236 MINERALOGY 



The alteration and relation of many orthosilicates is easily 

 explained, and their formulae may be written as substitutions in 

 the general formula of an aluminium orthosilicate, in which the 

 aluminium is replaced by other bases. 



Aluminium orthosilicate AK[ Si0 4 = Al ; Al 4 (Si0 4 )3. 



\SiO 4 =Al 



r A i r^' Ca 3 Al 2 (Si0 4 ) 3 



Garnet = Al< SiO 4 L 



Si0 4 =Al 



'/ H 

 /Si0 4 -K 



Biotite = Ak / M g . HKMg 2 Al 2 (Si0 4 ) 3 . 



/Si0 4 =(AlF 2 ) 3 

 Topaz = Al<-SiO 4 = Al ; A1 3 (A1F 2 ) 3 (Si0 4 ) 3 = Al (A1F 2 ) Si0 4 . 



c. Columbates and tantalates. 



Salts of columbic acid, HCbO 3 , as columbite, (Fe.Mn) (Cb0 3 ) 2 , 

 an orthocolumbate of iron and manganese. 



Tantalates, salts of tantalic acid, HTaO 3 , as tantalite, Fe(TaO 3 ) 2 . 



d. Phosphates, arsenates, and vanadates, etc. 



Salts of the acids, H 3 PO 4 , H 3 As0 4 , H 3 VO 4 ; as xenotime, YPO 3 ; 

 berzelite, (Ca.Mg.Mn) 3 (AsO 4 ) 3 ; and pucherite, BiV0 3 . 



e. Borates and uranates. 



Salts of boric acid, as sassolite, H 3 B0 3 ; also biborates, as borax, 

 Na 2 B 4 7 . 



/. Sulphates, chromates, and tellurates. 



Salts of sulphuric acid, as barite, BaSO 4 . Salts of chromic acid, 

 as crocoite, PbCrO 4 . 



g. Tungstates and molybdates. 



Salts of tungstic acid, H 2 WO 4 , as wolframite, (Fe.Mn) W0 4 . 



Salts of molybdic acid, as wulfenite, PbMo0 4 . 



In most cases the above acid groups include, in addition to the 

 normal salts, hydrated, acid, and basic salts, as well as complex 

 molecules, the exact structure of which has not in all cases been 

 determined. 



