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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
Full text linkThe 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 of 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, dictated2only 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 from the high-pressure transition.The analysis of natural and synthetic pyroxenes shows that the transition with composition occurs in strain free pyroxenes for a critical radius of 0.85 Å. Increasing strain stabilizes the P21/c structure to higher temperature and larger cation radius.Finally, our results show that the monoclinic P21/c Ca-poor clinopyroxene, i.e the pigeonite mineral, crystallizes only at conditions where the structure is HT-C2/c, and changes to the P21/c symmetry during cooling
Major element zonation following rapid heating of homogeneous glass in wire-loop experiments_full dataset
No full textData used for the publication "Major element zonation following rapid heating of homogeneous glass in wire-loop experiments" [https://doi.org/10.1016/j.chemer.2025.126383]. Data obtained as part of a systematic investigation of magmatic degassing process in analogue lunar magma, funded by the Leverhulme Trust through Research Project Grant RPG-2021-015, and the Carnegie Trust through PhD Scholarship award PHD01069
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
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
Importance of grain boundary diffusion : an experimental study
Full text linkThis research is concerned with the mechanisms of diffusion in the Earth and the implications of such an understanding. Specifically, this work is concerned with one particular aspect of diffusion: Grain Boundary Diffusion (GBD). An experimental investigation of GBD has been conducted by considering three specific scenarios; GBD of H in stoichiometric Mg-spinel, GBD of Ti in Quartz and GBD of Li in olivine. By considering the GBD of three very different elements it has been possible to synthesise an understanding of some of the mechanisms involved in the process. GBD is potentially a very important process within the Earth with wide ranging implications. Grain boundaries may provide fast pathways for transportation of a range of compatible and incompatible diffusing species in the Earth’s interior – potentially acting as storage locations and also as efficient pathways between different geological reservoirs. It is also potentially very important in the application of a number of techniques including dating and geothermometry and geobarometry. Here, an experimental study of the GBD of H has been carried out with the overall finding that GBD appears to occur at slightly greater yet broadly similar rates to lattice diffusion. This finding is considered in terms of the mantle properties which are affected by the presence and transport of H. A follow up series of experiments was conducted looking at Li diffusion. Li was chosen due to its volatile nature and larger atomic radius as compared to H. As such, it provided a useful test of the hypothesis that the radius of a diffusant might affect its chosen method of diffusion. A third set of experiments were carried out to investigate the GBD of Ti in quartz with particular reference to the TitaniQ geothermo(baro)meter. This set of experiments provided a very useful comparison to the data which had previously been obtained from lighter elements. This investigation has found that a combination of factors including charge, diffusant diameter and the specific mineralogical characteristics of the host phase will define the dominant diffusive mechanism and the size of the contribution made by that mechanism towards observed bulk diffusivities. A characterisation of the temperature dependency of diffusion within each setting has also been completed. As such, it also makes a useful contribution to the current dataset for GBD
Solid solutions along the synthetic LiAlSi2O6 (spodumene)-LiFeSi2O6 (ferri-spodumene) join: a general picture of solid solutions, bond lengths, lattice strains, steric effects, symmetries and chemical compositions of Li clinopyroexenes
Full text linkSeven clinopyroxene compositions along the joinM2LiM1AlTSi2O6(spodumene) toM2LiM1Fe3+TSi2O6(ferri-spodumene) were synthesized at 2 GPa, 800 °C under highly oxidizing conditions (using H2O2fluid) in an end-loaded piston cylinder. In addition, the LiFe3+TSi2O6composition was also synthesized under the intrinsically reducing conditions in a piston cylinder, to check the effect of fO2 on iron speciation. The run products were characterized by field emission scanning electron microscope (FE-SEM), Rietveld refinements on XRPD synchrotron data, and space groups were assigned using SAED-TEM patterns. Run products are composed mainly of lithium clinopyroxene (Li-Cpx), plus minor amounts of hematite (magnetite under reducing condition) and corundum, as independently detected by image analysis (area%) and Rietveld refinements (wt%); moreover, Rietveld results were used to derive cell parameters, M1-site occupancy (Al vs. Fe3+), atomic positions, and average bond lengths of all these Li-Cpx indexed in the C2/c space groups according to SAED-TEM. Li-Cpx with Al and Fe3+amounts close to 50:50 are actually slightly richer in Al apfu than nominal; the LiFe3+Si2O6grown under very oxidized and reducing conditions have very similar cell parameters, indicating that fO2 is unable to induce a significant incorporation of Fe2+ in these Li-Cpx. The replacement of Al with Fe3+induces a linear (%) increase of the cell edges following b ≥ a > c, whereas β is roughly constant and the cell volume increases linearly. Furthermore, the substitution of Al with Fe3+ only weakly affects the T-O average length ( b > a > c, whereas ? is roughly constant except for Ti-end-member and P21/c compositions. Lattice strains induced by X, T, and P for Li-Cpx in the C2/c stability field show that when the M1 site is progressively filled with a large cation, ε1 axis (ε1ε2ε3) increases along b, whereas ε2and ε3are nearly parallel to a and at about 30° from c. Conversely, T will provoke a similar enlargement of ε1and ε2along b and a edges, respectively, whereas ε3is again oriented at about 30° from c; the increasing of P will instead shorten all strain tensor components (ε1, ε2, and ε3) with a similar percentage amount; notably, high-P is the only stress that induces a strain component to be almost parallel to c edge. Moreover, finite lattice strains and orientation in C2/c LiMe3+Si2O6Li-Cpx induced by Me3+: Al-Fe3+, Fe3+-Sc, Sc-In are slightly different, with &1 invariably lying along b; conversely, Li-Na cation substitution is completely different with the highest and lowest deformations on the ac plane and ε2along b; ε3vector is negative and oriented at about 30° from T-chains. The ideal replacement of Al with larger cations up to In in Li-Cpx induces the M1-O, M2-O, and T-O average bond lengths to increase by 10.6, 4.3, and <0.5%. Steric effects in LiM1Me3+Si2O6and NaM1Me3+Si2O6 Cpx are significant and very similar, whereas several other Me1+and Me2+substitutions in Cpx at both the M1 and M2 site, keeping fixed the other site, display less or even the absence of steric effects. Our new data also better elucidate relationships between Li-Cpx composition, symmetry at room and non-Ambient conditions and Tc. The aggregate cation radii at the M1 site does not exclusively control the stability of C2/c and P21/c polymorphs; instead valence electrons can profoundly favor the stabilization of a polymorph
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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