The crystal structure of two synthetic orthopyroxenes, with compositions Ca0.04Mg1.96Si2O6 and Ca0.07Mg1.93Si2O6 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 P21/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 Ca0.07Mg1.93Si2O6. 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 C2/c clinopyroxene reduces the stability of the orthopyroxene phase.