CLA SSIFICA TION OF IGNE OUS RO CKS 6 5 I 



tributing Si0 2 tentatively to form leucite, nephelite and olivine 

 and noting the deficit of SiO s by means of the equation : 



v = - of the deficit of SiO, . 



3 

 y = molecules of akermanite. (4.Ca0.3Si0 2 ). 



CaO is to be taken from diopside, and the MgO and FeO so 



liberated are to be calculated as olivine. 



a) In case an excess of CaO has been set aside for wollas- 

 tonite this is first converted to akermanite by means of the 

 equations: y = the deficit of Si0 2 . 



y = molecules of akermanite (zi.CaO.3SiO,). 



c) If there are not sufficient molecules of wollastonite to 

 satisfy the deficit of silica, recalculate the molecules of diopside 

 and wollastonite so as to make olivine, diopside and akermanite 



by means of the formulae. 



2 

 2x -\- 2>y ~\ — = available Si0 9 . 

 2 



x -\- 4y = molecules of CaO. 



x -f- z = molecules of MgO + FeO. 



Where x = molecules of new diopside, y — molecules of aker- 

 manite (4CaO . 3SiO s ) , and z = molecules of olivine. 



21. If there is still not enough Si0 3 ,all the CaO of the diopside 

 and wollastonite must be calculated as akermanite, the (Mg, Fe)0 

 being reckoned as olivine and the K 2 distributed between 

 leucite and kaliophilite by the equations : 



x -\- y = molecules of K 2 0. 

 4X -\- 2y = available Si0 2 . 

 where x is K 3 in leucite and y = K 3 in kaliophilite. 



22 In case there is insufficient Si0 3 and an excess of A1 3 3 

 and (Mg,Fe)0, which might form aluminous spinel, an alferric 

 mineral, the excess of A1 3 3 is to be calculated as corundum, 

 and the uncombined (Mg,Fe)0 is to be estimated as femic min- 

 erals, being placed with the nonsilicate, mitic, group, magnetite, 

 ilmenite, etc. 



It will be noted that as a result of the methods given above 

 the following minerals are not found together in the norm, 

 standard mineral composition, of igneous rocks ; in other 



