286 J. H. Pratt — Occurrence of Chromite, etc. 



The author does not agree with Dr. Wadsworth that the 

 chromite represents an altered product of a mineral, of which 

 picotite is a purer form. The chromite is a mineral which 

 suffers alteration hut slightly and, as it is found at the present 

 time, represents the original mineral and not an altered form. 

 The difference in the microscopical properties can readily be 

 accounted for by the difference in the chemical composition. 

 With an increase in the ratio of the molecule, MgO . A1 2 3 

 and a corresponding decrease in the molecule, FeO . Cr 2 3 , the 

 more translucent the mineral will become. 



These two minerals belong to the same group and are closely 

 allied to each other, and they represent two primary minerals 

 and not different stages in the alteration of another mineral. 



In the analysis of the Webster chromite, the largest per- 

 centage of MgO was obtained and in the calculation of the 

 ratios the formula was shown to be FeO . Cr 2 3 ; MgO . Cr 2 3 ; 

 2MgO . A1 2 3 , this being the highest ratio of the molecule 

 MgO . Cr 2 3 of any chromite examined. The theoretical com- 

 position is here given, together with the analysis of the Webster 

 mineral : 



Theory for 

 Webster chromite found. 2 MgO . A1 2 3 ; 



MgO.Cr 2 3 ; FeO.Cr 2 3 . 



Cr 2 3 39-95 40-90 



A1 2 3 _ 2928 , 30-44 



FeO..., 13-90 10-75 



MgO _ 17-31 17-91 



100-44 100-00 



This analysis is similar to that described by Bock* for a 

 " magnochromite " from Grochau, Silesia, which contained 

 Cr 2 3 40-78, A1 2 3 29-92, FeO 15-30 and MgO 14-00. 



In appearance this Webster chromite is different from any 

 that has come under the author's observation, being much more 

 of a coarse-grained appearance than ordinary chromite. 



In order to designate this Webster chromite and others of 

 similar composition, the author proposes the name, Mitchellite, 

 in honor of the late Professor Elisha Mitchell of North Carolina. 



N. C. Geological Survey, December, 1898. 



Zs. G. Ges., xxv, p. 394, 1813. 



