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Five clinopyroxenes containing various amounts of six-coordinated Si (VISi) in the Na(Mg 0.5 Si 0.5 ) Si 2 O 6  (NaPx)-NaAlSi 2 O 6  (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  VI Si ≥ 0.45 apfu crystallize with P2/n symmetry. There is an obvious discontinuity in the unit-cell parameters a, β, and V as the  VI Si 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  VI Si 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 &lt;M1-O&gt; distance, the mean tetrahedral &lt;Si-O&gt; 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  VI Si. For C2/c clinopyroxenes, all Raman bands become progressively broader with the increased substitution of (Si 4+  + Mg 2+ ) for Al 3+ . In addition, several new Raman bands, attributable to the presence of  VI Si, are observed. Together with previous data, we suggest that the maximum  VI Si content allowed for an Mg/Al dominated octahedral site is close to ~35%, above which  VI Si and Mg/ Al are likely to be ordered into distinct sites.

X-ray diffraction and Raman spectroscopic study of clinopyroxenes with six-coordinated Si in the Na(Mg 0.5 Si 0.5 )Si 2 O 6 -NaAlSi 2 O 6 system