Volltext Seite (XML)
MICA. 389 each other a very small angle. Possibly these may have been uniaxal in their normal condition, and the separation of the single optic axis into two may be due to the state of strain produced in the crystal in the process of detaching and cleaving it. 224. MICA.—Mica (in part) ; Phillips, Hauy. Hemipris- rnatischer Talk-Glimmer ; Mohs. Glimmer; Ilausmann, Hai- dinger. Oblique. 101,100 = 25° 19'; 111,010 = 64° 46'; 101,001 = 64° 42'. i 010, c 001, e 021, m 110, s 111, r ill, v 112, x 131. be 90° 0' mb 60 23 sb 64 46 rb 61 27 vb 70 4 eb 24 46 xb 31 30 VC 43 1 sc 68 32 me 81° 20' re 107 5 mm 120 46 she 54° 42' mho 80 1 rbe 109 34 FIG-. 402. c k«\ ~~ Combinations, cmb, embvsrex. Cleavage, c, very perfect; m t b traces. Practure conchoidal, seldom observable. Trans parent...in thin leaves translucent. Lustre, c pearly inclining to metallic; the other faces vitreous inclining to adamantine. 11 = 1-600. The optic axes lie in a plane perpendicular to c. The angle between either axis and a normal to c varies in different varieties from 3° to 22° 3l'. Colourless, white, various shades of , grey, brown, green, black. Colours by reflected and trans mitted light frequently different. Streak white... grey. Sectile ; *0 thin leaves elastic, it = 2 p 6. G = 2'8...3'i. In the matrass yields water and a trace of hydrofluoric acid. Hefore the blowpipe melts into a yellowish-grey blebby glass. Different varieties possess very different degrees of fusibility ; those containing fluorino become dull w'hen heated. Is dis solved readily in borax and in salt of phosphorus, leaving a skeleton of silica in the latter. Is not decomposed by hydro chloric or sulphuric acid, the chrome-mica from the Zillerthal excepted, which is acted upon by hydrochloric acid. Analyses of mica a from Uto, b from Broddbo, c from the Neighbourhood of Falilun, d from Kimito, e from Ochotzk, all H. Rose,/brown mica from Cornwall by Turner s 3 .ill
