle, li = 6'0...6'0. G = 2'90.. .3'40. Before the blowpipe melts with intumescence into a grey, green or black bead. Its fusibility increases with the propor tion of iron contained in it. Dissolves slowly in borax. The varieties which are rich in iron are slightly acted on by hydro chloric acid. 3RS + 3t s S 3 , where B. is magnesia, lime and frequently protoxide of iron. The green and black varieties frequently contain a considerable quantity of alumina, amounting in some instances to 16 per cent., which appears to form an aluminate of magnesia or protoxide of iron capable of crystallizing with tho silicates in all proportions. Analyses of the white, grey and bright green varieties ex hibiting the forms b, m, g = 2'93...3‘0l (grammatite or tremo- lite), a from Fahlun, b from Gullsjo by v. Bonsdorff, c from Cziklowa by Beudant, of the green varieties having the forms