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Vittorio Tazzoli - One of the best experts on this subject based on the ideXlab platform.
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High-pressure displacive phase transition of a natural Mg-rich Pigeonite
Physics and Chemistry of Minerals, 2011Co-Authors: Matteo Alvaro, Fernando Camara, Fabrizio Nestola, M. Chiara Domeneghetti, Vittorio TazzoliAbstract:A high-pressure single-crystal X-ray diffraction study has been carried out on a P 2_1/ c natural Mg-rich Pigeonite sample with composition ca. Wo_6En_76Fs_18 using a diamond anvil-cell. The unit-cell parameters were determined at 14 different pressures to 7.14 GPa. The sudden disappearance of the b -type reflections ( h + k = odd) and a strong discontinuity (about 2.8%) in the unit-cell volume indicated a first-order P 2_1/ c – C 2/ c phase transition between 4.66 and 4.88 GPa. The P ( V ) data of the P 2_1/ c phase were fitted to 4.66 GPa by a third-order Birch–Murnaghan equation of state (BM3 EoS), whereas the limited number of experimental data collected within the C 2/ c phase between 4.88 and 7.14 GPa were fitted using the same equation of state but with K′ constrained to the value obtained for the P 2_1/ c fitting. The equation of state coefficients are V _0 = 424.66(6) Å^3, K _T0 = 104(2) GPa and K ′ = 8(1) for the P 2_1/ c phase, and V _0 = 423.6(1) Å^3, K _T0 = 112.4(8) GPa, and K ′ fixed to 8(1) for the C 2/ c phase. The axial moduli for a , b, and c for the P 2_1/ c phase were obtained using also a BM3-EoS, while for the C 2/ c phase only a linear calculation could be performed, and therefore the same approach was applied for comparison also to the P 2_1/ c phase. In general the C 2/ c phase exhibits axial compressibilities ( β _c > β _a >> β _b) lower than those of the P 2_1/ c phase ( β _b > β _c ≈ β _a; similar to those found in previous studies in clinopyroxenes and orthopyroxenes). The lower compressibility of the C 2/ c phase compared with that of the P 2_1/ c could be ascribed to the greater stiffness along the b direction. A previously published relationship between P _c and M2 average cation radius (i.r.) has been updated using all the literature data on P 2_1/ c clinopyroxene containing large cations at M2 site and our new data. The following weighted regression was obtained: P _c (GPa) = 26(4) − 28(5) × i.r (Å), R ^2 = 0.97. This improved equation can be used to predict the critical pressure of natural P 2_1/ c clinopyroxene samples just knowing the composition at M2 site.
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high pressure phase transition of a natural Pigeonite
American Mineralogist, 2010Co-Authors: Matteo Alvaro, Fernando Camara, Fabrizio Nestola, Tiziana Boffa Ballaran, Chiara M Domeneghetti, Vittorio TazzoliAbstract:High-pressure and room-temperature single-crystal X-ray diffraction (XRD) studies have been performed on crystals of a natural Pigeonite sample with composition ca. Wo 10 En 43 Fs 47 using diamond-anvil cells. The unit-cell parameters were determined at 18 different pressures up to about 6 GPa. A first-order P 2 1 / c-C 2/ c phase transition was found between 3.5 and 3.6 GPa, associated with the disappearance of the b -type reflections ( h + k = odd) and a strong discontinuity (about 1.7%) in the unit-cell volume. At the transition, a small hysteresis (~0.3 GPa) was observed. A third-order Birch-Murnaghan equation of state (BM3-EoS) fit to the 10 P - V data of the low- P phase yielded V 0 = 431.93(2) A 3 , K T 0 = 96.8(8) GPa and K ′ = 8.5(6). A second-order Birch-Murnaghan EoS fit to the 8 P - V data (between 3.6 and 6 GPa) of the C 2/ c high- P phase yielded V 0 = 423.6(1) A 3 and K T 0 = 112.4(8), indicating that the high- P C 2/ c phase is significantly stiffer than the low- P phase. In a separated experiment with crystals of the same sample, intensity data were collected and crystal structures were refined at 13 pressures up to 9.4 GPa. The M1-O and M2-O mean bond lengths of the low- P P 2 1 / c phase decrease by 0.7 and 2.1%, respectively. The two non-equivalent A and B tetrahedral chains become more kinked with pressure, with a reduction of their angle by 2.2 and 5.1%, respectively. At the transition the A-chain changes sense of rotation and both chains become equivalent and more kinked, with a further reduction of their angle by 2.5% up to 9.4 GPa. Strain calculations have been performed and the evolution of the spontaneous strain and the order parameter variation with pressure are discussed, considering geometrical parameters of the structure and comparing our results with the available data for other compositions.
