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 to 1 ; 

 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. Bisilicates ; II. Uintsilicates ; 

 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 3 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 R 3 Si, or R 9 Si 3 , or, if elements in both the pro- 

 toxide and sesquioxide state are present, (R 3 R) 9 Si 3 , 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 2 , Ca 2 , Fe 2 ; or with two-thirds of a molecule in 

 the sesquioxide state, that is, two-thirds of Al, Fe, Mn. 

 The formula of these silicates is hence R 2 4 Si, or Rf 4 

 Si, or, in order to remove the fraction in the last, R 2 12 

 Si 3 ; which becomes, when elements in the protoxide and 

 sesquioxide state are both present, (R 3 , R) 2 12 Si 3 . 



Among the species referred to the Unisilicates there are 

 some that vary from the uni silicate ratio. This occurs 

 especially in species in which an alkali is present, as in the 

 feldspars, micas, and scapolites. 



The Sabsilicates vary in the proportion of the silicon to 

 the basic elements, and graduate into the unisilicates. 



