β-diopside, a new ultrahigh-pressure polymorph of CaMgSi ₂O₆ with six-coordinated silicon

Minerals containing silicon in four-fold coordination (^IVSi ⁴⁺) are common in crustal rocks, while those involving six-coordinated silicon (^VISi⁴⁺) dominate the Earth's lower mantle and determine its properties. Here we show a new type of phase transition determined by single-crystal high pressure X-ray diffraction experiments in a diamond anvil cell (DAC) using natural diopside (CaMgSi ₂O₆), the archetypic member of clinopyroxene family, and one of the most abundant minerals of the Earth's upper mantle. Above 50GPa at ambient temperature diopside transforms to a previously unknown post-clinopyroxene phase, β-diopside, with half of the tetrahedral ^IVSi⁴⁺ layers converted to octahedral ^VISi ⁴⁺coordination. This phase is most probably a metastable state that is kinetically accessible at room temperature and the transformation is fully reversible on decompression. This new type of phase transition provides important clues to the exact mechanisms of breakdown of clinopyroxene in the Earth's mantle and may be expected to take place in other pyroxenes at pressures higher than previously explored.