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kinetics of fe2 mg order disorder in p21 c Pigeonite
American Mineralogist, 2005Co-Authors: Chiara M Domeneghetti, Michele Zema, Vittorio TazzoliAbstract:The kinetics of the Fe-Mg intracrystalline exchange reaction in P 2 1 / c Pigeonite (Wo 10 En 47 Fs 43 ) free of exsolved augite, from the Parana rhyodacite sample BTS308, was studied by single-crystal X-ray diffraction (XRD). Isothermal disordering annealing experiments, with oxygen fugacity controlled at the IW buffer, were performed on two crystals at 650, 700, 750, and 800 °C until the Fe-Mg exchange equilibrium was reached. The XRD data were collected from the two untreated crystals and after each annealing experiment. Structure refinements were carried out taking into account the recently discovered stronger preference of Mn for the M2 site compared to Fe 2+ . The linear regression of ln k D * vs. 1/ T yielded the following equation: \[ln\mathit{k}_{D}^{{\ast}}\ =\ {-}2925({\pm}110)/\mathit{T}(K)\ +\ 0.574\ ({\pm}0.111);\ (\mathit{R}^{2}\ =\ 0.997)\] The T c values calculated using this equation were 566 (±6) and 571 (±6) °C for the two crystals. Analysis of the kinetic data was performed according to Mueller’s model, which allowed retrieval of the disordering rate constants C 0 K dis + for all four temperatures. The Arrhenius relation: \[ln\mathit{K}_{dis}^{+}\ =\ ln\ \mathit{K}_{0}\ {-}\ \mathit{Q}/(R\mathit{T})\ =\ 20.45({\pm}1.91)\ {-}\ 25191\ ({\pm}1900)/\mathit{T(K)};\ (\mathit{R}^{2}\ =\ 0.989)\] yielded an activation energy of 50.03 (±3) kcal/mol for the Fe-Mg exchange process. Cooling time constants, calculated at the QMF buffer conditions of the host rock were, for the two crystals, η = 0.94 × 10 −1 K −1 year −1 and η = 1.10 × 10 −1 K −1 year −1 , and gave cooling rates on the order of 10 °C/h consistent with very fast lava cooling.
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coupling between non convergent ordering and transition temperature in the c2 c p21 c phase transition in Pigeonite
American Mineralogist, 2003Co-Authors: Fernando Camara, Michael A. Carpenter, Chiara M Domeneghetti, Vittorio TazzoliAbstract:A Landau potential with linear-quadratic coupling has been developed to describe interactions between a non-convergent order parameter, Q OD, for Fe/Mg ordering, and the order parameter, Q D, for the C 2/ c - P 21/ c phase transition in Pigeonite. Spontaneous strain relationships, and expressions for the effect of ordering on the transition temperature derived from this expansion, have been tested by single crystal X-ray diffraction methods. Lattice parameters collected from a natural Pigeonite crystal with composition En47Fs44Wo9, in situ at temperatures up to 1050 °C, reveal that increasing Q OD could act to suppress Q D by a mechanism which includes overlapping and opposing strain fields. In a second experiment, the intensities of superlattice reflections ( h + k = 2 n + 1) were followed in situ at temperatures up to 500 °C. The crystal was heated ex situ successively at 700, 750, 800, and 850 °C between repeated in situ measurements in order to produce changes in the degree of cation order. The resulting data sets, giving the temperature dependence of Q D2 for different fixed values of Q OD, are consistent with the initial Landau model. In particular, they show a strong and linear dependence of transition temperature on Q OD. The fourth order coefficient of the expansion describing the phase transition is perhaps also renormalized by changes in Q OD. It is suggested that the influence of Q OD on the phase transition could be greater than the influence of the phase transition on the equilibrium variation of Q OD.
