In-situ high-temperature thermo-elastic behavior and structural response of a mantle orthopyroxene by neutron powder diffraction up to 1200°C

Orthopyroxene (Opx) (Mg,Fe)2Si2O6, one of the most common rock-forming minerals of the lithosphere and a fundamental constituent of the Earth's upper mantle, has Pbca symmetry and its structure consists of alternating tetrahedral and octahedral layers. The tetrahedral layer has two non-equivalent tetrahedra TA and TB, TA being smaller and more distorted than TB; the layer consists of tetrahedral single chains parallel to c. The octahedral layer consists of zig-zag chains of smaller and nearly regular M1 octahedra alternately joined with larger and more distorted M2 octahedra also running parallel to c. Si-Al substitution occurs only in TB tetrahedra. Mg2+ and Fe2+ occupy the M1 and M2 octahedral sites respectively and with a strong ordering preference. At high temperature, orthopyroxene has been reported to transform reconstructively into different phases depending on chemical composition. Enstatite, Mg2Si2O6, transforms into protoenstatite with the space group Pbcn at about 1030 °C, whereas ferrosilite, Fe2Si2O6, transforms reversibly into clinoferrosilite with the space group C2/c at the same temperature. Intermediate orthopyroxenes with Fe/(Fe+Mg)>13% also show transformations to C2/c clinopyroxenes, the transition being irreversible and non-topotactic, with transition temperatures decreasing from approx. 1230 to 980°C with increasing Fe content. At temperatures above 1100°C, orthopyroxene becomes unstable and transforms into a protopyroxene with the Pbcn structure. This phase is unquenchable. In order to give new insights into the crystal structure of the protopyroxene phase, the temperature induced structural response and thermoelastic behaviour of a natural (Pbca) orthopyroxene (Opx), with chemical formula M2(Mg0.856Ca0.025Fe2+0.119) M1(Mg0.957Fe2+0.011Fe3+0.016Cr0.011Al0.005) Al0.032Si1.968O6, from a suite of high pressure ultramafic nodules of mantle origin, have been investigated by in-situ neutron powder diffraction at several temperatures starting from 1200°C down to 150°C. Unit-cell parameter variations as a function of T show no phase transition within this temperature range. The volume thermal expansion coefficient, α=V -1(V/T)P0, varies linearly with T. The axial thermal expansion coefficients, αj=lj-1(lj/T)P0, increase non-linearly with T. The principal Lagrangian unit-strain coefficients (ε//a, ε//b, ε//c), increase continuously with T. However, the orientation of the unit-strain ellipsoid appears to change with T. With decreasing T, the values of the unit-strain coefficients along the b and c axes tend to converge. The orientation at ΔT=1080°C is maintained down to the lowest temperature (150°C). The two non-equivalent tetrahedral chains, TAnOA3n and TBnOB3n, are kinked differently. At room-T, the TBnOB3n chain is more strongly kinked by about 23° than the TAnOA3n chain. With increasing T, the difference decreases by 3° for the TBnOB3n chain. The intersite cation exchange reaction between M1 and M2 (Mg2+ and Fe2+) shows a slight residual order at 1200°C followed by reordering with decreasing temperature although seemingly not with a definite progressive trend. At the lowest temperature reached (150°C), the same value of partitioning coefficient KD as that found before heating is also reached. The experimental results obtained in this study indicate that the isomorphic substitution of small amounts of Ca, Fe3+, Al and Cr may play a crucial role on the thermoelastic behaviour and phase-stability fields of orthopyroxenes with consequent important petrologic and geological implications, in particular on the thermodynamic modelling of the Earth mantle processes. A full report of the present investigation has been accepted for publication in Physics and Chemistry of Minerals.