Omphacite is an clinopyroxene with chemical composition intermediate within the diopside - jadeite solid solution. Its nominal composition corresponds to the 50:50 term [i.e. (Ca0.5Na0.5)(Mg0.5Al0.5)Si2O6]. An ordered omphacite has a P2/n space group and its structural setting includes two tetrahedral sites (T1 and T2) which accommodate Si, two octahedral sites (M1 and M11) that host Mg and Al, and two eight-fold coordinated sites (M2 and M21) over which Ca and Na distribute. Omphacite is one of the main mineralogical species in high pressure rocks as eclogites and thus plays an important role in petrological processes taking place at deep environments. The elastic behaviour of (Ca0.5Na0.5)(Mg0.5Al0.5)Si2O6 omphacite has been investigated so far only for an ordered sample [1] using synchrotron X-ray powder diffraction method. In such study the authors used a Birch-Murnaghan equation of state to fit the pressure volume data obtaining the following coefficients: K0=117(3) and K´0=6.0(6). Concerning the structure evolution of omphacite as a function of pressure no data are so far available. In this work we have determined the pressure volume equation of state and performed a high-pressure structural study on a natural omphacite sample (from the Münchberg Mass eclogites, Germany, [2]) with composition (Ca0.49Na0.51)(Al0.54Mg0.39Fe2+0.07)Si2O6. The sample was characterized at room-P by means of single-crystal X-ray diffraction in order to determine the order degree of Al/Mg and Ca/Na in the M1/M11 and M2/M21 sites. We obtained the following order parameters QM1 = 0.900 and QM2 = 0.440, in agreement with [2], indicating a strong ordering. The evolution with P at room temperature was investigated using a diamond anvil cell (the lattice parameters were measured at 13 different pressures for EoS determination and 5 complete intensity data collections for structural refinement) up to ca. 7.5 GPa. Fitting the pressure volume data by a third-order Birch-Murnaghan EoS and refining simultaneously V0, K0 and its first pressure derivative K´ we obtained the following coefficients: V0 = 421.42(4) Å3, K0 = 122(1) GPa and K´=5.1(3). The structural evolution with P shows that the volumes of M1 and M11 polyhedra decrease by 5.1 and 5.5% respectively, and those of M2 and M21 decrease by 6.7 and 7.0%, respectively. The tetrahedral volumes, as expected for clinopyroxenes, do not show significant variations throughout the pressure range investigated. The single chain of tetrahedra shrinks with pressure, as expressed by the variation of the kinking angle (O32-O31-O32), which changes from 168.52 to 166.09°. The two tetrahedral sites become less tilted with P, with a reduction of their TILT angles from 4.14 to 3.14° and from 2.37 to 1.30°, for T1 and T2, respectively. References. [1] Pavese A., Diella V., Levy D., Hanfland M. (2001) Phys Chem Minerals, 28, 9-16 [2] Boffa Ballaran T., Carpenter M.A., Domeneghetti M.C., Tazzoli V. (1998) Am Min, 83, 419-433

Equation of state and structural evolution of a natural ordered omphacite.

CAMARA ARTIGAS, Fernando;
2010-01-01

Abstract

Omphacite is an clinopyroxene with chemical composition intermediate within the diopside - jadeite solid solution. Its nominal composition corresponds to the 50:50 term [i.e. (Ca0.5Na0.5)(Mg0.5Al0.5)Si2O6]. An ordered omphacite has a P2/n space group and its structural setting includes two tetrahedral sites (T1 and T2) which accommodate Si, two octahedral sites (M1 and M11) that host Mg and Al, and two eight-fold coordinated sites (M2 and M21) over which Ca and Na distribute. Omphacite is one of the main mineralogical species in high pressure rocks as eclogites and thus plays an important role in petrological processes taking place at deep environments. The elastic behaviour of (Ca0.5Na0.5)(Mg0.5Al0.5)Si2O6 omphacite has been investigated so far only for an ordered sample [1] using synchrotron X-ray powder diffraction method. In such study the authors used a Birch-Murnaghan equation of state to fit the pressure volume data obtaining the following coefficients: K0=117(3) and K´0=6.0(6). Concerning the structure evolution of omphacite as a function of pressure no data are so far available. In this work we have determined the pressure volume equation of state and performed a high-pressure structural study on a natural omphacite sample (from the Münchberg Mass eclogites, Germany, [2]) with composition (Ca0.49Na0.51)(Al0.54Mg0.39Fe2+0.07)Si2O6. The sample was characterized at room-P by means of single-crystal X-ray diffraction in order to determine the order degree of Al/Mg and Ca/Na in the M1/M11 and M2/M21 sites. We obtained the following order parameters QM1 = 0.900 and QM2 = 0.440, in agreement with [2], indicating a strong ordering. The evolution with P at room temperature was investigated using a diamond anvil cell (the lattice parameters were measured at 13 different pressures for EoS determination and 5 complete intensity data collections for structural refinement) up to ca. 7.5 GPa. Fitting the pressure volume data by a third-order Birch-Murnaghan EoS and refining simultaneously V0, K0 and its first pressure derivative K´ we obtained the following coefficients: V0 = 421.42(4) Å3, K0 = 122(1) GPa and K´=5.1(3). The structural evolution with P shows that the volumes of M1 and M11 polyhedra decrease by 5.1 and 5.5% respectively, and those of M2 and M21 decrease by 6.7 and 7.0%, respectively. The tetrahedral volumes, as expected for clinopyroxenes, do not show significant variations throughout the pressure range investigated. The single chain of tetrahedra shrinks with pressure, as expressed by the variation of the kinking angle (O32-O31-O32), which changes from 168.52 to 166.09°. The two tetrahedral sites become less tilted with P, with a reduction of their TILT angles from 4.14 to 3.14° and from 2.37 to 1.30°, for T1 and T2, respectively. References. [1] Pavese A., Diella V., Levy D., Hanfland M. (2001) Phys Chem Minerals, 28, 9-16 [2] Boffa Ballaran T., Carpenter M.A., Domeneghetti M.C., Tazzoli V. (1998) Am Min, 83, 419-433
2010
89° SIMP meeting “L'evoluzione del Sistema Terra dagli atomi ai vulcani”
Ferrara
13-15 settembre
“L'evoluzione del Sistema Terra dagli atomi ai vulcani”
Società Italiana di Mineralogia e Petrologia
52
52
http://www.socminpet.it/SIMP2010/home.htm
omphacite; X-ray diffraction; high-pressure; equation of state
Pandolfo F; Nestola F; Cámara F; Domeneghetti M.C
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2318/84016
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