1,721,202 research outputs found
Average and local structure in P21/c clinopyroxenes along the join diopside-enstatite (CaMgSi2O6- Mg2Si2O6)
No full textTHE STRUCTURE OF PBCA ORTHOPYROXENES ALONG THE JOIN DIOPSIDE-ENSTATITE (CAMGSI2O6-MG2SI2O6)
No full textThe thermodynamics of the I -P phase transition in Ca-rich plagioclase from an assessment of the spontaneous strain
No full textThe high-pressure P21/c - C2/c phase transition for the CMS (CaO-MgO-SiO2) clinopyroxenes
No full textJournal of Conference Abstract
Rietveld refinement of clinopyroxenes with intermediate Ca-content along the diopside-enstatite join
No full text
Pyrometamorphic processes at the magma-hydrothermal system interface of active volcanoes: evidence from buchite ejecta of Stromboli (Aeolian Islands, Italy)
No full textPyrometamorphic ejecta erupted during recent paroxysmal explosions and hydrothermally altered volcanic and subvolcanic rocks from Stromboli volcano (Aeolian Islands, Italy) were studied by combined polarizing microscopy, electron microprobe, scanning electron microscope, X-ray diffraction, transmission electron microscope and whole-rock inductively coupled plasma optical emission spectrometry-mass spectrometry analysis. The pyrometamorphic ejecta are compositionally heterogeneous at the macro- and micro-scale, and are classified as buchites; that is, glass coupled with a high-temperature, low-pressure modal mineralogy. Three distinct facies (i.e. cordierite-plagioclase, mullite-plagioclase palisade and tridymite-clinopyroxene) are recognized among the buchites, whose bulk-rock chemistry is compatible with aluminous- to silica-rich protoliths. Abundant mineral growth (e.g. plagioclase palisades and clinopyroxene coronae) occurs at the contact between the ejecta and the crystal-rich host shoshonitic basalt, which typically coats the pyrometamorphic blocks. Mineral constraints, pseudoporphyritic microstructures, plagioclase relict phenocrysts and major and trace element compositions strongly support the origin of the buchite ejecta as hydrothermally altered rocks from theVancori edifice (a period of activity spanning >10 kyr), which underwent high-temperature, low-pressure contact metamorphism. In the uppermost magmatic system of an active island arc volcano, such as Stromboli, volcanic and subvolcanic rocks usually undergo alteration processes induced by acid-sulphate hydrothermal systems. The circulation of acidic fluids results in leaching of the Stromboli rocks (in the upper part of the volcano edifice) and, depending on the pH, temperature and exposure time to the hydrothermal fluids, production of different alteration facies. The hydrothermally altered Stromboli rocks show facies consistent with argillic to silicic alteration, containing abundant hydrous sulphate and hydroxy-sulphate minerals such as the alunite group minerals, hexahydrite, millosevichite, pickeringite and kieserite. The hydrothermally altered wall-rocks, heated by contact with the basaltic magma of the feeder dyke system of Stromboli, can easily undergo pyrometamorphic processes, inducing partial melting and subsolidus crystallization of the volcanic protoliths. This results in the development of buchite rocks at the magma-hydrothermal system interface
Single-crystal thermometric calibration of Fe-Mg order-disorder in pigeonites
No full textThe single-crystal X-lay technique was used to calibrate a new intracrystalline geothermometer based on equilibrium Mg-Fe* fractionation (Fe* = Fe2+ + Mn2+) between M1 and M2 sites of natural P2(1)/c pigeonite, Suitable crystals free of exsolution textures and sharp diffraction maxima were selected by careful TEM and XRD investigations from a large number of samples. Two single crystals, PCA82506-3 (Wo(6)En(76)Fs(18)) from the Pecora Escarpment 82506 Antarctic ureilite, and BTS308-2 (Wo(10)En(47)Fs(43)) from the BTS308 Parana rhyodacite, were annealed at temperatures ranging from 600 to 1000 degrees C. The TEM investigation, carried out on fragments of selected single crystals both before and after thermal treatment, shows heating-induced texture modifications preliminary to spinodal decomposition in both crystals and a size increase in antiphase domains in BTS308-2, The two geothermometric equations calculated by linear regression of In K-D* vs. 1/T are:
In K-D* = -3291(+/- 269)/T(K) + 0.971(+/-0.253); (r(2) = 0.974)
ln K-D* = -2816(+/- 83)/T(K) + 0.542(+/-0.083); (r(2) = 0.995)
respectively for PCA82506-3 (X-Fe* = 0.20) and BTS308-2 (X-Fe* = 0.49) [X-Fe* = Fe*/(Fe* + Mg)]. These results imply negligible compositional effects on Mg-Fe* site partitioning within the range of compositions encompassed by the samples. Comparison of the intracrystalline fractionation data of pigeonites with those of Pbca orthopyroxenes shows a similar degree of ordering for both at a given temperature. This result suggests only a small effect of Ca on Fe*-Mg ordering in pyroxene with Ca content up to Wo(10)
HIGH-PRESSURE BEHAVIOR, TRANSFORMATION AND CRYSTAL STRUCTURE OF THE SYNTHETIC IRON-FREE PIGEONITE
No full textA single-crystal high pressure X-ray investigation was performed up to P = 6.5 GPa on a synthetic clinopyroxene of composition Ca0.15(1)Mg1.85(1)Si2.00(1)O6 [Di(15)En(85), unit-cell parameters at room pressure: a = 9.6525(6)Angstrom, b = 8.8461(2)Angstrom, c = 5.2036(5)Angstrom, beta = 108.370(5)degrees, V = 421.68(4)Angstrom(3)]. A first order P2(1)/c-C2/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-Mumaghan equation of state with V-0 = 421.68(8) Angstrom(3), K-T0 = 102(2) GPa, and K' = 8(1) for the low-symmetry phase (P2(1)/c) and with V-0 = 411.06(3) Angstrom(3) and K-T0 = 108(2) GPa for the high-symmetry phase (C2/c), with K' fixed to the value obtained for the low-symmetry form. The axial compressibility shows the following scheme: beta(b) > beta(A) = beta(c) > beta(asinbeta) 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 P2(1)/c phase and at P = 6.2 GPa for the C2/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 P2(1)/c phase at high temperature and high pressure shows opposite behavior for M2-O bond lengths and O3-O3-O3 kinking angle
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