BISILICATES. 243 



The same three grand divisions exist more or less satis- 

 factorily among the hydrous silicates. 



A. ANHYDROUS SILICATES. 



I. BISILICATES. 



The bisilicates, when the base is in the protoxide state, 

 and hence have the general formula R 3 Si, are resolved in 

 analyses into protoxides and silica in the ratio of 1RO to 

 1 Si O a , in which, as the term bisilicate implies, the oxygen 

 of the silica is twice that of the protoxides. If the base is 

 in both the protoxide and sesquioxide states, giving the f or- 

 mula R 3 ,R0 9 Si 3 , the mineral is resolved in analyses into 

 protoxides, sesquioxides and silica. If the ratio of the pro- 

 toxides to sesquioxides is 1 : 1, the formula will become 

 JR 3 £R 9 Si 3 ; and analyses give then, for the oxides and 

 silica 3 RO, lR0 3 6Si0 2 . 



Among the following bisilicates the species from ensta- 

 tite to spodumene and amphibole make a natural group 

 called the hornblende, or hornblende and augite group. 

 They are closely related in composition and also in crystal- 

 lization. The cleavage prism is rhombic, and has either an 

 angle of about 124£° or of about 87°; and the former of 

 these two rhombic prisms has just twice the breadth of the 

 other ; that is, if the lateral axis from the front to the 

 back edge in each be taken as unity, the other lateral axis 

 is twice as long in the prism of 124J° as it is in that of 87°. 

 The forms are either trimetric, monoclinic or triclinic ; 

 and yet the close relations just stated exist between them. 

 Enstatite is a magnesium or magnesium and iron species ; 

 wollastonite, a calcium species ; rhodonite, a manganese 

 species ; pyroxene and hornblende contain calcium with 

 magnesium or iron ; spodumene contains lithium and alu- 

 minum, aluminum replacing the elements that in other 

 species are in the protoxide state. 



