308 SILICATES. 102, f 103, O 110, k 210, n 111, X 211, y 112, 2 i 111. im 34° 21' nri O o 33' xm 61° 65* sm 45 37 acb 48 21 nm 75 11 rm 63 43 96 41 qm 89 42 Im 89 27 km 39 9 zm 104 16 fm 98 38 68 26 dm 117 49 tm 115 24 bin 90 O y* 45 5 em 145 17 kk’ 101 43 nt 69 1 ut 35 27 oo' 63 8 ot 102 59 zt 64 66 dd’ 96 10 td’ 130 11 ib 90 O uu 109 6 FIG. 323. zz 70 9 38 103 30 64 46 6 4} 44 nr PIG. 322. 71 Combinations. rntm, Irztumb, rntuomb, Irnxyqtuzdom. Twins, l. Twin-face t; 2. twin-face m, seldom. Cleavage. m, perfect; t, less perfect; I, in thulite. Fracture uneven. I, y, q, z striated parallel to their intersections with each other. Semi-transparent...transparent on the edges. Lustre vitreous. e = 1'7. Pearly on the faces of perfect cleavage. Green, yel low, brown, red, black. Streak grey; in the variety from St. Marcel red. Exhibits pleochroism. A ray traversing the crystal in a plane parallel to b and polarized in that plane is almost entirely absorbed. The optic axes lie in a plane parallel to b, and make angles of about 46° 30' with a normal to t. On looking through a crystal in the direction of either optic axis a light-coloured stripe is seen between two dark spots. Brittle. II = 6‘6. G = 30...36. Before the blowpipe the green varieties swell and exfoliate ; they melt with great difficulty on the edges into a clear glass ;