492 , MINERALOGY 



produced by the fusion of a mixture of KAlSiO 4 and Mg 2 Si0 4 with 

 fluorides ; by a variation of the components, phlogopite, zinnwaldite, 

 and muscovite have also been produced, but none of these arti- 

 ficial products contains hydrogen, which all natural micas do. 

 Micas have been found as a crystalline product in some furnace 

 slags. 



Biotite. HK(Mg . Fe) 2 Al 2 (SiO 4 ) 3 ; K 2 O = 11.21; (Mg.Fe)O = 

 19.21; A1 2 3 = 24.35; Si0 2 = 43.08; H 2 O = 2.14; Monoclinic; 

 Type, Digonal Equatorial ; a : b : c = .5774 : 1 : 3.2743 ; p = 

 90 = 001 A 100; lbO A 110 = 30;001 A 101 =80; 001 A Oil = 73 1'; 

 001.221=85 38'; Common forms, c (001), b (010), M (221), 

 r (101), o(112); Twinning plane, 110; Cleavage, basal perfect; 

 Laminae, flexible, elastic, and tough ; H. = 2.5-3; G. = 2.7-3.1; 

 Color, greenish black to black in thick crystals; Streak, colorless; 

 Luster, splendent, transparent to translucent; a = 1.504; p = 

 1.589; -y = 1.589; -y - a = .085; Optically ( - ); Axial plane 

 JL 010 or II 010 ; Bx a X 001 ; 2 V = 0-40. 



B.B. Whitens and fuses, some with difficulty, while specimens 

 with much iron fuse easily and become magnetic. In the closed 

 tube yields water. Decomposes with sulphuric acid without gelat- 

 inizing. 



General description. In crystalline habit twinning and cleav- 

 age like muscovite. 



Chemically biotite differs from muscovite in that two atoms of 

 aluminium have been replaced by magnesium and iron. Ferric 

 iron may replace the Al in part ; lepidomelane is a black mica rich 

 in both ferrous and ferric iron. 



In rock sections biotite is brown, greenish, or reddish, rarely with 

 distinct outline. Pleochroism very marked, increasing with the 

 depth of color ; the absorption is the strongest with the ray vibrat- 

 ing parallel to the cleavage cracks. Relief is high in those sections 

 in which the cleavage cracks appear, and is very low in the basal 

 section, in which the indices of refraction are but little above that 

 of Canada balsam. Interference colors in sections parallel to the 

 vertical axis are high, but in the basal section where the two in- 

 dices are so nearly equal, the section reacts like a uniaxial crystal, 

 appearing as if dark during a complete revolution. The plane of 

 the optic axes varies in its position from perpendicular to the plane 



