472 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



OOl 



The scheelite from the Coeur d'Alene district is for the most part 

 massive and it all has a translucent buff color. Distinct crystals 

 have been seen by the writer only as minute druses lining cracks in 

 the massive mineral. The scheelite is associated with auriferous 

 pyrite in quartz with usually small amounts of other sulphides. 

 A typical specimen showing these associations is illustrated in Plate 

 19 (lower). 



POWELLITE (816) 

 Calcium molybdate, Tetragonal. 



CaO.Mo0 3 . 



The calcium molybdate, powellite, corresponding to the calcium 

 tungstate, scheelite, was first described as a new mineral from Idaho 

 and named in honor of Maj. J. W. Powell, director of the United 

 States Geological Survey, by W. H. Melville, whose description is 

 abstracted below. 88 



ADAMS COUNTY 



A piece of ore, friable from weathering, picked up on the dump of 

 one of the tunnels of the Peacock claim, Seven Devils district, in 



1890, consisted of light-brown gar- 

 net, bornite, and a mineral suggest- 

 ing scheelite in appearance, which 

 upon examination was found to be 

 the new species, calcium molybdate. 

 The mineral resembles scheelite and 

 shows a poor cleavage. The color is 

 yellow with a decided green tinge. 

 The crystals are semitransparent 

 and are brittle and the hardness 

 is less than that of scheelite, or 

 about 3.5. The specific gravity is 

 4.526 the mean of two determin- 

 ations. The blowpipe character- 

 istics are those commonly given 

 under molybdates and tungstates, 

 the molybdenum obscuring the 

 tungsten associated with it. It 

 fuses at 5 to a gray mass and is decomposed by nitric and hydro- 

 chloric acids. Associated with the powellite is a small amount 

 of olive-green molybdic ocher apparently resulting from its decom- 

 position. Crystallographically the powellite is tetragonal and iso- 

 morphous with scheelite. The best crystals are about 1 mm. long 

 and others reach a maximum length of nearly 3 mm. They are 

 closely allied to scheelite in form and habit, the appearance and 

 development being essentially like Figure 164. The angles are so 



Fio. 164.— Powellite. Seven Devils dis 

 trict, Adams County. After Melville 



W. H. Melville. Auier. Journ. Sci., vol. 41, p. 138, 1891. 



