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TOPAZ. 355 D . . . . 1-61375 1-61161 1-62109 C . . . . 1-61144 1-60935 1-61880 B . . . . 1-61049 1-60840 1-61791 The optic axes He in a plane parallel to a. In the variety- examined by Rudberg they made angles of 28° i9'-5 with a normal to c within the crystal, and angles of 50° 2', when seen m air through planes parallel to c. In Brazilian topaz they make angles of 24° 56' with a normal to c. In topaz from Schneckenstein they make angles of 57° 6' with a normal to c, when seen in air through planes parallel to c. This angle be comes larger when the crystal is heated. Colourless, yellowish- white... yellow, reddish-white...hyacinth-red, blue-green. Ex hibits trichroism. A variety from the Brazils, by transmitted light polarized in a plane parallel to a, straw-yellow, by light polarized in a plane perpendicular to a, c, honey-yellow, and by light polarized in a plane parallel to c, wine-yellow. Streak white. 11 = 80. q = 3 - 4...3'6. Pyroelectric, the middle of c being analogous, and the edges mm' antilogous. _ Heated strongly in the open tube with salt of phosphorus yields the reaction of fluorine. Infusible before the blowpipe ; in a strong heat blisters form on the surface, but not on the faces of cleavage, and burst immediately. Soluble in salt of phosphorus leaving a skeleton of silica. With borax fuses slowly into a clear glass. With solution of cobalt becomes hlue. Digested with sulphuric acid yields hydrofluoric acid. Hy ignition the dark yellow varieties become red, and the pale yellow varieties colourless, retaining their transparency. 2AIF 3 + 3SiF s + l2A18i, fluorine 17-28, oxygen 36-65, silicon 16-94, aluminium 29-14. The silicon and aluminium correspond to silica 35-26, alumina 54'80. Analyses of topaz a from Schneckenstein, I from the Brazils, c from Finbo, all by Berzelius, d from the Brazils, e from Trum bull in Connecticut,/from Finbo, aH by Forchhammer :— a b c d e f Silica 34-24 34-01 34-36 — 35'39 36'66 ■Alumina 57‘45 58‘38 67‘74 54-88 66-96 65'16 Fluorine 14-99 16-06 16-02 17-33 17-35 17’79 The quantities of fluorine in d, e,f deduced from the loss 8 ustaincd by exposure to a very high temperature, which drives °ff the whole of the fluorine in combination with silicon, were to*50, 16 86, 17-84 respectively. Usually in attached crystals, seldom in imbedded crystals, massive or disseminated, in veins and druses, in granite, gneiss and porphyry. Is found, green, blue and colourless at Alabaschka near
