102,198 research outputs found
A low temperature X-ray single-crystal diffraction and polarised infra-red study of epidote
Full text linkThe effects of low-temperature on the crystal structure of a natural epidote [Ca1.925Fe0.745Al2.265Ti0.004 Si3.037O12(OH), a = 8.8924(7), b = 5.6214(3), c = 10.1547(6) angstrom and beta = 115.396(8)degrees at room conditions, Sp. Gr. P2(1)/m] have been investigated with a series of structure refinements down to 100 K on the basis of X-ray single-crystal diffraction data. The reflection conditions confirm that the space group is maintained within the T-range investigated. Structural refinements at all temperatures show the presence of Fe3+ at the octahedral M(3) site only [%Fe(M3) = 70.6(4)% at 295 K]. Only one independent proton site was located and two possible H-bonds occur, with O(10) as donor and O(4) and O(2) as acceptors. The H-bonding scheme is maintained down to 100 K and is supported by single crystal room-T polarised FTIR data. FTIR Spectra over the region 4,000-2,500 cm(-1) are dominated by the presence of a strongly pleochroic absorption feature which can be assigned to protonation of O(10)-O(4). Previously unobserved splitting of this absorption features is consistent with a NNN influence due to the presence of Al and Fe3+ on the nearby M(3) site. An additional relatively minor absorption feature in FTIR spectra can be tentatively assigned to protonation of O(10)-O(2). Low-T does not affect significantly the tetrahedral and octahedral bond distances and angles, even when distances are corrected for "rigid body motions". A more significant effect is observed for the bond distances of the distorted Ca(1)- and Ca(2)-polyhedra, especially when corrected for "non-correlated motion". The main low-T effect is observed on the vibrational regime of the atomic sites, and in particular for the two Ca-sites. A significant reduction of the magnitude of the thermal displacement ellipsoids, with a variation of U-eq (defined as one-third of the trace of the orthogonalised U-ij tensor) by similar to 40% is observed for the Ca-sites between 295 and 100 K. Within the same T-range, the U-eq of the octahedral and oxygen sites decrease similarly by similar to 35%, whereas those of the tetrahedral cations by similar to 22%.</p
Effects of temperature on the crystal structure of epidote: a neutron single-crystal diffraction study at 293 and 1070K
Full text linkThe effects of temperature on the crystal structure of a natural epidote [Ca-1.925 Fe0.745Al2.265Ti0.004Si3.037O12(OH), a = 8.890(6), b = 5.630(4), c = 10.150(6) and beta = 115.36(5)A degrees, Sp. Gr. P2(1) /m] have been investigated by means of neutron single-crystal diffraction at 293 and 1,070 K. At room conditions, the structural refinement confirms the presence of Fe3+ at the M-3 site [%Fe(M3) = 73.1(8)%] and all attempts to refine the amount of Fe at the M(1) site were unsuccessful. Only one independent proton site was located. Two possible hydrogen bonds, with O(2) and O(4) as acceptors [i.e. O(10)-H(1)center dot center dot center dot O(2) and O(10)-H(1)center dot center dot center dot O(4)], occur. However, the topological configuration of the bonds suggests that the O(10)-H(1)center dot center dot center dot O(4) is energetically more favourable, as H(1)center dot center dot center dot O(4) = 1.9731(28) , O(10)center dot center dot center dot O(4) = 2.9318(22) and O(10)-H(1)center dot center dot center dot O4 = 166.7(2)A degrees, whereas H(1)center dot center dot center dot O(2) = 2.5921(23) , O(10)center dot center dot center dot O(2) = 2.8221(17) and O(10)-H(1)center dot center dot center dot O2 = 93.3(1)A degrees. The O(10)-H(1) bond distance corrected for "riding motion" is 0.9943 . The diffraction data at 1,070 K show that epidote is stable within the T-range investigated, and that its crystallinity is maintained. A positive thermal expansion is observed along all the three crystallographic axes. At 1,070 K the structural refinement again shows that Fe3+ share the M(3) site along with Al3+ [%Fe(M3)(1,070K) = 74(2)%]. The refined amount of Fe3+ at the M(1) is not