The role of aluminum in igneous clinopyroxenes with relation to their parentage

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Abstract
A study of the distribution of Al in soda-poor pyroxenes in the range Ca (super 35-55) MgFe (sub 0-18) is occurring in igneous rocks reveals characteristics which differ in the various magma types. Kushiro has already shown that the more alkaline the magma-type, the more aluminous the clinopyroxene. It is further shown here for groundmass pyroxenes of the above range that the proportion of the Z group filled by Al atoms (Al Z ) decreases with differentiation in tholeiitic and related rocks but increases with differentiation in alkaline rocks, and that the percentage of Al atoms in tetrahedral coordination (Al IV x 100/Al IV + Al VI ), tends to vary inversely with the silica concentration of the host magma. Under solid-solid equilibrium conditions, increased load pressure favors the entry of Al into octahedral rather than tetrahedral sites in the clinopyroxene.

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