F. JSf. Guild — Composition of Molybdite from Arizona. 455 



Art. XLIV. — The Composition of Molybdite from Arizona ; 

 by F. K- Guild. 



Me. W. T. Schallee lias recently shown* that the rare min- 

 eral known as molybdic ocher, or molybdite, from at least four 

 different localities, does not possess the chemical composition of 

 the trioxide which for over fifty years has been attributed to it, 

 but is in reality a hydrated ferric molybdate. These investiga- 

 tions suggest the probability of the non-occurrence of the 

 uncombined trioxide in nature, all samples hitherto so described 

 being an iron molybdate instead of mixtures of the pure oxide 

 with limonite. The matter cannot be definitely settled, how- 

 ever, until a large number of analyses from various localities 

 have been reported. Owing to the extreme rarity of the min- 

 eral and its usual mode of occurrence as a dusty incrustation 

 associated with limonite, chances for good analytical results 

 are not many. 



The Museum of the University of Arizona contains some 

 excellent specimens collected from the Santa Rita mountains 

 about thirty miles south of the city of Tucson. The mineral 

 occurs in milky quartz with the usual associations of molyb- 

 denite and limonite. Occasionally small cavities are found in 

 the pure quartz filled with pure molybdite not mingled with an 

 appreciable amount of the oxide of iron. Here the mineral con- 

 sists of tufts of fine fibers and acicular crystals sometimes two 

 millimeters in length. They possess the usual silky luster and 

 bright yellow color. 



Sufficient amount of this material for analysis was collected 

 by means of a pair of sharp-pointed pincers and the following 

 results obtained: — 



No. 1 No. 2 



Insoluble residue 4-86 2-66 



Water 16-83 16-61 



Ferric oxide,Fe„0 3 20-88 21-18 



Molybdic trioxide, MoO„ 5*7-38 59-79 



Total 99-95 100-24 



These two samples were collected independently from two 

 different cavities in the same hand specimen. The insoluble 

 residue was examined under a microscope and found to consist 

 of small crystals of quartz. 



* This Journal, vol. xxiii, p. 297, April, 1907. 



