X-ray diffraction and Raman spectroscopic study of clinopyroxenes with six-coordinated Si in the Na(Mg_0.5Si_0.5)Si₂O₆-NaAlSi₂O₆ system

Five clinopyroxenes containing various amounts of six-coordinated Si (^VISi) in the Na(Mg_0.5Si_0.5)Si₂O₆ (NaPx)-NaAlSi₂O₆ (jadeite) system have been synthesized at 15 GPa and 1600 °C and their structures studied with single-crystal X-ray diffraction and Raman spectroscopy. The results show that clinopyroxenes with ^VISi ≤ 0.33 atoms per formula unit (apfu) possess C2/c symmetry, whereas those with ^VISi ≥ 0.45 apfu crystallize with P2/n symmetry. There is an obvious discontinuity in the unit-cell parameters a, β, and V as the ^VISi content increases from 0.33 to 0.45 apfu, suggesting that the C2/c-P2/n transition is first-order in character, rather than tricritical as reported for the transition from jadeite/diopside/augite to omphacite. The Mg and ^VISi cations in the P2/n structure are completely ordered into two nonequivalent octahedral sites, M1 and M1(1), respectively, with M1 being appreciably more distorted than M1(1). With increasing mean <M1-O> distance, the mean tetrahedral <Si-O> distance increases, whereas the O3-O3-O3 angle of the tetrahedral chain decreases systematically, consistent with the structural variation trends found in the jadeite-diopside system. A comparison of the Raman spectra reveals that the C2/c-P2/n transition is characterized by the splitting of many Raman bands in C2/c clinopyroxenes into doublets in P2/n, and such splitting becomes more pronounced with increasing ^VISi. For C2/c clinopyroxenes, all Raman bands become progressively broader with the increased substitution of (Si⁴⁺ + Mg²⁺) for Al³⁺. In addition, several new Raman bands, attributable to the presence of ^VISi, are observed. Together with previous data, we suggest that the maximum ^VISi content allowed for an Mg/Al dominated octahedral site is close to ~35%, above which ^VISi and Mg/Al are likely to be ordered into distinct sites.