Abstract: Subsolidus diopside compositions and coexisting diopside and liquid compositions which are also in equilibrium with anorthite and forsterite in the system CaO-MgO-Al₂O₃-SiO₂ were determined by X-ray diffraction and by electron microprobe in samples equilibrated at 1 bar and at 7 kbar. Along with previous data from the literature and using a recently published grid relating diffraction peaks to composition, results from both techniques are satisfactorily reconciled. At 1 bar, diopside composition (moles, Di = CaMgSi₂O₆, En = Mg₂Si₂O₆, CaTs = CaAl₂Si₂O₆) in equilibrium with anorthite, forsterite, and spinel (analogous to alkali basalts) are close to Di₇₉CaTs₂₀En₁ and those in equilibrium with anorthite, forsterite, and pigeonite (analogous to tholeiitic basalt) are close to Di₇₅CaTs₃En₂₂. At 7 kbar the equivalent compositions are Di₆₉CaTs₂₈En₃ and Di₆₉CaTs₂₆En₅ respectively.

In the system CaO-Na₂O-MgO-Al₂O₃-SiO₂, electron microprobe analyses of augites and liquids at 1 bar confirm the changes expected in the loci of liquids and show that the low Na₂O (< 0.50 wt.%) augites are similar to those in CaO-MgO-Al₂O₃-SiO₂. At 7 kbar the orthopyroxene field has expanded sufficiently and augite was not encountered in the limited range of samples studied.

In remelted rocks the augite compositions at 1 bar are similar to the compositions of augites in low pressure effusive rocks. At 2–15 kbar, the available data in the literature for dry basalts show that experimentally recrystallized augites are very sub-calcic and very aluminous, and no natural equivalents exist in plutonic gabbros or in ophiolitic rocks. This discrepancy is not resolved but either the experimental pyroxenes are metastable or the pyroxenes presently in the rocks are not the pristine compositions.