Abstract

The tetrahedron silica-nepheline-diop-side-olivine is used for chemical definition of alkali basalts and tholeiites in conformity with their petrographic definition. In an earlier paper, the plane separating the two types of basalt was found and an equation given for this plane. A second equation has now been derived which allows more rapid classification of olivine-normative basaltic rocks directly from their norms. This equation is  
formula
or, by nearest approximation,  
formula
For alkali basalts, the difference (A′ = Ab – 2 Enhy – 11/2 Ofhy) is positive; for olivine tholeiites it is negative. Basaltic rocks without normative hypersthene are alkali-basaltic, whereas those with normative quartz are tholeiitic.

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