The only available HP data on P21/c pyroxene, concern so far synthetic samples of clinoenstatite, Mg2Si2O6 clinoferrosilite, Fe2Si2O6, and of a sample with composition Ca0.15Mg1.85Si2O6[1,2,3]. Although these studies have concluded that composition affects the pressure of transformation (Pc), the mechanism remains to be fully characterized. Moreover, there are as yet no high pressure data on natural pigeonite samples. In this study a sample of natural pigeonite from the Paranà ryodacite with composition M2(Ca0.20Mg0.15Fe0.65)M1(Mg0.80Fe0.20)Si2O6 and free of exsolved augite was studied by means of single-crystal X-ray diffraction using a diamond anvil cell at 13 different pressure values up to about 8 GPa and room temperature. In the P21/c LPphase the unit-cell parameters showed a non-linear decrease almost isotropic linear axial compressibility with a ratio of _a:_b:_c = 1.0:1.3:1.0, while the unit-cell parameters of the C2/c HP-phase decreased almost linearly and with an isotropic linear axial compressibility (_a:_b:_c =1.02:1.26:1.00). Between 3.3 and 3.4 GPa the variation in the unit-cell volume showed a strong discontinuity (about 2%) typical of a first order behavior phase transformation. A small hysteresis (_0.2 GPa) was observed. Diffraction data clearly indicate that the transition occurs from P21/c to C2/c symmetry as expected for this composition. By fitting the pressure-volume data for the P21/c phase using a third-order Birch- Murnaghan equation of state (EoS) we could refine simultaneously its unit-cell volume V0, the bulk modulus KT0 and its first pressure derivative, K’, obtaining the following EoS coefficients: V0 = 431.9(2)Å3, KT0 = 96.8(8) GPa and K’= 8.5(6). For the C2/c phase KT0, calculated fixing the K’ to 4, was significantly higher than that of the P21/c phase, with values of V0= 423.64(11) and KT0= 111.9(8) GPa. We used the fitted EoS parameters (a0, b0, c0, V0 and csin_0) of the HP-phase to calculate the spontaneous strain due to the phase transition to the LP-phase. The evolution of every single component of the strain tensor with pressure compares well with the observed evolution for the same phase transition in ZnSiO3 [4]. Scalar spontaneous strain ("s) scales almost linearly with volume strain (Vs) as "s= 0.169(16) - 5.8(9) •Vs. Crystal structure evolution with pressure in the low-symmetry phase (P21/c) indicated that M1-O and M2-O mean bond lengths decreased as 0.006 Å•GPa−1 and 0.013 Å•GPa−1 respectively, with no significant change in tetrahedral T-O bond lengths. For the high-symmetry phase (C2/c) the M1-O and M2-O mean bond lengths showed a decrease of 0.006 Å•GPa−1 and 0.008 Å•GPa−1 respectively. A shrinking of 2.2 and 5.1% were observed in the two non-equivalent A and B tetrahedra chains respectively. At the transition, the A-chain changed from S-rotated to O-rotated reaching a value of 142.4° for the single chain of the HP-phase. References [1] Hugh-Jones D. A. et al. (1994) Am. Mineral. 79, 405-410 [2] Hugh-Jones D. A. et al. (1994) Am. Mineral. 79, 1032-1041 [3] Nestola F. et al. (2004) Am. Mineral. 89, 189–196 [4] Arlt, T., and Angel, R.J. (2000) Phys. Chem. Minerals, 27(10), 719-731.
HP-phase transition of natural P21/c pigeonite
CAMARA ARTIGAS, Fernando;
2008-01-01
Abstract
The only available HP data on P21/c pyroxene, concern so far synthetic samples of clinoenstatite, Mg2Si2O6 clinoferrosilite, Fe2Si2O6, and of a sample with composition Ca0.15Mg1.85Si2O6[1,2,3]. Although these studies have concluded that composition affects the pressure of transformation (Pc), the mechanism remains to be fully characterized. Moreover, there are as yet no high pressure data on natural pigeonite samples. In this study a sample of natural pigeonite from the Paranà ryodacite with composition M2(Ca0.20Mg0.15Fe0.65)M1(Mg0.80Fe0.20)Si2O6 and free of exsolved augite was studied by means of single-crystal X-ray diffraction using a diamond anvil cell at 13 different pressure values up to about 8 GPa and room temperature. In the P21/c LPphase the unit-cell parameters showed a non-linear decrease almost isotropic linear axial compressibility with a ratio of _a:_b:_c = 1.0:1.3:1.0, while the unit-cell parameters of the C2/c HP-phase decreased almost linearly and with an isotropic linear axial compressibility (_a:_b:_c =1.02:1.26:1.00). Between 3.3 and 3.4 GPa the variation in the unit-cell volume showed a strong discontinuity (about 2%) typical of a first order behavior phase transformation. A small hysteresis (_0.2 GPa) was observed. Diffraction data clearly indicate that the transition occurs from P21/c to C2/c symmetry as expected for this composition. By fitting the pressure-volume data for the P21/c phase using a third-order Birch- Murnaghan equation of state (EoS) we could refine simultaneously its unit-cell volume V0, the bulk modulus KT0 and its first pressure derivative, K’, obtaining the following EoS coefficients: V0 = 431.9(2)Å3, KT0 = 96.8(8) GPa and K’= 8.5(6). For the C2/c phase KT0, calculated fixing the K’ to 4, was significantly higher than that of the P21/c phase, with values of V0= 423.64(11) and KT0= 111.9(8) GPa. We used the fitted EoS parameters (a0, b0, c0, V0 and csin_0) of the HP-phase to calculate the spontaneous strain due to the phase transition to the LP-phase. The evolution of every single component of the strain tensor with pressure compares well with the observed evolution for the same phase transition in ZnSiO3 [4]. Scalar spontaneous strain ("s) scales almost linearly with volume strain (Vs) as "s= 0.169(16) - 5.8(9) •Vs. Crystal structure evolution with pressure in the low-symmetry phase (P21/c) indicated that M1-O and M2-O mean bond lengths decreased as 0.006 Å•GPa−1 and 0.013 Å•GPa−1 respectively, with no significant change in tetrahedral T-O bond lengths. For the high-symmetry phase (C2/c) the M1-O and M2-O mean bond lengths showed a decrease of 0.006 Å•GPa−1 and 0.008 Å•GPa−1 respectively. A shrinking of 2.2 and 5.1% were observed in the two non-equivalent A and B tetrahedra chains respectively. At the transition, the A-chain changed from S-rotated to O-rotated reaching a value of 142.4° for the single chain of the HP-phase. References [1] Hugh-Jones D. A. et al. (1994) Am. Mineral. 79, 405-410 [2] Hugh-Jones D. A. et al. (1994) Am. Mineral. 79, 1032-1041 [3] Nestola F. et al. (2004) Am. Mineral. 89, 189–196 [4] Arlt, T., and Angel, R.J. (2000) Phys. Chem. Minerals, 27(10), 719-731.
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