52 C. W. Cook — New Occurrence of Ilsemannite. 



denite from Gibson is molybdenum dioxide. All material 

 so far examined indicates a thickness for the coating no 

 greater than that of one of the cleavage laminae of the 

 molybdenite. Hence, the definite determination of its 

 composition or of its properties other than color and 

 luster, the latter being metallic, has been impossible. 



One other observed phenomenon should- also be men- 

 tioned since it may have a direct bearing not only upon 

 the composition of both the coating and ilsemannite but 

 also upon the nature of the decomposition process. In 

 some instances the blue colored material which has been 

 designated as ilsemannite does not appear to be water 

 soluble. This might be expected if the following stages 

 were passed through in the alteration of molybdenite to 

 molybdite. 



If the molybdenite was first altered by the oxidation 

 of the sulphur, molybdenum dioxide, a brown substance 

 insoluble in water (Muthmann), 13 would result. This 

 would represent the tarnish stage mentioned above. The 

 subsequent oxidation of a small portion of the molyb- 

 denum to the hexavalent form would result in a change 

 of color from brown to blue although the compound would 

 still remain insoluble in water, and would correspond to 

 the substance referred to in the preceding paragraph. 

 The existence of such an insoluble blue compound might 

 be expected from the fact that for many years after its 

 original preparation, the color of molybdenum dioxide 

 was thought to be blue. Further oxidation, with an 

 increase in the relative amount of the trioxide, and hydra- 

 tion would then transform this insoluble blue compound 

 into the soluble blue compound, ilsemannite. Finally, 

 complete oxidation to the trioxide, on combination with 

 iron, would yield the yellow compound, molybdite. 



The above interpretation of the observed facts would 

 seem to indicate the composition proposed for the coating 

 and also to support Yancey's suggestion regarding the 

 composition of ilsemannite. Further, it has a direct 

 bearing upon the question of the secondary enrichment 

 of molybdenite, a subject now under investigation by the 

 writer. 



University of Michigan, 

 Ann Arbor, Mich., 

 February 14, 1922. 



13 The process of Ullik described by Muthmann (Liebigs Annalen vol. 238, 

 p. 114, 1887) involves purification with hydrochloric acid and potassium 

 hydroxide so that it seems probable that it is insoluble in water. 



