Intracrystalline cation distribution in natural clinopyroxenes of tholeiitic, transitional, and alkaline basaltic rocks.

It is possible to estimate Fe-Mg site occupancies on M1 and M2 sites in clinopyroxenes of intermediate composition through crystal structure refinements. There are distinct correlations between crystal-structure parameters and concentrations of trivalent and tetravalent cations in the M1 site and Si in the T site; thus it is possible to determine completely the M1 and M2 site occupancy by Al3+, Fe3+, Ti4+, Cr3+, Fe2+, Mg, Ca and Na. The composition-structural parametric relationships make it possible to predict the bulk composition and site occupancies in crystals that have not been chemically analysed. A new type of M2 site in intermediate clinopyroxenes (the M2' site) is a function of pyroxene chemistry and may have a bearing on the process of unmixing in calcic pyroxenes. Basaltic rocks may be distinguished by the crystallographic parameters of their pyroxenes.-R.A.H.