Chemical Composition of Chromite. 285 



oxide, and the second chromite from Yache Island, West 

 Indies, analyzed by J. Clouet.* 



I (Garrett). II (Olouet). 



Cr 2 3 41-55 51-53 



FeO 48-46 



Fe 2 3 62-02 



SiO , 1-25 



104-82 99-99 



A greater part of the iron in Garrett's analyses is undoubt- 

 edly in the form of ferrous oxide, as is indicated by the high 

 per cent obtained. From the above, it is seen that a pure 

 chromite having the composition FeO . Cr 2 3 is not common 

 in nature. 



The MgO . A1 2 3 occurs nearly pure in nature as normal 

 spinel. The normal MgO . Cr a 3 has not been found in nature, 

 but, as indicated from the above analyses, there is good reason 

 to believe that this molecule does exist, and we may expect to 

 find normal MgO . Cr 2 3 occurring as a definite mineral. Under 

 this theory of the composition of the chromite, the formulae 

 for the three chromites described in this 'paper would be as 

 follows : 



1. 



Price Creek 



lOFeO. 



, Cr A ; 



MgO. 



, Ct 



■A; 



2MgO, 



. Al n O a . 



o. 



Corundum Hill 



9FeO 



..Or.O,; 



MgO. 



Ci 



A; 



2MgO. 



.A1„0,. 



3. 



Webster 



FeO 



.Cr 2 3 ; 



MgO . 



, Cr 



A; 



2MgO . 



A1A- 



The first two formulae will represent approximately those of 

 the majority of the chromites that have been found, and they 

 are approaching the normal chromite FeO . Cr 2 3 as their limit. 



As the FeO . Cr 2 3 molecule decreases and the MgO . A1 2 3 

 increases the mineral would approach normal spinel MgO . 

 A1 2 3 as its limit, the mineral picotite or chrome spinel being 

 representative of a mineral near the spinel end. 



M. E. "Wadsworthjf in comparing the chromite with the 

 picotite associated with peridotites, says : 



"It is probable that picotite and chromite belong to the 

 same mineral series, the term picotite being more commonly 

 applied to the freshest states, and that of chromite to those 

 forms more altered, and to the local aggregations arising from 

 the migration of the chromic oxide during the alteration of 

 the associated peridotite rocks. As a further extreme in 

 the alteration a change to a more or less magnetite occurs." 

 These conclusions are deduced from a microscopical study of 

 these minerals." 



* Ann. Chim. Phys., vol. xvi, pp. 90-100, 1869; and Wadsworth's Lithological 

 Studies, appendix, p. iv. 



f Lithological Studies, p. 184. 



