Pyroxene analyses of superior quality, from Dsdp Legs 11, 15, 17, 34, and 37, have been used to identify the various basalt groups from pyroxene chemistry. Cluster analysis, factor analysis, and variation plots give insight into the crystal chemistry of the pyroxenes. The tholeiitic and alkalic pyroxene groups differ primarily in Cr2O3, TiO2, and Na2O contents. Tholeiitic pyroxenes are high in Cr2O3 and low in Na2O and TiO2, relative to an average pyroxene. For the same oxides, pyroxenes from alkalic basalts are high in TiO2 and Na2O, and low in Cr2O3. Two pairs of cations, Ti–IVAl and Fe3+IVAl, have high correlation coefficients in all of the groups. It appears that common substitutions which occur in deep-sea pyroxenes are: Al for Si in a tetrahedral site combined with Ti or Fe3+ substituting for Mg or Fe2+ (respectively) in the octahedral M1 site.

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