Hugh-Jones, Demelza Alice; (1995) High pressure behaviour of pyroxenes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Pyroxenes make up almost 25% by volume of some mineralogical models of the Earth's upper mantle, with their transformation to the higher density garnet structure at ∼ 400 km contributing to the seismic discontinuity at this depth. At depths shallower than this, both orthorhombic and monoclinic structures of (Mg,Fe)SiO3 pyroxenes are stable. In order to try to understand their behaviour at depth, a single crystal X-ray diffraction study has been carried out on both synthetic and natural ortho- and clinopyroxenes along or near the (Mg,Fe)SiO3 join using a diamond anvil cell. Synthetic (Mg,Fe)SiO3 orthopyroxenes undergo a distinct change in compression mechanism at ∼ 4 GPa and ambient temperature, characterised by the onset of shortening of the previously incompressible Si-O bonds, with associated changes in the degrees of kinking and tilting of the silicate chains. This change in compression mechanism causes a discontinuous break in their equation of state (EOS). The addition of impurities such as Ca2+ to the orthopyroxene causes the M2 cation sites to become significantly stiffer, thus inhibiting any change in compression mechanism of the natural orthopyroxenes at any pressure, and also any break in their EOS'S. Both low-clinopyroxene (space group P21/c) and orthopyroxene (Pbca) transform to the higher pressure C2/c clinopyroxene structure at high pressures; both transitions are first order in character involving a ∼ 3 % volume discontinuity. However, whilst the low- to high-pressure clinopyroxene transition is displacive, involving a major reconfiguration of the silicate chains, the ortho- to high-pressure clinopyroxene transformation is highly reconstructive, with the newly-formed C2/c phase reverting preferentially to the P21/c polymorph upon pressure release. This high-pressure C2/c structure differs from that of the C2/c phase stable at high temperatures predominantly in the degrees of kinking of their tetrahedral chains. The possibility of a "cross-over" transition between these structurally distinct C2/c phases at extreme conditions is discussed. Changing the composition of a pyroxene from MgSiO3 to FeSiO3 has very little effect on the topology of the equilibrium phase diagram, although the triple point and phase boundaries are shifted to lower pressures with increasing iron content.

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