
BISILICATES. PAD 
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 O;Si, are resolved 27 
analyses into protoxides and silica in the ratio of 1RO to 
18: O,, in which, as the term distlicate 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 for- 
mula R;,R O,8i;, 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 
$R; $R O,Si; ; and analyses give then, for the oxides and 
silica 3 RO, 180, 6 81 O,. 
Among the following bisilicates the species from ensta- 
tite to spodumene and amphibole make a natural group 
called the hornblende, or hornblende and augite group. 
They are closely related in composition. and also in crystal- 
lization. ‘The cleavage prism is rhombic, and has either an 
angle of about 124° or of about 87°; and the former of 
these two rhombic prisms has just twice the breadth of the 
other ; that is, if the lateral axis from the front to the 
back edge in each be taken as unity, the other lateral axis 
is twice as long in the prism of 124$° 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. 
Knstatite 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. 
