544 



MINERALOGY 



001 A 101 =56 55'; 111 A 111 = 79 55'; 101*011 = 72 40'; Com- 

 mon forms, p(lll), e(10I), c (001), h(313), s (311) ; Twinning 

 plane, 100, both contact and interpenetrating; Cleavage, 111 

 distinct, e interrupted ; Brittle ; Fracture, uneven ; H. = 4.5-5 ; 

 G. = 5.9-6.1 ; Color, white, pale yellowish white, pale yellow to 

 brown; Streak, white; Luster, adamantine to vitreous; Trans- 

 parent to opaque; o> = 1.934; = 1.918; o> = .016; 

 Optically (-). 



B.B. Fuses at 5. When dissolved in S. Ph., the bead is 

 yellow in O. F., blue in R. F. when cold ; the same bead reduced 

 beside tin on coal shows tungsten, page 587. Soluble in HN0 3 , or 

 HC1, leaving a yellow powder (WO 3 ), which is soluble in ammonia. 



General description. Crystals octahedral in habit, combina- 

 tions in which the pyramid of the second order usually predomi- 



FIG. 523. Scheelite and Fluorite from Schwarzenberg, Saxony. 



nates. Crystals from Turnbull, Connecticut, are combinations 

 of the first and second order pyramids in which the former pre- 

 dominates. 



Scheelite is interesting as an example of the tetragonal equa- 

 torial type, and pyramids of all three orders occur on crystals from 

 Schlackenwald and Zinnwald, Bohemia. It also occurs massive. 



Chemically copper may replace some of the calcium ; the pure 

 copper tungstate is the mineral, cuprotungstite, CuWO 4 , from La 

 Paz, Lower California. Molybdenum may replace the tungsten, 

 as in the variety, or mineral, powellite of western Idaho. 



Tungstic acid (W0 3 ) was discovered in scheelite, in 1781, by the 



