High temperature structural and thermoelastic behaviour of mantle orthopyroxene: an in situ neutron powder diffraction study

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• 作者：G. Diego Gatta ; Romano Rinaldi ; K. S. Knight ; G. Molin and G. Artioli
• 关键词：Orthopyroxene ; High temperature ; Neutron powder diffraction ; Structural refinement ; Thermoelastic behaviour ; Cation ; ordering
• 刊名：Physics and Chemistry of Minerals
• 出版年：2007
• 出版时间：April, 2007
• 年：2007
• 卷：34
• 期：3
• 页码：185-200
• 全文大小：435 KB
• 刊物类别：Earth and Environmental Science
• 刊物主题：Earth sciences
  Mineralogy
  Crystallography
  Geochemistry
  Mineral Resources
  
• 出版者：Springer Berlin / Heidelberg
• ISSN：1432-2021

文摘

The temperature induced structural evolution and thermoelastic behaviour of a natural (Pbca) orthopyroxene (Opx), with chemical formula M2(Mg_0.856Ca_0.025Fe2+ _0.119) M1(Mg_0.957Fe2+ _0.011Fe3+ _0.016Cr_0.011Al_0.005)Al_0.032Si_1.968O₆, 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 1,200°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, α_j = l _j?1(?l _j/?T)_P0, increase non-linearly with T. The principal Lagrangian unit-strain coefficients (?//a, ?//b, ?//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 = 1,080°C is maintained down to the lowest temperature (150°C). The two non-equivalent tetrahedral chains, TA _n OA_3n and TB _n OB_3n , are kinked differently. At room-T, the TB _n OB_3n chain is more strongly kinked by about 23° than the TA _n OA_3n chain. With increasing T, the difference decreases by 3° for the TB _n OB_3n chain. The intersite cation exchange reaction between M1 and M2 (Mg2+ and Fe2+) shows a slight residual order at 1,200°C followed by reordering with decreasing temperature although seemingly not with a definite progressive trend. At the lowest temperature reached (150°C), reordering has occurred with the same value of partitioning coefficient K _D as that before heating. The absence of the expected phase transition is most likely due to the presence of minor amounts of Fe3+, Al, Ca and Cr which must play a crucial role on the thermoelastic behaviour and phase stability fields in natural Opx, with consequent important petrologic and geological implications.