CLA SSIFICA TION OF IGNE OUS RO CKS 649 



potash compound, kaliophilite, is rare and not a rock-making 

 mineral, though K 3 enters into the composition of nephelite to 

 some extent. If potash and soda are present and there is insuf- 

 ficient silica to form polysilicates of both, then as a general rule, 

 K s O takes all the silica it can get to form orthoclase, or ortho- 

 clase and leucite, the soda taking the rest to form nephelite, 

 together with albite in some cases. The occurrence of K s O in 

 the micas appears at first thought to be an exception to this rule, 

 but further consideration shows that in the muscovite molecule, 

 (H,K) 3 0. AI0O3 ,2Si0 2 , when H = 2K, the Si0 3 is six times 

 K 3 0, as in orthoclase ; and when H = K, the Si0 3 is four times 

 K 2 0, as in leucite. 



CaO forms the orthosilicate anorthite with the ratio CaO: 

 Si0 3 :: 1 : 2, and controls an amount of Si0 3 equal to itself in 

 the wollastonite molecule. In akermanite the CaO : Si0 3 :: 4 : 3. 



MgO and FeO can, at most, control only their own amounts 

 of silica, in the hypersthene molecule, and also form the ortho- 

 silicate olivine, with the ratio (Fe, Mg)0 : Si0 3 :: 2 : 1. Finally, 

 FeO crystallizes out in non-siliceous and non-aluminous min- 

 erals, magnetite and ilmenite. 



It is noteworthy that the order of affinity of these oxides 

 for alumina is the same in relative order as that for silica. So 

 K 2 forms muscovite with an excess of alumina over potash, 

 the analagous paragonite being rare and only found in meta- 

 morphic rocks, Na 3 being slightly in excess of A1 2 3 in the 

 sodalites, CaO largely so (3:1) in garnet, and MgO and FeO 

 having little affinity for it. 



MgO and FeO can, it is true, form the non-siliceous, alumin- 

 ous mineral, spinel, RO. R 2 O s . 



In the calculation of the standard mineral composition the 

 allotment of silica to the alternative molecules is, therefore, as 

 follows : 



16. To CaO set aside for wollastonite or akermanite is allot- 

 ted tentatively Si0 2 to form wollastonite (CaO.Si0 3 ). 



17. To extra (Mg,Fe)0 is allotted Si0 2 to form orthosilicate, 

 olivine (2(Mg,Fe)O.Si0 3 ). 



