CLA SSIFICA TION OF IGNEO US RO CKS 653 



them, the molecular weights of the minerals which have fixed 

 molecules, as orthoclase, anorthite, magnetite, etc., are multiplied 

 by the number of molecules of the unit oxide in each case to 

 arrive at the percentage weight. 



For general purposes very small amounts of the component 

 oxides may be neglected. But for close work it is necessary to 

 take into account even small percentages of P 2 O g ,Ti0 2 , SO s , 

 etc., that is, where any one of them amounts to two units in the 

 scale of molecular proportions ; when P 2 O g = 0.28, TiO g = 0.16, 

 S0 3 = 0.16, CO s = 0.08, CI = 0.07, F = 0.04 of a per cent. 



The introduction of these into the calculation is important 

 in proportion as the amounts of the bases with which they com- 

 bine are small in the rock. 



A check on the results is furnished by the agreement of the 

 sum of the calculated mineral components and the components 

 not included in the calculation with the sum total of the analy- 

 sis. This check cannot be absolutely exact because of errors 

 in the determination of the last decimal figure in the calcula- 

 tion of each component throughout the process. 



The rocks chosen to illustrate the method represent several 

 cases. That of the toscanose (granodiorite), Table I, is the sim- 

 plest case. A1 2 3 and Si0 2 , being in excess, yield normative 

 corundum and quartz; the femic silicate being hypersthene. 



The hessose (amphibole-gabbro), Table II, illustrates the 

 method of adjusting Si0 2 between hypersthene and olivine. 



The nordmarkose (litchfieldite), Table III, illustrates the 

 method of distributing Si0 2 between albite and nephelite after 

 reckoning the femic silicate as olivine. All the Na 2 is first 

 allotted to A1 2 3 , and Si0 2 is allotted to orthoclase, anorthite, 

 and olivine ; the remainder, 0.680 mol., is distributed according 

 to the formulae. 



In the case of the laurdalose (laurdalite), Table IV, insuffi- 

 cient A1 2 3 necessitates the formation of acmite molecules. 

 After allotting Si0 2 to orthoclase, acmite, diopside, and olivine, 

 the remainder, 0.522 mol., is distributed between albite and 

 nephelite. 