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leverage analysis of x ray single crystal diffraction data from orthopyroxene and Pigeonite
European Journal of Mineralogy, 2002Co-Authors: Marcello Merli, Fernando Camara, Chiara Domeneghetti, Vittorio TazzoliAbstract:Leverage analysis, a statistical procedure for calculating the influence of each reflection on the estimate of each refined variable, was applied to the structure refinement of a Pbca orthopyroxene (Wo 1 En 48 Fs 51 ) and that of a P 2 1 / c Pigeonite (Wo 10 En 47 Fs 43 ). For both minerals this procedure was carried out on single-crystal X-ray diffraction data collected up to 55° and, for Pigeonite, also on data collected in the range 2–28° during in situ annealing at 750 and 850°C. Leverage analysis revealed the role of certain groups of reflections in constraining some crucial variables of the structure refinement, such as atomic positions, atomic displacement parameters and site occupancies. The problem of which reflections to choose to achieve reliable results in terms of atomic coordinates and site population, and thus of Fe/Mg ordering degree, is discussed. The treatment of data collected on Pigeonite during in situ annealing to induce P 2 1 / c ⟺ C 2/ c displacive phase transition is described in terms of leverage analysis. Some suggestions for correct data collection strategy and structure refinement are made.
J H Jones - One of the best experts on this subject based on the ideXlab platform.
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an experimental study of rare earth element partitioning between a shergottite melt and Pigeonite implications for the oxygen fugacity of the mart ian interior
Geochimica et Cosmochimica Acta, 2004Co-Authors: M C Mccanta, Malcolm J Rutherford, J H JonesAbstract:Abstract A series of experiments on a synthetic, Pigeonite-saturated, basaltic shergottite were carried out to constrain the variation of D (Eu/Gd) Pigeonite/melt and D (Eu/Sm) Pigeonite/melt with oxygen fugacity ( f O2 ). The experiments have been run under both dry and hydrous conditions. The shergottite was doped with 0.1, 0.5, and 1.0 wt.% Eu, Gd, and Sm oxides in different experiments and was equilibrated at liquidus conditions for 24 hours. D (Eu/Gd) Pigeonite/melt in dry melts ranges from 0.156 ± 0.014 ( f O2 = IW − 1) to 0.630 ± 0.102 (IW + 3.5). D (Eu/Sm) Pigeonite/melt in dry melts ranges from 0.279 ± 0.021 (IW − 1) to 1.114 ± 0.072 (IW + 3.5). Due to difficulties with low- f O2 experiments, hydrous distribution coefficients were measured, but were not used in the calibration of the Eu-oxybarometers. These two Eu-oxybarometers provide an accurate way to measure f O2 recorded during Pigeonite crystallization, thereby yielding a record of f O2 during the earliest period of Martian meteorite parent magma crystallization history. Using this new calibration, Martian meteorite Pigeonite cores record f O2 values of IW − 0.6 (±0.3) (QUE94201) to IW + 1.9 (±0.6) (Shergotty). These new values differ in magnitude, but not trend, from previously published data. The Pigeonite Eu-oxybarometer yields an f O2 range in the basaltic shergottites of 2 to 3 orders of magnitude. Several processes have been proposed to explain the origin of this f O2 range, the majority of which rely on assimilation of an oxidized source. A potential correlation between this new Pigeonite data and recent Fe-Ti oxide data, however, is consistent with intrinsic f O2 differences in the magma source region being responsible for the measured f O2 variations. This implies that the Martian meteorite source region, the mantle or lithosphere, may be heterogeneous in nature. However, the process of assimilation cannot be completely ruled out in that an assimilation event that took place before crystallization commenced would result in the overprinting of the source region f O2 signature.
