348 



WASHINGTON: CALCIUM ORTHOSILICATE IN ROCKS 



amounts of the minerals (mostly minor constituents), of fixed 

 composition, as apatite, acmite, magnetite and ilmenite, are first 

 written down, and then the salic K2O, Na20 and CaO are assigned 

 to orthoclase, albite and anorthite. Diopside is then formed 

 from the balance of the CaO with an equal amount of MgO + 

 FeO, and the rest of these last two oxides, which are usually in 

 excess over CaO, are assigned to hypersthene, care being taken 

 to have their relative proportions the same in both femic minerals. 

 A slide rule is very useful for this, and for obtaining other ratios. 

 It is to be remembered that only the FeO (+ MnO) remaining 

 after the formation of magnetite and ilemite is to be used, as is 

 seen in the lower left hand corner. 



The various amounts of Si02 assigned for these various mole- 

 cules are written down on the right and the sum is compared with 

 the total Si02 of the rock. If there is an excess it is calculated 

 as quartz and the other mineral molecules are calculated directly 

 from their molecular amounts by the use of the regular tables. 

 If there is a deficit in Si02, the Si02 of the tentative hypersthene 

 is subtracted from the sum total of used Si02, the balance sub- 

 tracted from total Si02, and hypersthene and olivine formed by 



A. Nephelinite, Vosges, France. A. Michel-Levy, C. R., CXLVIII, p. 1530, 

 1909. Aa = mol numbers. 



B. Nephelinite, Etinde, Kamerun. E. Esch, Sb. Pr. Ak. W., 1901, p. 415. 

 Ba = mol numbers. 



