C. W. Cook — New Occurrence of Ilsemannite. 51 



broken rock. It would seem, therefore, that the altera- 

 tion of the molybdenite to ilsemannite had been relatively 

 rapid. 



According to different authorities, ilsemannite has 

 been formed in different ways. Cahen and Wooten' 5 state 

 that it is formed by the alteration of jordisite ; Dana 7 says 

 that ilsemannite is "a product of the decomposition of 

 metallic molybdates"; while Lindgren and Ransome 8 

 indicate their belief that at Cripple Creek it has been 

 formed by the direct oxidation of molybdenite. 



The exact composition of ilsemannite is likewise sub- 

 ject to discussion. Dana 9 gives the formula (Mo0 2 . 

 4Mo0 3 ) ; Schaller 10 has proposed the formula (Mo0 3 . 

 S0 3 .3H 2 Q) ; while Yancey 11 believes that Guichard's 

 formula for the synthetic blue oxide (Mo0 2 .4Mo0 3 .6H 2 G) 

 most closely, although not exactly, expresses the compo- 

 sition of the blue of ilsemannite and for the present 

 prefers to consider it is a chemical mixture of molyb- 

 denum dioxide with relatively larger amounts of the tri- 

 oxide. In support of the latter supposition, the writer 

 wishes to advance some observational evidence which 

 may also shed some light on the nature of the alteration 

 process in the case of molybdenite. 



On the crystal and cleavage faces of the molybdenite in 

 specimens from Gibson, a phenomenon has been observed 

 which the writer has not seen on specimens from any 

 other locality, namely a tarnish or iridescence which 

 immediately reminds one of the "peacock colors" on 

 bornite. A closer examination shows the color to vary 

 from a bronze-brown to a violet brown to blue. Appar- 

 ently the product is identical with that obtained near 

 the assay on charcoal when molybdenite is subjected to 

 the action of the oxidizing flame. Also the writer has 

 produced this tarnish artificially by gently touching 

 crystals and cleavage faces of molybdenite with an oxi- 

 dizing flame. Guichard 12 has shown that the true color 

 of molybdenum dioxide is brown or violet brown although 

 this color is easily obscured by the blue of the mixed 

 oxides. ^ It, therefore, seems to the writer quite probable 

 that this bronze -to violet brown coating on the molyb- 



c The Mineralogy of the Bare Metals. 1912, p. 51. 



7 System of Mineralogy. 6th edition, p. 202. 



8 Loc. cit. 



9 Loc. cit. 



10 Wash. Acad. Sci., vol. 7, p. 417, 1917. 



11 Chem. and Met. Engr., vol. 19, p. 189, 1918. 



12 Compt. Eend., vol. 129, p. 722, 1899. 