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oxygen fugacity of the martian mantle from Pigeonite melt partitioning of samarium europium and gadolinium
Lunar and Planetary Science XXXV: Oxygen in the Solar System, 2004Co-Authors: S Musselwhite, J H Jones, C ShearerAbstract:This study is part of an ongoing effort to calibrate the pyroxene/melt Eu oxybarometer for conditions relevant to the martian meteorites. There is fairly good agreement between a determinations using equilibria between Fe-Ti oxides and the estimates from Eu anomalies in shergottite augites in tenns of which meteorites are more or less oxidized. The Eu calibration was for angrite composition pyroxenes which are rather extreme. However, application of a calibration for martian composition augites 113 does not significantly reduce the discrepancy between the two methods. One possible reason for this discrepancy is that augites are non-liquidus. The use of Pigeonite rather than augite as the oxy-barometer phase is considered. We have conducted experiments on martian composition Pigeonite/melt REE partitioning as a function of fO2.
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an experimental study of eu gd partitioning between a shergottite melt and Pigeonite implications for the oxygen fugacity of the martian interior
Lunar and Planetary Science Conference, 2002Co-Authors: M C Mccanta, Malcolm J Rutherford, J H JonesAbstract:We experimentally investigated the partitioning behavior of Eu/Gd between a synthetic shergottite melt and Pigeonite as a function of oxygen fugacity. This has implications for the oxidation state of the source region of the martian meteorites. Additional information is contained in the original extended abstract.
Mario Tribaudino - One of the best experts on this subject based on the ideXlab platform.
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raman investigation on Pigeonite in ureilite
Spectroscopy Letters, 2011Co-Authors: Mario Tribaudino, Luciana Mantovani, Danilo Bersani, P P LotticiAbstract:ABSTRACT The Raman spectra of 10 samples containing the mineral Pigeonite, a Ca-poor P21/c pyroxene, were measured. Seven of the studied samples come from the achondritic meteorites belonging to the family of ureilites, two are from volcanic rocks, and one is synthetic. The studied samples have an mg content [mg = 100 Mg/(Mg + Fe)] between 52 and 100. The differences in Raman spectra with C2/c and Pbca pyroxenes are highlighted; changes in peak position with mg of the major peaks at 340 and 670 cm−1 are significantly lower in C2/c compared to Pbca and P21/c pyroxenes. Peak width in natural Pigeonite is higher than in synthetic one, owing to the defective texture of the former. The present results may provide a basis for in situ characterization of pyroxenes by means of Raman spectroscopy of planetary bodies in space exploration.
