1,721,195 research outputs found
A transmission electron microscope investigation on the C2/c -P21/c phase transition in clinopyroxenes along the diopside-enstatite (CaMgSi2O6-Mg2Si2O6) join.
No full textHigh-temperature crystal chemistry of C2/c clinopyroxenes along the join CaMgSi2O6-CaAl2SiO6
No full textSingle-crystal in-situ data have been collected at high temperature on synthetic clinopyroxenes of composition Di(70)CaTs(30) and Di(50)CaTs(50). Sample Di(70)CaTs(30) was refined at 25, 300, 500 and 700 degrees C and sample Di(50)CaTs(50) at 700 degrees C. A strong deformation with temperature in the M2 polyhedron is induced by a shift in the tetrahedral chains along z, decreasing the M2 polyhedral coordination, similar to the high temperature behaviour of pure diopside and other C2/c clinopyroxenes. This deformation is opposite to that produced at room temperature by the entrance of CaTs into the diopside structure. The amount of the deformation in the M2 polyhedron is comparable to that observed in pure diopside and lower than in Ca-poorer clinopyroxene. The behaviour of B-eq vs T suggests the presence of positional disorder in the studied samples, mostly affecting the oxygen atoms
COORDINAMENTO SCUOLA Nazionale “Le Microstrutture: Analisi ed Applicazioni in Materiali Geologici” tenuta a Campiglia (LI) dal 19 al 23 settembre 2006,
No full textMicrotextures and crystal chemistry of pigeonite in the ureilites ALHA77257, RKPA80239, Y-791538 and ALHA81101.
No full textThe 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 (approximately equal to 5 nm). Single crystal refinement yielded R4-sigma = 5.71%, with Fe2+-Mg partitioning coefficient kD = 0.077(8) and Tc = 658(35) degrees 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 is approximately equal to 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.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202
High temperature structure and thermal expansion of Co(3)Al(2)Si(3)O(12) garnet
No full textSynchrotron radiation powder diffraction patterns were taken on synthetic Co(3)Al(2)Si(3)O(12) garnet at BM8-GILDA (ESRF), at T = 298, 423, 573, 723 and 873 K. The cell parameters and atomic positions were refined by Rietveld analysis; the volume thermal expansion coefficient is alpha(V) = 28.5(9) x 10(-6) K(-1). The results on synthetic Co(3)Al(2)Si(3)O(12) garnet were compared with the high temperature structure data of X(3)Al(2)Si(3)O(12) garnets with X = Mg, Ca, Mn and Fe taken from literature. The thermal expansion of the unit cell is very similar in X(3)Al(2)Si(3)O(12) garnets, whereas the thermal expansion of the longest bonds in the distorted cubic cage surrounding the X cation decreases with increasing cation size. Contrarily to the compression behaviour, the thermal expansion in X(3)Al(2)Si(3)O(12) garnets does not change significantly as a function of composition
A high temperature in situ single-crystal study of P4/n vesuvianite RID B-5536-2008
No full textSingle crystal X-ray diffraction data were collected at T = 570, 770, 1070 K on a sample of a P4/n vesuvianite from Val d'Ala (Piedmont, Italy). Structure refinements yielded R values of 6.7, 6.8 and 8.5% for the three temperatures, respectively; the P4/n symmetry was retained throughout. The volume thermal expansion coefficient was 33.7(7) . 10(-6) K-1; different expansion rates were observed along the a and c directions [10.4(3) . 10(-6) K-1 and 12.9(3) . 10(-6) K-1 respectively]. The volume expansion is larger than that observed in the structurally similar garnet, due to a higher expansion rate along the c axis. An analysis of the atomic displacement parameters (ADP's) with temperature performed on Ca(2) and Ca(3) atoms did not indicate significant positional disorder. As well no evidence of changes in the cation distribution was found in the Ca(4)a and b and M(2)a and b sites even after prolonged heating at 1070 K
Average and local structure in P21/c clinopyroxenes along the join diopside-enstatite (CaMgSi2O6-Mg2Si2O6).
No full textEffect of Al On Enstatite Solubility In Cmas Clinopyroxenes .1. Experimental Results In the Clinopyroxene-ortho-pyroxene 2 Phase Field At P=18 Kbar
No full textClinopyroxene coexisting with orthopyroxene in the univariant (at constant P and T) two pyroxene field in the CMAS system was reversed at P = 18 kbar and T = 1350 and 1430-degrees-C, in order to compare the En content in Al-rich clinopyroxenes of CMAS system at the same P and T and in corresponding Al-free clinopyroxenes of the CMS system. Reversal runs were performed with compositions from 5 to 20 mole % CaTs to determine the solubility variation of En vs CaTs in clinopyroxenes at constant P and T. The obtained results show that the En solubility in clinopyroxenes at T = 1430-degrees-C is progressively lower than that observed along the Al-free join in the CMS system at the same P-T conditions (10 mol % less in enstatite for a CaTs content close to 20 mol %). At T = 1350-degrees-C, however, the change in En solubility in clinopyroxenes with increasing CaTs content is not significant. Therefore, at higher T, mixing on M2 and M1 sites in clinopyroxenes is not independent. As suggested by previous authors the presence of Al affects the En solubility in clinopyroxenes. As a consequence, geofliermometers based on clino-orthopyroxene equilibria in the CMS system - for a P-T range close to that of spinel lherzolite stability field - can be used to a good approximation for Al-rich two pyroxene assemblages only at T < 1300-degrees-C. At higher T, the CaTs content strongly affects the En solubility and limits its thermometric application
The structure of Pbca orthopyroxenes along the join diopside-enstatite (CaMgSi2O6-Mg2Si2O6).
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