CLA SSIFICA TION OF IGNEO US RO CKS 645 



2. Before undertaking the distribution of the chemical com- 

 ponents as mineral molecules, small amounts of MnO and NiO 

 are to be united with FeO, and of BaO and SrO with CaO ; of 

 Cr 2 3 with Fe 3 3 , unless these unusual components occur in 

 sufficient amounts to make their calculation as special mineral 

 molecules desirable. These amounts are indicated in connec- 

 tion with examples of calculation given later. 



3. Establish the fixed molecules by allotting: 



a) to Cr 2 3 , if present in notable amount, FeO to satisfy 

 the ratio Cr 3 O s : FeO : : I : 1 for chromite ; 



b) to TiO s enough FeO to satisfy the ratio Ti0 3 : FeO : : 

 1 : 1 for ilmenite. If there is excess of Ti0 3 , allot to it equal 

 CaO for titanite or perofskite according to available silica, to 

 be determined later. If there is still an excess of Ti0 3 it is to 

 be calculated as rutile ; 



c) to P 2 O g allot enough CaO to satisfy the ratio P 2 5 : 

 CaO : : 1 : 3.33 for apatite. Allot F to satisfy CaO = 0.33P 3 O 5 ; 



d) to F not used in apatite allot CaO to form fluorite, 

 CaO : F : : 1 : 2 ; 



e) to CI allot Na 3 in the ratio Cl 8 : Na 3 (0) : : I : 1 for 

 sodalite ; 



/) to S0 3 allot Na 3 in proportion SO s : Na 3 : : 1 : 1 for 

 noselite ; 



g) to S allot FeO in proportion S : Fe(O) : : 2 : 1 for pyrite ; 



h) to C0 3 in undecomposed rocks allot CaO in the propor- 

 tion 1 : 1 for calcite. C0 3 may occur in primary calcite and 

 cancrinite. If these minerals are secondary, the C0 3 is to be 

 neglected, since it is understood that analyses of decomposed 

 rocks are not available for purposes of classification. 



Having adjusted the minor, inflexible, molecules, there 

 remain the more important but variable silicate molecules, which 

 form the great part of the standard mineral composition, or 

 norm, of most rocks. 



4. To A1 3 3 are allotted all the K 3 and Na 3 0, not already 

 disposed of, in the proportion of Al 2 O s : K 3 -f- Na 3 : : 1 : 

 I for alkali feldspathic and feldspathoid (lenad) molecules. 



