242 DESCRIPTIONS OF MINERALS. 
II. SILICATES. 
The silicates are here divided into the anhydrous and 
the hydrous. 
In part of the anhydrous silicates, the combining value 
(or quantivalence, see page 77) of the silicon is to that of 
the basic elements as 2 to 1; in another part, as 1 to1; 
and in a third division, as less-than-1 to 1. On this ground 
the mineral silicates may be arranged in three groups, 
named respectively: I. BistLicatres ; I]. UNISILICATES ; 
and III. SuBSILICATES. 
In the Bisilicates, one molecule of silicon is combined 
with one molecule of an element in the protoxide state, as 
Mg, Ca, Fe, etc., or one-third of a molecule of an element 
in the sesquioxide state, as Al, Fe, Mn, etc.; or, what is 
the same thing, 3 molecules of silicon, with 5 of an element 
in the protoxide state, or 1 of an element in the sesquiox- 
ide state. The general formulas of such compounds is 
hence RO;S$i, or 8 O, Siz, or, if elements in both the pro- 
toxide and sesquioxide state are present, (R,; BR) O,8i;, as 
explained on page 81. 
In the Unisilicates, one molecule of silicon is combined 
with two of an element in the protoxide state, that is, for 
example, Mg», Ca, Fe, ; or with two-thirds of a molecule in 
the sesquioxide state, that is, two-thirds of Al, Ee, Mn. 
The formula of these silicates is hence R,O,8i, or RB O, 
Si, or, in order to remove the fraction in the last, Ry Or 
Siz; which becomes, when elements in the protoxide and 
sesquioxide state are both present, (R;, R). On Si. 
Among the species referred to the Unisilicates there are 
some that vary from the unisilicate ratio. This occurs 
especially in species in which an alkali is present, as in the 
feldspars, micas, and scapolites. 
The Subdsilicates vary in the proportion of the silicon to 
the basic elements, and graduate into the unisilicates. 
