Schaller — Chemical Composition of Molybdic Ocher. 297 



Art. XXIX. — The Chemical Composition of Molybdic Ocher / 

 by Waldemae T. Schaller.* 



Summary. — It is shown that the natural molybdic ocher, 

 called molybdite, is not the trioxide of molybdenum, as stated 

 in the literature, but a hydrous ferric molybdate with the for- 

 mula, Fe 2 O s .3Mo0 3 .7iH 2 0. 



The composition of natural molybdic ochre is given in all the 

 text books on Mineralogy as agreeing in composition with the 

 artificial product, molybdenum trioxide, Mo0 3 , which may be 

 obtained by oxidizing the natural sulphide, molybdenite. So 

 far as the writer is aware, but one analysis was ever made of 

 the natural ocher, and while that suggested the desirability of 

 further examination, such has never been made. This is 

 doubtless clue to the fact that whereas the yellow molybdic 

 ocher is very widespread in its occurrence, it seldom occurs in 

 sufficient quantity and of such purity as to warrant any chem- 

 ical investigation. The writer was fortunate enough to receive 

 a sample of supposed autunite from the Foote Mineral Com- 

 pany which, on investigation, was found to be molybdic ocher. 

 As the • mineral occurs in a pure state some qualitative tests 

 were made and it was found that beside the molybdenum, 

 considerable ferric iron and water were present. The material 

 was then examined under the microscope and found to be pure 

 and especially free from limonite. Some artificial crystals of 

 Mo0 3 were prepared, and on comparing the two substances 

 under the microscope, such differences were found in their prop- 

 erties as to indicate that the mineral examined was not 

 molybdite, but a new species, — a hydrated ferric molybdate. 

 The investigation thus opened was extended, and through the 

 generosity of several people, it has been possible to make 

 analyses of the natural molybdic ocher from four different local- 

 ities, and to show that the natural ocher is not Mo0 3 but 

 Fe 2 3 .3Mo0 3 .7pi 2 0. 



A summary of the literature on molybdite is so well given 

 by Hintze that only such points as bear directly on the ques- 

 tion at hand will be mentioned. Owenf described a deep yel- 

 low iron molybdate from Nevada City, California, and W. J. 

 Taylor^ described a similar occurrence from Heard County, 

 Georgia, but neither article contained quantitative data of any 

 value. Owen found 35 per cent Fe 2 3 , but Genth§ in a later 

 publication, made a determination on the same mineral and 

 obtained 24'3 per cent Fe 2 3 . He says, "That which could 



* Published by permission of the Director of the IT. S. Geological Survey. 



+ Proc. Ac. Phila., vi, 108, 1852. 



J This Journal, xix, 429, 1855. ilbid., xxviii, 248, 1859. 



