PETROLOGY 



323 



Chemical Analysis of Hornblende from Camptonite 



(J. G. Dinwiddie, analyst) 



Si0 2 39.65 



Al 2 O a 13.61 



Fe 2 3 - 3.96 



FeO 10.58 



MgO 11.36 



CaO 11.16 



Na 2 



K 2 



H 2 



Ti0 2 



MnO 



1.99 

 2.19 

 1.43 

 5.01 

 0.06 



101.00 



As will be observed, the ferrous iron and soda in this analysis are too 

 low for arfvedsonite or barkevikite. The hornblende is zonally built, as 

 shown in figure 4. It is usually quite fresh, but where alteration occurs, 

 pale green chlorite and magnetite are noted 

 as the secondary products. Twinning is 

 common, the composition plane being in 

 most cases 100. Sometimes several lamellae 

 occur in the same individual. Figure 3 

 shows a case of abnormal twinning. 



Augite is developed as an important con- 

 stituent in all the thin-sections studied, but 

 is much less abundant than hornblende. It 

 occurs in short, stout, well formed crystals, 

 with the faces 100, 110, 010, and often 011, 

 and in crystal aggregates. It is almost col- 

 orless except in the border zones, which are 

 sometimes grass green. The inclination of 

 c to z is about 44°. Zonary growth is com- 

 mon (figure 4), the colorless augite of the 

 centra] zone of some crystals passing into 

 an outer zone of grass green aegirite, while 



in others the entire crystal is colorless and normal Twinning in Hornblende o\ 



Camptonite 

 the zones can be detected only by differences 



m optical orientation. In the latter case the outer zone sometimes shows 



an extinction angle 2 to '-\ degrees larger than that of (he inner /one. 



In some crystals, where there are numerous concentric zones, a wave o( 



extinction passes from the center to (he margin of the crystal. In other 



eases extinction starts midway between I he center and margin and moves 



Figure :?. — sketch showing ah- 



