284 J. H. Pratt — Occurrence, Origin and 



The analyses were made by Dr. Chas. Baskerville* and Dr. 

 H. W. Foote.f 



The result of the analyses were as follows : 





Price Creek4 



Ratio. 



Corundum HilLJ 



Ratio. 



Webster § 



Ratio. 



OA 



... 59-20 



•386 



57-80 



•377 



3995 



•261 



A1 2 3 



... 7-15 



•070 



7'82 



•076 



29-28 



•287 



FeO . 



... 25-02 



•347 



25-68 



•356 



13-90 



•193 



MgO. 



... 4-42 



•111 



5-22 



•131 



1731 



•433 



Si0 2 - 



... 3-20 







2-80 



.... 







■ 



MnO. 



•92 







•69 















In the above analyses the ratio of the bivalent oxides to the 

 trivalent oxides is uniformly high and can probably be accounted 

 for by reason that some of the iron calculated as ferrous oxide 

 was in the ferric state, as was proved in the Webster chromite. 



Taking enough of the Cr 2 3 and MgO to unite with the FeO 

 and A1 2 3 respectively to form the molecules FeO . Cr 2 3 , and 

 MgO . A1 2 3 , there remains approximately enough of the Cr 2 3 

 to unite with the excess of the MgO to form the molecule 

 MgO . Cr 2 3 . The nearer the ratio of the bivalent oxides 

 equals that of the trivalent oxides, the nearer the excess of the 

 Cr 2 3 and MgO equal each other. The inability to determine 

 the ratio of the ferrous to the ferric oxide in the above analyses 

 prevents the obtaining of a sharp ratio in the excess of the 

 Cr 2 3 and MgO. 



With the exception of two cases, in all the terrestrial chro- 

 mite analyses examined, alumina and magnesium were invari- 

 ably present, varying from a small per cent in some samples to 

 others that showed the presence of more than 20 per cent of 

 these oxides. In the above analyses and in most of the others 

 examined, it was noticed that the magnesia usually varied with 

 the alumina, those rich in alumina being correspondingly rich 

 in magnesia. 



This constant occurrence of magnesia and alumina in the chro- 

 mite would seem to indicate that the molecule of the mineral 

 now called chromite is not pure FeO . Cr 2 3 but is a combina- 

 tion of the three isomorphous molecules, FeO . Cr 2 3 ; MgO . 

 Cr 2 3 and MgO . A1 2 3 . 



But two analyses of chromite (terrestrial) have been found 

 that do not show the presence of magnesia and alumina, the 

 first a magnetic chrome sand from Chester, Pa., analyzed by 

 T. H. Garrett,|| in which all the iron is calculated as ferric 



* Of the Chemical Laboratory, N. C Geological Survey. 

 f Of the Sheffield Laboratory, Yale University. 

 % Baskerville. § Foote. 



|| This Journal, vol. xiv, p. 4T, 1852. 



