AM E SITE AND CORUNDOPHILITE— SHANNON. 



379 



The ratios derived from the analysis then yield the formula: 

 6(Mg,Fe)0. (Al,Cr)203- ^SiOj. ffl^O, 

 which may be compared with the clinochlore formula as follows: 



This, like the preceding chromium bearing chlorite from California 

 while differing from the formula usually assigned to clinochlore, is 

 similar to several chrome-chlorites which have been referred to 

 kammererite or kotschubeite. 



Under the microscope the present mineral is biaxial and negative, 

 with a very small optic axial angle, and having the acute bisectrix 

 normal to the perfect cleavage. Basal plates are dark in all posi- 

 tions between crossed nicols. The indices of refraction are: 



a=1.587±.003 



^=1.590±.003 



7=1.590±.003 



a-7= .003 ±.003 



The mineral is distinctly pleochroic, the scheme being: 



a=pale red purple. 

 /3=red purple. 

 7=red purple. 



Under existing classifications both the above chromium-bearing 

 <jhlorites would be referred to clinochlore, or to the chromium-bear- 

 ing varieties kammererite and kotschubeite. These two examples 

 serve to illustrate the wide variation, not only in chemical composi- 

 tion, but also in indices of refraction which may exist between dif- 

 ferent specimens now included under the same species. It is highly 

 desirable that optical properties, especially refractive indices, be in 

 all cases determined upon analyzed material if work on minerals of 

 this sort is to be made of any value in the intorpretation of isomor- 

 phous relationships. 