significant [%Fe(M1)(1,070K) = 1(2)%]. The tetrahedral and octahedral bond distances and angles show a slight distortion of the polyhedra at high-T, but a significant increase of the bond distances compared to those at room temperature is observed, especially for bond distances corrected for "rigid body motions". The high-T conditions also affect the inter-polyhedral configurations: the bridging angle Si(2)-O(9)-Si(1) of the Si2O7 group increases significantly with T. The high-T structure refinement shows that no dehydration effect occurs at least within the T-range investigated. The configuration of the H-bonding is basically maintained with temperature. However, the hydrogen bond strength changes at 1,070 K, as the O(10)center dot center dot center dot O(4) and H(1)center dot center dot center dot O(4) distances are slightly longer than those at 293 K. The anisotropic displacement parameters of the proton site are significantly larger than those at room condition. Reasons for the thermal stability of epidote up to 1,070 K observed in this study, the absence of dehydration and/or non-convergent ordering of Al and Fe3+ between different octahedral sites and/or convergent ordering on M(3) are discussed.</p
Development of crystallographic preferred orientation and microstructure during plastic deformation of natural coarse-grained quartz veins
Full text linkThe microstructure and crystallographic preferred orientation (CPO) of quartz were quantified in 17 samples of natural monomineralic tabular veins. The veins opened and were deformed, up to shear strain γ > 15, in a small temperature window (about 25°C) above 500°C, as established by Ti-in-quartz thermometry. The veins filled a set of fractures within the Adamello tonalite (southern Alps, Italy) and localized homogeneous simple shear during postmagmatic cooling. The local (square millimeter scale) and bulk (square centimeter) CPO were investigated by computer-integrated polarization microscopy (CIP) and X-ray texture goniometry. Weakly deformed veins (WDV: γ <1) consist of millimeter- to centimeter-sized crystals with a strong CPO showing a c-axis girdle slightly inclined, mostly with the shear sense, to the foliation (XY) plane and a strong maximum close to the lineation (X). Moderately deformed veins (MDV: 2 <γ <3) consist of elongated nonrecrystallized ribbon grains and most have a CPO showing a strong Y maximum of c axes some with weak extension into a YZ girdle. Strongly deformed veins (SDV: γ = 4 to 15) are pervasively to completely recrystallized to fine (34-40 μm grain size) aggregates with a strong CPO similar to that of MDV. The slip systems during plastic deformation were dominantly prism 〈a〉 with subordinate rhomb and basal 〈a〉 slip. Recrystallization occurred rather abruptly for 3 <γ <4. In contrast to dislocation creep experiments in quartz (and other minerals), a steady-state recrystallized fabric is achieved at early stages of deformation (γ ≈ 4) as there is no evidence, with increasing strain, of strengthening of the CPO, of rotation of the fabric skeleton, or of change in grain size. WDV represent weakly deformed relicts of veins with an initial CPO believed to have developed during crystal growth but unsuitably oriented for prism 〈a〉 slip during subsequent shear. MDV and SDV appear to derive from veins different from WDV, where the vein crystals grew with orientation favorable for prism 〈a〉 slip. The relationship between the initial growth CPO and the kinematic framework suggests that veins opened at a temperature close to that at which there is a switch between the activity of prism 〈c〉 and prism 〈a〉 slip, with the temperature of growth causing growth of crystals well oriented for slip. The initial CPO of veins, from which quartz mylonites are commonly derived, plays a critical role in the fabric evolution. The strong growth- and strain-induced CPOs of these sheared veins inhibited significant reworking during lower temperature stages of pluton cooling when basal 〈a〉 slip would have been dominant
Solid solutions and phase transitions in (Ca,M2+)M2+Si2O6 pyroxenes (M2+ = Co, Fe, Mg).
