SII.K 'ATI-S, TITANATES, ETC. 459 



imperially characterise of pegmatites containing cassiterite, in 

 which it is also associated with beryl, tourmaline, fluorite, and apa- 

 tite. At Schnrrkeiisteiu, Saxony, it is associated with apatite 

 chalropyrite, and cassiterite. 



The large l>lue crystals from Mursinka, in the Urals, are asso- 

 ciated with smoky quartz, lepidolite, and feldspars. At Nathrop, 

 ( 'olorado, and in the Thomas Range, I'tali, well-developed crystals, 

 both white and wine-colored, are associated with quartz, in cavi- 

 ties in rhyolite. 



At Stoneham, Maine, it occurs in granite. The topazes of Minas 

 ( ier.-trs, Brazil, occur in a decomposed schist ; they are a light brown 

 color with numerous elongated cavities, containing liquid carbon 

 dioxide. 



Topaz, owing to its hardness, transparency, and delicate coloring, 

 has for a long time been used as a precious stone, especially those 

 from Siberia and Brazil. The Brazilian topazes may be improved 

 in color and the yellow and brown shades changed to a delicate 

 pink by careful heating ; this process is known as pinking. 



In the process of weathering, topaz takes up water and alkalies, 

 forming micas. 



The synthesis of topaz has been accomplished by heating a mix- 

 ture of silica and aluminium fluoride to a red heat and then igniting 

 the mixture in a current of silicon fluoride. Both the synthesis and 

 the associations indicate that in many cases topaz has been the 

 result of pneumatolytic reactions in which volatile fluorides were 

 the direct agent. 



ANDALUSITE 



Andalusite. Al(A10)SiO 4 ; A1 2 O 3 = 63, SiO 2 = 37; Ortho- 

 rhombic ; Type, Didigonal Equatorial ; a : b : c = .986 : 1 : .702 ; 

 100 A 110 = 44 36'; 001 A 011 = 35 5'; Common forms, c (001), 

 m (110), s (Oil) ; Cleavage, prismatic distinct, a less so; Brittle; 

 Fracture, uneven ; H. = 7.5; G. = 3.16-3.20; Color, gray, reddish, 

 pink, blue, and green; Streak, white; Luster, vitreous; Trans- 

 parent to opaque; a = 1.632; p = 1.638; -y = 1.643; -y-a = 

 .011;. Optically (-); Bx a = c; 2V = 83 37'. 



B.B. Infusible, not attacked by acids. The fine powder 

 becomes blue with cobalt solution. 



General description. Crystals are coarse, prismatic, simple 

 combinations of the nearly square unit prism and the base or dome ; 



