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STAPUOLITE. 283 a 100, c 001, r on, m no, x 302 twin-face, z 322 twin-face. x truncates the edge ac ; z truncates the solid angle cma. rc 65° 22' ma 64° 40' ZC 60° 37' rr 110 44 mm 60 40 za 69 42 xe 45 48 me 90 0 FIG. . 304. FIG, , 305 FIG. 306. m‘ Combinations, cm, cma, crma. Twins. 1. Twin-face x, (fig. 305.) co = 91° 36'. 2. Twin-face z. (fig. 306.) co == 68° 46', av = — 60° 36'. The faces c sometimes rough. Cleav age. a perfect, but intcrrusted ; m traces. Fracture con- choidal...uneven. Translucent...translucent on the edges. Lustre vitreous, inclining to resinous. For red rays in a plane parallel to c, and polarized in that plane, g — 1-7526. The optic axes make angles of 42° 30' with a normal to c, in a plane perpendicular to the edge ac. Reddish-brown.. .blackish-brown. Streak white, n = 7'0...7'5. G = 3'52...3'79. Nearly infusible before the blowpipe. In borax melts with difficulty into a glass coloured green by iron. Soluble with very great difficulty in salt of phosphorus into a colourless opalescent glass. With soda melts with effervescence into a vellow slag. Partially decomposed by sulphuric acid either before or after ignition. Analyses of staurolite a, b from Airolo, G = 3'66...3'73, c, d from Rretagne, G = 3'627...3'529, e, f from Polewskoi in the Ural, G = 3-547...3-688, all by Jacobson :— a b c d e 33-45 32-99 39-19 40-35 38-68 47-23 47-92 44-87 44-22 47-43 16-61 16-65 15*09 15-77 15-06 — — 0-17 o-io — 1-99 1-66 0-32 — 2-44 Silica .... Alumina . . . Red oxide of iron Ox. mangan. Magnesia . . Analyses of staurolite y, h, k, I from St. Gottliardt, g = 3-737 in fragments, G = 3'744 in powder, by Jacobson, m by Marig- / 38-33 45-97 14-60 2-47