An orthopyroxene with composition (Li0.5Zn0.5)(Sc0.5Zn0.5)Si2O6 and space group Pbca was investigated by in situ high-pressure single-crystal X-ray diffraction using a diamond anvil cell in order to evaluate the effect of trivalent and monovalent cations on the elasticity of the orthopyroxene phase. Preliminary data (measured up to 7.12 GPa at 7 different pressures) showed that no phase transition occurs throughout the pressure range investigated and that such Li-bearing orthopyroxene results to be the softest among the Pbca pyroxenes studied so far. The pressure – volume data fitted using a 3rd-order Birch-Murnaghan equation of state gave a bulk modulus KT0 = 79(2) GPa with the first pressure derivative K’ = 8(1). The evolution of the unit-cell volume with pressure is shown in Figure A (upper left) while the evolution of the single unit-cell parameters with pressure is shown in Figure B (upper right) indicating a significant anisotropic deformation with a being the most rigid direction and b the softest one. The evolution of the crystal structure with pressure indicates that the M2 polyhedron is the most deformed with a volume decrease by about 13% compared to that of about 5% for M1 polyhedron. (Figure C, lower left). For such calculation we assumed a 6-fold coordination for the M2 polyhedron: the O3B oxygen positioned at 3.014Å from M2 site at room pressure enters in coordination between 6 and 7 GPa with a distance reduced to 2.528Å at 7.12 GPa. The TA tetrahedron is practically uncompressible whereas TB undergoes a limited deformation compared to that of M2 and M1 polyhedra (-1.8%). The strong deformation of M2 is accompanied by a strong kinking (O3-O3-O3 angle) of the B tetrahedral chain with a contraction rate of 1.97°/GPa against an extension rate of 0.68°/GPa for the A tetrahedral chain. An analysis of the individual M2-O distances clearly shows that the main deformation for the M2 site is represented by the strong reduction of M2-O3B (-19%) and M2-O3A (-7%) distances (Figure D, lower right) whereas the other M2–O distances as well as M1–O distances show only minor deformations.
High-pressure behavior of a Li-bearing orthopyroxene
CAMARA ARTIGAS, Fernando;
2008-01-01
Abstract
An orthopyroxene with composition (Li0.5Zn0.5)(Sc0.5Zn0.5)Si2O6 and space group Pbca was investigated by in situ high-pressure single-crystal X-ray diffraction using a diamond anvil cell in order to evaluate the effect of trivalent and monovalent cations on the elasticity of the orthopyroxene phase. Preliminary data (measured up to 7.12 GPa at 7 different pressures) showed that no phase transition occurs throughout the pressure range investigated and that such Li-bearing orthopyroxene results to be the softest among the Pbca pyroxenes studied so far. The pressure – volume data fitted using a 3rd-order Birch-Murnaghan equation of state gave a bulk modulus KT0 = 79(2) GPa with the first pressure derivative K’ = 8(1). The evolution of the unit-cell volume with pressure is shown in Figure A (upper left) while the evolution of the single unit-cell parameters with pressure is shown in Figure B (upper right) indicating a significant anisotropic deformation with a being the most rigid direction and b the softest one. The evolution of the crystal structure with pressure indicates that the M2 polyhedron is the most deformed with a volume decrease by about 13% compared to that of about 5% for M1 polyhedron. (Figure C, lower left). For such calculation we assumed a 6-fold coordination for the M2 polyhedron: the O3B oxygen positioned at 3.014Å from M2 site at room pressure enters in coordination between 6 and 7 GPa with a distance reduced to 2.528Å at 7.12 GPa. The TA tetrahedron is practically uncompressible whereas TB undergoes a limited deformation compared to that of M2 and M1 polyhedra (-1.8%). The strong deformation of M2 is accompanied by a strong kinking (O3-O3-O3 angle) of the B tetrahedral chain with a contraction rate of 1.97°/GPa against an extension rate of 0.68°/GPa for the A tetrahedral chain. An analysis of the individual M2-O distances clearly shows that the main deformation for the M2 site is represented by the strong reduction of M2-O3B (-19%) and M2-O3A (-7%) distances (Figure D, lower right) whereas the other M2–O distances as well as M1–O distances show only minor deformations.
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