Abstract
The crystal structure of two synthetic orthopyroxenes, with compositions Ca 0.04 Mg 1.96 Si 2 O 6 and Ca 0.07 Mg 1.93 Si 2 O 6 has been refined by single-crystal X-ray diffraction [R(4σ) = 2.7 and 3.6 %, respectively]. TEM observation has shown the presence of irregularly repeated P 2 1 / c clinopyroxene lamellae, oriented parallel to (100), in agreement with the presence of reflections with 1 = 2n violating the b glide plane. Significant electron density residuals close to the M2 and O2B sites were found, allowing split site refinement for the sample with composition Ca 0.07 Mg 1.93 Si 2 O 6. M2-O bond lengths for average and local configurations for Ca and Mg have been found to be very similar to those of clinopyroxenes along the join diopside-enstatite. The increasing presence, due to the substitution of Ca for Mg in the M2 site, of Ca configurations in orthopyroxene that are similar to the average ones in Ca-richer C 2/ c clinopyroxene reduces the stability of the orthopyroxene phase.
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