134 Penfield and Forbes — Chrysolite-Fay alite Group, etc. 



Per Cent Axial Angle 2V 



Material. 



Locality. 



of FeO. 



measured over a. 



/?■ 



Fayalite, 



Rockport, 



68-1 



49° 50' 



1-864 



Hortonolite, 



Monroe, 



47*3 



69 24 



1-791 



Chrysolite, 



Auvergne, 



13-0 



89 36 



1-692 



a 



Vesuvius, 



12-6 



89 42 





a 



Hawaii, 



10-3 



91 2 





a 



Egypt, 



9 -2 



91 19 



1-678 



it 



New Mexico, 



8-6 



91 24 





a 



Unknown, 



? 



92 14 



1-678 



t<. 



East Indies, 



? 



92 45 



1-670 



Fosterite, 



Vesuvius, 



2? 



93 50 



1-657 



The effect of a decrease in iron is to constantly increase the 

 value of 2V, which, at the iron end of the series, changes much 

 more rapidly than at the magnesia end, as may be seen by the 

 curve which has been plotted, and where the percentages of 

 FeO have been taken as ordinates and the values of 2V as 

 abscissas. 



50 



30 



20 



\ 













\ 











\ 



\ 











\ 













\ 











\ 













\ 



The decrease in FeO is 

 accompanied by a decrease in 

 the value of /9 and also in the 

 strength of the double refrac- 

 tion. With the FeO about 12 

 per cent 2V for yellow equals 

 nearly 90°. Chrysolites con- 

 taining less than 12 per cent 

 FeO have c, or the crystallo- 

 graphic axis a for the acute 

 bisectrix and are optically 

 positive with dispersion p <u, 

 and those richer in iron are 

 optically negative with disper- 

 sion p > u. 



50 



100 



Monticellite. 



In order to make the investigation of the chrysolite group 

 more complete, we have included monticellite CaMgSi0 4 , 

 which in its crystal lographic relations is very close to chrysolite, 

 the crystals from Magnet Cove, Arkansas, investigated by 

 Genth and Pirsson* furnishing excellent material for the pur- 

 pose. An abundant supply of this rare mineral was generously 

 furnished to us by Messrs. Geo. L. English & Co. of New 

 York, to whom we take pleasure in expressing our thanks. As 



*This Journal, vol. xli, p. 399, 1891. 



