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The single-chain silicates of the pyroxene group are major rock forming minerals in the Earth's crust and upper mantle. Clinopyroxenes with a primitive lattice, including clinoenstatite-clinoferrosilite (MgSiO₃-FeSiO₃), pigeonite (Mg,Fe,Ca)SiO₃ and kanoite MnMgSi₂O₆, exhibit a complicated polymorphism with pressure and temperature. At high pressure these clinopyroxenes with space group P2₁/c transform to C-centered symmetry (HP-C2/c) and phase transitions with the same change in space group occur at high temperatures (HT-C2/c). The major structural change at these transitions is a rotation of the chains of SiO₄ tetrahedra within the structure and the breaking (and re-making) of a small number of bonds between the bridging oxygen atom of these chains and the cation on the M2 site. Thus these transitions are mainly displacive in character, normally non-quenchable, and must be studied in-situ.

The results that we reported last year for kanoite (1998 Annual Report, Fig 3.3-4) suggest that although the HT and HP C2/c structures have the same general structural topology, they have distinct stability fields separated in P,T space by the stability field for the P2₁/c phase. However, no single experiment at either high temperature or at high pressure had been performed that spanned all three phase fields in a single traverse of pressure or temperature. Previous reports in the literature suggested that the C2/c clinopyroxene ZnSiO₃ might undergo both transitions within the pressure range easily accessible to single-crystal diamond-anvil cell experiments. We therefore synthesised crystals of ZnSiO₃, and measured the evolution of the unit-cell parameters with pressure. At 1.9 GPa weak but significant steps in the unit-cell parameters were observed. Reflections with indices h + k = odd, forbidden in space group C2/c but characteristic of a primitive lattice, appeared suddenly at the phase transition and grew in intensity on further increase in pressure. A structure refinement based on a full set of intensity data collected at 4.258 GPa, confirmed the space group as P2₁/c and the structure as a distorted but otherwise normal low-clinopyroxene. This intermediate P2₁/c phase further transforms at about 5.0 GPa, to a second C2/c phase (HP C2/c), again confirmed by data collection and refinement. The volume change at this transition is about 2.6%, three times larger than in the first phase transition, and typical of the P2₁/c -> HP-C2/c phase transitions found previously on MgSiO₃, FeSiO₃, MnMgSi₂O₆ etc. These results have therefore provided the first direct evidence that the HP-C2/c and the HT-C2/c structures are distinct polymorphs with the same space group.