Calculation schemes of end-member proportions of pyroxenes presented previously haverequired the knowledge of Fe2O3 content. Most mineral analyses currently being presented are electron microprobe analyses, in which ferrous and ferric iron content are not determined separately.

An alternative scheme is suggested here which minimizes the significance of Fe2O3 contents and allows for a more rigorous comparison of end-member concentrations, especially between minerals where Fe2O3 content is determined and those where it is not available.

Examination of 344 clinopyroxene analyses with Fe2O3content quoted suggests that for certain parageneses the sum of cations based on 6 oxygens approaches the ideal value of 4.00.However, for sub-calcic and jadeitic pyroxenes, deviations from ideality become significant.Hence, calculation of ferric and ferrous iron proportions, on the assumption that the sum of cations equals 4.00, is not valid for these particular pyroxenes.

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