72 W. T. Schaller — Notes on Powellite and Molybdite. 



The ratio of CaO to Mo0 3 is 1 : 0'96, giving the formula 

 CaMo0 4 . 



What is probably a similar occurrence of powellite has 

 recently been described* as possibly a natural occurrence of 

 Mo0 3 , the mineral differing in its physical properties from the 

 hydrous ferric molybdate. It forms a white or grayish altera- 

 tion product of molybdenite, which it sometimes covers and 

 after which it is pseudomorphous, preserving the form of the 

 molybdenite. The luster is pearly and the mineral is semi- 

 translucent, difficultly fusible to a gray scoria, soluble in nitric 

 acid and does not contain any iron or water. The material was 

 too scanty for analysis. 



2. Powellite from Nye Co., Nevada. — The powellite from 

 Nevada was received through Mr. F. L. Hess of the U. S. 

 Geological Survey, who states that the specimens were sent 

 him by Mr. F. O. Byor of Columbia, Nevada. The locality is 

 given as 2 miles south of Oak Springs, Nye County, Nevada. 



The powellite occurs in a vein about l^ cm wide in a soft 

 earthy mass which seems to be some altered rock. In this rock 

 are occasionally found irregular masses of powellite which 

 sometimes are several centimeters across. The mineral is dull in 

 appearance, grey in color, and occurs in platy masses often 

 bent and twisted in different directions. 



Imbedded in this vein of powellite and also as small masses 

 and veins in the altered rock matrix, are found grey to white 

 masses of scheelite showing good cleavage surfaces with a 

 highly vitreous to adamantine luster, often having also a decided 

 greasy appearance. The association of scheelite with pow- 

 ellite side by side was thought to be a most unusual one, but 

 otber specimens showed that the powellite, like the Texas min- 

 eral, was a secondary one being formed, in situ, from molyb- 

 denite; it is in fact a pseudomorph after the sulphide of 

 molybdenum. Some other specimens showed the various 

 stages of alteration very well, the amount of unaltered molyb- 

 denite varying from considerable to none at all. The 

 association of molybdenite with scheelite is one well known 

 and when the molybdenite alters to powellite the association, 

 powellite-scheelite, necessarily follows. The agencies affecting 

 the change from molybdenum sulphide to calcium molybdate 

 are apparently without effect on the scheelite. This pseudo- 

 morphous character of the powellite accounts for its occurrence 

 in platy masses, the structure of the original molybdenite being 

 retained. On some of the specimens, notably those on which 

 there is still a considerable amount of molybdenite remaining, 

 the powellite often has a reddish color due to iron stain. 



*Molybdite froni the Ilnien Mts., G. Gagarine, Bull. Acad. Imp. Sci., St. 

 Petersburg, 6 ser., 287-88, 1907. 