No full textThe effect of the substitution of Ca with Co, on the phase transition and on the extension of the miscibility gap, was studied to model the general mechanism of phase transitions and solid solutions in (Ca,M2+)M2+Si2O6 pyroxenes. Eleven pyroxenes with composition Ca1-xCo1+xSi2O6, (0 ≤ x ≤ 1) were therefore synthesized by piston cylinder at P = 3 GPa, and T between 1100 and 1350 °C. The samples were characterized by SEM-EDS, XRD powder diffraction, and TEM. The results were compared with those of Ca-Fe and Ca-Mg pyroxenes. The phase diagram of Ca-Co pyroxenes is similar to that of Ca-Fe and Ca-Mg ones, with a wide asymmetric miscibility gap, and higher solubility in the Ca-rich side of the gap. The solubility on the Ca-rich side of the gap is related to the radius of the cation substituting.
The cell parameters of the Ca-Co pyroxenes undergo a sudden change at the composition of about 0.4 Ca apfu, due to the C2/c-P21/c phase transition. The change in volume with composition follows an ideal trend, in the C2/c phase, dictated by the ionic size of the substituting cation. Deviation from the C2/c behavior are instead observed in the P21/c field and ascribed to volume strain. The same turnover was found in Ca-Mg, Ca-Fe, and Ca-Mn pyroxenes. The C2/c-P21/c transition occurs with decreasing the M2 average cation radius, down to a critical value between 0.85 and 0.88 Å, depending on the series. A stabilization of the C2/c phase related to crystal field in Ca-Fe and Ca-Co pyroxenes is suggested by the analysis of the volume strain in the P21/c field. A key finding is that a miscibility gap may develop either by lattice strain related to cation substitution, within a series where all endmembers have the same structure, or for the combined effect of lattice strain and a phase transition, as is the case for pyroxene
A single-crystal neutron diffraction study of hambergite, Be2BO3(OH,F)
No full textThe crystal chemistry and crystal structure of hambergite from the Anjanabonoina mine, Madagascar [Be 2BO 3(OH) 0.96F 0.04, Z = 8, a = 9.762(2), b = 12.201(2), c = 4.430(1) Å, V = 527.6(2) Å 3, space group Pbca], were reinvestigated by means of electron microprobe analysis in wavelength-dispersive mode, secondary-ion mass spectrometry, single-crystal X-ray and neutron Laue diffraction. Chemical analyses show only a small amount of F (0.7-0.8 wt%, approximately 0.04 atoms per formula unit) substituting OH and no other substituent at a significant level. An anisotropic neutron structural refinement has been performed with final agreement index R 1 = 0.0504 for 76 refined parameters and 1430 unique reflections with F o > 4σ(F o). The geometry of the hydroxyl group and hydrogen bonding in hambergite is now well defined: (1) only one independent H site was located and the O4-H distance, corrected for "riding motion," is ∼0.9929 Å; (2) only one hydrogen bond appears to be energetically favorable, with a symmetry-related O4 as acceptor and with O4...O4 = 2.904(1) Å, H...O4 = 1.983(1) Å, and O4-H...O4 = 157.5(1)°. In other words, O4 sites act both as donor and as acceptor of the hydrogen bond, with a zigzag chain of H-bonds along [001]. The hydrogen-bonding scheme in hambergite found in this study is consistent with the pleochroic scheme of the infrared spectra previously reported, with two intensive modes ascribable to stretching vibrations of the hydroxyl group, at 3415 and 3520 cm -1, respectively. The two modes suggest at least two distinct hydrogen-bonding environments, ascribable to the presence of oxygen and fluorine at the acceptor site
The real topological configuration of the extra-framework content in alkali-poor beryl: a multi methodological study
No full textThe crystal structure of alkali/water-poor beryl (1120 + Na2O + Cs2O 4 sigma(F-0). The topological configuration of the H2O molecule into the channel is confirmed by the spectroscopic investigation. Polarized single-crystal IR spectra show that the H2O molecule is oriented with the molecular symmetry axis perpendicular to the hexagonal axis and (HH)-H-... vector parallel (or quasi-parallel) to [001]
The high temperature P21/c-C2/c phase transition in Fe-free Ca-rich P21/c clinopyroxenes
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