X-ray diffraction and Raman spectroscopic study of clinopyroxenes with six-coordinated Si in the Na(Mg0.5Si0.5)Si2O6-NaAlSi2O6 system

Five clinopyroxenes containing various amounts of six-coordinated Si (VISi) in the Na(Mg0.5Si0.5) 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 distance, the mean tetrahedral 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.