324 AUSTRALASIAN ANTARCTIC EXPEDITION 



In all four rocks the hypersthene is pleockroic in light tones of green and pink ; 

 Z is always pale green and X light pink. The absorption is not strong, the formula is 



The monoclinic pyroxene in all four cases is a pale green to colourless augite. 

 It is optically positive, 2V large, the extinction (Z A c) is about 45, the D.R., 0-024. 



In No. 792 occasional simple twins on Oil occur among the pyroxenes. 



The pyroxenes are usually clear, but the augite of No. 792 exhibits good schiller 

 structure, and not infrequently holds inclusions of magnetite (both as abundant dust 

 and larger grains), flakes of biotite, felspar (and possibly a little quartz the augite is 

 here poikiloblastic), and hornblende. 



Granulation, usually peripheral, of the pyroxene is evident in all four rocks, but 

 more especially in No. 792. 



Certain genetic relationships between the pyroxene and other minerals hold, 

 but these will be discussed below. 



The hornblende of No. 792 has as limits for its refractive indices n a = 1-66, 

 np = 1-67. n r = 1-69. The extinction (Z A c) is 15. These data, according to 

 Winchell (op. cit., fig. 139, p. 224), correspond to that member of the pargasite-hornblende 



series whose chemical composition is approximately : 



Per cent. 



NaFeSi 2 6 + Fe 2 3 ............... 16 



CaMgSi 2 6 + MgSi0 3 ............... 42 



CaFeSi 2 6 + FeSi0 3 ............... 42 



That is to say, it is a true hornblende. It is pleochroic, with Z brown-green, Y 

 green-yellow, and X pale straw-yellow. The absorption is strong. The formula is 

 the usual one, Z > Y > X. 



In the other two rocks in which hornblende occurs to any appreciable amount 

 (it is almost absent entirely from No. 783, see Rosiwal analysis above) it has the same 

 characteristics as those given for No. 972. This rock was used as the standard of reference 

 because of the greater abundance of hornblende in it. 



The texture of the amphibole is usually granoblastic, although in places it is 

 found in flakes and shreds. It holds frequent inclusions of magnetite (ilmenite). Its 

 border is usually granulitic, in which respect it resembles the pyroxene. 



The biotite of the rocks is deep reddish-brown in colour. It is pleochroic in lighter 

 and darker shades, and has very strong absorption, Z > Y > X. 