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microtextures and crystal chemistry of Pigeonite in the ureilites alha77257 rkpa80239 y 791538 and alha81101
Meteoritics & Planetary Science, 2006Co-Authors: Mario TribaudinoAbstract:The microtextures of Pigeonite in four ureilites, Allan Hills (ALH) 77257, Reckling Peak (RKP) A80239, Yamato (Y-) 791538, and Allan Hills A81101, chosen to span a range of composition and shock level, were investigated by transmission electron microscopy (TEM); two of the samples were also investigated by single crystal X-ray diffraction to determine Fe2+-Mg cation site partitioning.The low-shock and compositionally homogeneous Pigeonites in ALHA77257 and RKPA80329 (Wo 6.4 for both, mg 86.3 and 84.3 respectively) display irregularly spaced, shock-induced stacking faults oriented parallel to (100), and largeantiphase domains (50-100 nm). Antiphase domains have no preferential orientation. No evidence of exsolution was observed.The low-shock Y-791538 Pigeonite is homogeneous and has higher Ca and mg (Wo 9.4, mg 91.2). TEM investigation showed spinodal decomposition, indicative of incipient exsolution; small antiphase domains were observed (?5 nm). Single crystal refinement yielded R4σ = 5.71%, with Fe2+-Mg partitioning coefficient kD = 0.077(8) and Tc = 658(35) °C. ALHA81101 has compositionally heterogeneous pyroxenes, with large local variations in Wo and mg (Wo = 4-13, mg = 86-68). No compositional gradients from core to rim of grains were observed, and the heterogeneity is interpreted as related to cation migration during shock. In one relatively Ca-rich region (Wo ≈ 12), TEM analysis showed augite-Pigeonite exsolution lamellae, with spacing 145(20) nm.Results for ALHA77257, RKPA80239, and Y-791538 support a model of rapid cooling following breakup of the ureilite parent body. The presence of exsolution lamellae in ALHA81101 can be related to a local shock-induced Ca enrichment and provides no constraint on the late cooling history.
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high pressure behavior transformation and crystal structure of synthetic iron free Pigeonite
American Mineralogist, 2004Co-Authors: Fabrizio Nestola, Mario Tribaudino, Tiziana Boffa BallaranAbstract:A single-crystal high pressure X-ray investigation was performed up to P = 6.5 GPa on a synthetic clinopyroxene of composition Ca 0.15(1) Mg 1.85(1) Si 2.00(1) O 6 [Di 15 En 85 , unit-cell parameters at room pressure: a = 9.6525(6)A, b = 8.8461(2)A, c = 5.2036(5)A, β = 108.370(5)°, V = 421.68(4)A 3 ]. A first order P 2 1 - C 2/ c displacive phase transition was found at P = 5.1 GPa; the transition was revealed by the disappearance of the b reflections ( h + k = odd) and by sharp changes in the unit-cell parameters. Reversals through the transformation show that, if present, hysteresis is smaller than 0.1 GPa. The volume variation has been described by a third-order Birch-Murnaghan equation of state with V 0 = 421.68(8) A 3 , K T0 = 102(2) GPa, and K ′ = 8(1) for the low-symmetry phase ( P 2 1 / c ) and with V 0 = 411.06(3) A 3 and K T0 = 108(2) GPa for the high-symmetry phase ( C 2/ c ), with K ′ fixed to the value obtained for the low-symmetry form. The axial compressibility shows the following scheme: β b > β a ≅ β c >β a sinβ for both phases. In comparison with pure clinoenstatite, Di 15 En 85 shows a similar step in unit-cell parameters at the transition, the disappearence of hysteresis and a decrease of transition pressure and of bulk modulus. Full intensity data sets were collected at room pressure, 2.6 and 4.5 GPa for the P 2 1 / c phase and at P = 6.2 GPa for the C 2/ c phase. A slight increase of the intensity of h + k odd reflections and of the difference in the A and B chain kinking angles were observed. A comparison of the structural behavior of the P 2 1 / c phase at high temperature and high pressure shows opposite behavior for M2-O bond lengths and O3-O3-O3 kinking angle.
Philippe Claeys - One of the best experts on this subject based on the ideXlab platform.
