458 MINERALOGY 



condition. At times by hydration it becomes dull and greasy in 

 appearance, forming several varieties, as the malacon from Hit- 

 teroe, Norway. 



Jargon, jacinth, and hyacinth are clear varieties usually obtained 

 from Ceylon, which are cut and polished as gems. 



Artificial zircon may be produced by heating gelatinous silica 

 and zirconia to a red heat under pressure. 



TOPAZ 



Topaz. Al . Al(O,F 2 )SiO 4 ; A1 2 O 3 = 55.44, SiO 2 = 32.61, 

 F = 20.65 ; Orthorhombic ; Type, Didigonal Equatorial ; & : b : c 

 = .528; 1 : .477; 110 A 110 = 55 43'; 120 A 120 = 86 49'; 001 

 A lll = 4535'; 001,223 = 34 14'; 001 A 221 = 63 54'; 001 

 A 041 = 62 20'; 001 A 043 = 32 27'; 001 A 201 = 61 30"; 001 

 A 021 = 43 39'; Common forms, a (100), b(010), c(001), m(110), 

 1(120), d(201), f(021), y(041), x(043), o(221), u(lll), i(223); 

 Cleavage basal perfect; Brittle; Fracture, subconchoidal ; H. = 8 ; 

 G. = 3.4-3.65 ; Color, white and pale shades of yellow, pink, 

 blue, green; Streak, white ; Transparent to translucent ; a = 1.615; 

 P = 1.618; -y = 1 - 625 ; <y-a = .010; 2V = 62 33'; Axial 

 plane = 010; Bx a = c; Optically (+). 



B.B. Changes color, but infusible. Yields a fluorine reaction 

 in the closed tube. The powdered mineral becomes blue with 

 cobalt solution. Very little affected by acids. 



General description. Crystals are prismatic in habit, combi- 

 nations of the two prism, m and 1, both of which are striated length- 

 wise, but 1 more than m; terminated by one or two domes and 

 pyramids, rarely doubly terminated. 



In chemical composition the fluorine is variable, as it may be 

 replaced by hydroxyl (OH) ; both of these are driven off at a 

 white heat, transforming the topaz to sillimanite. 



In rock sections topaz appears with crystalline outlines or lath- 

 shaped, elongated parallel to 6 ; the basal cleavage is well marked 

 by cracks. Relief is medium and the interference colors are first 

 order gray or yellow. The interference figure is shown in the basal 

 section ; the angle 2 E varies with the fluorine and decreases as OH 

 increases. For .93 per cent. H 2 O, 2 E = 114. Cavities elongated 

 parallel to the vertical axis and containing fluids are common. 



Topaz is an accessory mineral in many granites, and is also 



