30 Washington and Merwin — Augite from Vesuvius, etc. 



occasion, in connection with those of other Italian augites. 

 It may, however, be of interest to give the composition of 

 the Etna augite in terms of the nsual molecules, which 

 is as follows (No. 1) : 



1 2 



CaMgSi.A 70.0 | 80.1 



CaFeSiobc 16.5 ] 



CaSiOo 1.5 none 



(Mg,Fe) SiOs none 7.5 



(Mg,Fe)ALSi06 7.0 7.0 



Na(re,Al)SL06 5.0 5.4 



It will be seen that the Etna augite (1) is composed 

 very largely of diopside and hedenbergite molecules, with 

 a little wollastonite and acmite, and a small amount of 

 the aluminous Tschermak molecule. In general it much 

 resembles the augite (2) of Stromboli,^"^ though this 

 carries somewhat less of the diopside molecule, and 

 considerable hypersthene instead of wollastonite. The 

 respective refractive indices are as follows : 



Augite, Etna a=1.704, /?=1.711, y=1.735 



Augite, Stromboli, a=1.693, ^8=1.699, y=1.719 



This is not the place to discuss the differences, but it 

 may be mentioned that the higher indices of the Etna 

 augite are to be connected with its higher wollastonite 

 and titanium content, which seem to more than counter- 

 balance the higher ferrous oxide of the Stromboli augite. 

 The Stromboli augite, furthermore, is slightly higher in 

 magnesia, which would tend to lower the refractive index. 

 On the whole, it may be said that the chemical and optical 

 data for both of these augites are quite in accord, and bear 

 out observations made on the pyroxene molecules gener- 

 ally. "^ • 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 July, 1920. 



" Kozu and Washington, this Journal, 45, 468, 1918. 