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Two generations of exsolution lamellae in pyroxene from Asuka 09545: Clues to the thermal evolution of silicates in mesosiderite
American Mineralogist, 2019Co-Authors: Lidia Pittarello, Akira Yamaguchi, S. Mckibbin, Dominique Schryvers, Vinciane Debaille, Philippe ClaeysAbstract:Abstract Mesosiderite meteorites consist of a mixture of crustal basaltic or gabbroic material and metal. Their formation process is still debated due to their unexpected combination of crust and core materials, possibly derived from the same planetesimal parent body, and lacking an intervening mantle component. Mesosiderites have experienced an extremely slow cooling rate from ca. 550 °C, as recorded in the metal (0.25–0.5 °C/Ma). Here we present a detailed investigation of exsolution features in pyroxene from the Antarctic mesosiderite Asuka (A) 09545. Geothermobarometry calculations, lattice parameters, lamellae orientation, and the presence of clinoenstatite as the host were used in an attempt to constrain the evolution of pyroxene from 1150 to 570 °C and the formation of two generations of exsolution lamellae. After Pigeonite crystallization at ca. 1150 °C, the first exsolution process generated the thick augite lamellae along (100) in the temperature interval 1000–900 °C. By further cooling, a second order of exsolution lamellae formed within augite along (001), consisting of monoclinic low-Ca pyroxene, equilibrated in the temperature range 900–800 °C. The last process, occurring in the 600–500 °C temperature range, was likely the inversion of high to low Pigeonite in the host crystal, lacking evidence for nucleation of orthopyroxene. The formation of two generations of exsolution lamellae, as well as of likely metastable Pigeonite, suggest non-equilibrium conditions. Cooling was sufficiently slow to allow the formation of the lamellae, their preservation, and the transition from high to low Pigeonite. In addition, the preservation of such fine-grained lamellae limits long-lasting, impact reheating to a peak temperature lower than 570 °C. These features, including the presence of monoclinic low-Ca pyroxene as the host, are reported in only a few mesosiderites. This suggests a possibly different origin and thermal history from most mesosiderites and that the crystallography (i.e., space group) of low-Ca pyroxene could be used as parameter to distinguish mesosiderite populations based on their cooling history.
Geoffrey Bromiley - One of the best experts on this subject based on the ideXlab platform.
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the structure of p21 c ca0 2co0 8 cosi2o6 pyroxene and the c2 c p21 c phase transition in natural and synthetic ca mg fe2 pyroxenes
Mineralogical Magazine, 2017Co-Authors: M Tribaudino, Luciana Mantovani, Francesco Mezzadri, G Calestani, Geoffrey BromileyAbstract:A P21/c synthetic (Ca0.2Co0.8)CoSi2O6 pyroxene was synthesized by slow cooling from melt at high pressure. Single crystals suitable for X-ray diffraction were obtained and refined. The results were compared to those of C2/c pyroxenes along the series CaCoSi2O6–Co2Si2O6. Strong similarities in the crystal chemical mechanism of the transition with the synthetic CaFeSi2O6–Fe2Si2O6 and CaMgSi2O6–Mg2Si2O6 pyroxenes, both at an average and local level are apparent.The results, examined together with two new refinements of Pigeonite in the ureilites ALHA77257 and RKPA80239 and with a set of natural and synthetic C2/c and P21/c pyroxenes, show that the average cation radius in the M2 site is the driving force for the phase transition from C2/c to P21/c. The longest M2–O3 distances and the O3–O3–O3 angles follow the same trend, dictated only by the ionic radius in M2, in either synthetic or natural pyroxenes, regardless of the ionic radius of the M1 cations. The transition also affects the difference between bridging and non-bridging oxygen atoms and the extent of tetrahedral deformation, whereas the M1–O, M2–O1 and M2–O2 distances are unaffected by the transition and are determined only by the ionic radius of the bonding cation. The structural changes between the ionic radius and the high temperature C2/c and P21/c transitions are similar, and different to the high-pressure transition.Analysis of natural and synthetic pyroxenes shows that the transition with composition occurs in strain free pyroxenes for a critical radius of 0.85 A. Increasing strain stabilizes the P21/c structure to a higher temperature and larger cation radius.Finally, our results show that the monoclinic P21/c Ca-poor clinopyroxene, i.e the mineral Pigeonite, crystallizes only at conditions where the structure is HT-C2/c, and changes to the P21/c symmetry during cooling.