260 M. Rose on those Minerals which possess [April, 



It is, therefore, a bisilicate of protoxide of manganese com- 

 bined with a small quantity of bisilicate of lime. 



Besides the pyroxenes of these four divisions, there exist in 

 nature also some which contain alumina as a real constituent 

 part. They are rare, and the quantity of alumina which occurs 

 in them never amounts to more than seven per cent. The alu- 

 mina, which contains three atoms of oxygen, is not isomorphous 

 with the bases containing two atoms of oxygen, and its presence 

 in the pyroxenes is still enigmatical. It seems that in propor- 

 tion as the quantity of alumina increases, that of the silica 

 decreases, so that it is not improbable that it does not exist as 

 a silicate in the pyroxenes, but as an aluminate, of which the 

 electronegative constituent part may be isomorphous with silica. 

 Such pyroxenes which contain alumina are the black pyroxene 

 from Frascati analysed by Klaproth,* the pyroxene from 

 iEtna analysed by Vauquelin, and the black pyroxene from 

 Pargas analysed by Nordenskiold. 



The greater part of the pyroxenes analysed by M. Rose 

 were fragments taken from massive pieces, with a very distinct 

 foliated fracture. In this state, the minerals contain less 

 foreign admixtures than when they are crystallized. In this 

 case, the crystals are generally large, and have distinct faces ; 

 but between the laminae very distinct admixtures may be disco- 

 vered, and so many traces of the matrix that they may be seen 

 with the unassisted eye. In the beautifully crystallized pyroxene 

 from Frascati, the naked eye discovers many such foreign admix- 

 tures, while even with a glass none can be found in the massive 

 white malacolite from Orrijervi. 



The pyroxenes analysed by M. Rose were all measured by 

 Prof. Mitscherlich, and found to be real pyroxenes. The incli- 

 nations of the two sides, and of the terminal face, approached 

 to the numbers given by Haiiy, and agreed completely with the 

 measurement of Phillips. These cleavages occurred on the 

 malacolite from Orrijervi, the yellow variety from Langbanshyt- 

 tan, though they were not so clear in the hedenbergite, in 

 both malacolites from Bjormyreooeden, or the hard sahlite. 



The pyroxene from Taberg is the only one of those ana- 

 lysed by M. Rose, which is well crystallized. The red sili- 

 cate of manganese from Langbanshyttan shows only the 

 cleavage parallel to the sides, and the truncations of the 

 acute and obtuse solid angles. The inclination of the sides 

 is the same as that mentioned by Haiiy as belonging to 



* Klaproth found alumina in all pyroxenes, of which he has published an account 

 in the collection of- his analyses; however, among them all, the pyroxene from Fras- 

 cati only is a real pyroxene ; the augit from the Sanalpe is hornblende ; the slaggy augit 

 from Sicilia whicli is not crystallized is, according to Prof. Weiss, an obsidian ; the 

 common augits from the Rhon mountains are not crystallized, and no trace of foliated 

 fracture can be discerned in them. 



