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  
or, by nearest approximation,  
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.

First Page Preview

First page PDF preview
You do not currently have access to this article.