The structure of Pbca orthopyroxenes along the join diopside-enstatite
The structure of Pbca orthopyroxenes along the join diopside-enstatite (in Special issue dedicated to the memory of Luciano Ungaretti, Maria Chiara Domeneghetti (prefacer))
European Journal of Mineralogy (April 2003) 15 (2): 365-371
The crystal of two synthetic orthopyroxenes, with compositions Ca (sub 0.04) Mg (sub 1.96) Si (sub 2) O (sub 6) and Ca (sub 0.07) Mg (sub 1.93) Si (sub 2) O (sub 6) , has been refined to R 2.7 and 3.6% respectively. TEM shows the presence of irregularly repeated P2 (sub 1) /c clinopyroxene lamellae (100), in agreement with the presence of reflections with 1 = 2n violating the b glide plane. Significant electron density residuals were found close to the M2 and O2B sites, allowing split site refinement for the Ca (sub 0.07) Mg (sub 1.93) Si (sub 2) O (sub 6) sample. M2-O bond lengths for average and local configurations for Ca and Mg are very similar to those of clinopyroxenes along the diopside-enstatite join. The increasing presence, due to the substitution of Ca for Mg in the M2 site of configurations in orthopyroxenes that are similar to the average ones in Ca-richer C2/c clinopyroxenes reduces the stability of the orthopyroxene phase.
