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Crystal structure analyses of four tourmaline specimens from the Cleopatra's Mines (Egypt) and Jabal Zalm (Saudi Arabia), and the role of Al in the tourmaline group
No full textFe-rich "oxydravite" and dravite from the Late Proterozoic ophiolitic melange of the Arabo-Nubian Shield, located in Egypt and Saudi Arabia, were structurally and chemically characterized by using crystal structure refinement based on single-crystal X-ray diffraction data, electron microprobe analysis, and Mossbauer spectroscopy. Structural formulae obtained by optimization procedures indicate disordering of Al, Mg, and Fe(2+) over the Y and Z sites, and an ordering of Fe(3+) at Y. The disordering can be explained by the substitution mechanisms 2(Y)Mg+(Z)Al+(W)OH = 2(Y)Al+(Z)Mg+(W)O(2-) and 2(Y)Fe(2+)+(Z)Fe(3+)+(W)OH = 2(Y)Fe(3+)+(Z)Fe(2+)+(W)O(2-), which are consistent with reducing the mismatch in dimensions between YO(6) and ZO(6) octahedra. To explain the Mg-Al disordering process, as well as the occurrence of B at the T site in tourmaline, analogies have been drawn between the crystal structure of tourmaline and that of lizardite. A critical constraint in both structures is the geometrical fit of the six-membered tetrahedral ring with the attached group of three YO(6) octahedra. In tourmaline, the disordering of Mg and Al over Y and Z relieves the strain due to the misfit in dimensions of the larger triads of edge-sharing MgO(6), octahedra and the smaller Si(6)O(18) tetrahedral rings. In Al-rich tourmaline, where the octahedral cluster is smaller, the strain can be relieved by incorporating B in the tetrahedra. An opposite effect is observed by substitution of Al for Si at the tetrahedral site in Mg-rich tourmaline. Because the Al radius is intermediate between those of Mg and Si, Al plays an important structural role in accommodating the potential misfit between YO(6), ZO(6), and TO(4) polyhedra. The amount of Al and its distribution in the structure strongly affects the values of the unit-cell parameters of tourmaline and yields volume variations according to a quadratic model. This results from the effect of (Z)Al combined with the occurrence of B at Tin Al-rich tourmaline. (Z)Al has a greater effect than (Y)Al as long as Al does not fully occupy the Z site
Comparative compressibility and structural behavior of spinel MgAl2O4 at high pressures: The independency on the degree of cation order
No full textThe equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural MgAl2O4 spinels with different degrees of order. The two samples studied were cut from a larger single crystal and one of them was experimentally disordered at high temperature. The two crystals, showing an inversion parameter x of 0.27 and 0.15 at ambient conditions,
were loaded together in a diamond anvil cell and their unit-cell edge was measured up to about 7.5 GPa at 14 different pressures. The unit-cell volume, V0, the bulk modulus, KT0, and its first pressure derivative, K′, were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: V0 = 529.32(2) Å3, KT0 = 193(1) GPa, K′ = 5.6(3) for the ordered sample and V0 = 528.39(2) Å3, KT0 = 192(1) GPa, K′ = 5.4(3) for the disordered one. Complete
intensity data were collected at 0, 0.44, 2.92, 7.34, and 8.03 GPa in a separate experiment. For the ordered and disordered samples the oxygen atomic coordinate u remains practically unchanged inside the investigated pressure range with an average value of 0.2633(5) and 0.2614(2), respectively. As a consequence, the polyhedral compressibilities are similar and are not influenced by the Mg/Al distribution over the two crystallographic sites. This also suggests that pressure has little or no influence
on the degree of order in the MgAl2O4 spinel
THE STABILITY OF BLOEDITE (NA2MG(SO4)2•4H2O) UNDER HIGH PRESSURE BY SINGLE CRYSTAL SYNCHROTRON X-RAY DIFFRACTION: A CONTRIBUTION TO KNOWLEDGE OF ASTEROIDS AND ICI SATELLITES
No full textThe mineral-chemistry of the pyroclastic deposits from Orvieto-Bagnoregio formation, Roman Magmatic Province, Central Italy
No full textThe high-pressure behavior of bloedite:a synchrotron single-crystal X-ray diffraction study.
No full textHigh-pressure single-crystal synchrotron X‑ray diffraction was carried out on a single crystal of bloedite [Na2Mg(SO4)24H2O] compressed in a diamond-anvil cell. The volume-pressure data, collected up to 11.2 GPa, were fitted by a second- and a third-order Birch-Murnaghan equation of state (EOS), yielding V0 = 495.6(7) Å3 with K0 = 39.9(6) GPa, and V0 = 496.9(7) Å3, with K0 = 36(1) GPa and K′ = 5.1 (4) GPa-1, respectively. The axial moduli were calculated using a Birch-Murnaghan EOS truncated at the second order, fixing K′ equal to 4, for a and b axes and a third-order Birch-Murnaghan EOS for c axis. The results were a0 = 11.08(1) and K0 = 56(3) GPa, b0 = 8.20(2) and K0 = 43(3) GPa, and c0 = 5.528(5), K0 = 40(2) GPa, K′ = 1.7(3) GPa-1. The values of the compressibility for a, b, and c axes are ba = 0.0060(3) GPa-1, bb = 0.0078(5) GPa-1, bc = 0.0083(4) GPa-1 with an anisotropic ratio of ba:bb:bc = 0.72:0.94:1. The evolution of crystal lattice and geometrical parameters indicates no phase transition up to 11 GPa. Sulfate polyhedra are incompressible, whereas the Mg polyhedral bulk modulus is 95 GPa. The sodium polyhedron is the softest part of the whole structure with a bulk modulus of 41 GPa. Pressure decreases significantly the distortion of Na coordination. Up to 10 GPa, the donor-acceptor oxygen distances decrease significantly and the difference between the two water molecules decreases with an increase in the strengths of hydrogen bonds. At the same time, the bond lengths from Na and Mg to O atoms of the water molecules decrease faster than other bonds to these cations suggesting that there is a coupling between the Na-Ow and Mg-Ow bond strengths and the “hydrogen transfer” to acceptor O atoms
THE STABILITY OF BLOEDITE (NA2MG(SO4)2•4H2O) UNDER HIGH PRESSURE BY SINGLE CRYSTAL SYNCHROTRON X-RAY DIFFRACTION: A CONTRIBUTION TO KNOWLEDGE OF ASTEROIDS AND ICI SATELLITES
No full textThe mineral-chemistry of the pyroclastic deposits from Orvieto-Bagnoregio formation, Roman Magmatic Province, Central Italy.
No full textRare sulfosalts from Vulcano, Aeolian Islands, Italy. VII. Cl-bearing galenobismutite
No full textCl-bearing galenobismutite (with Cl ranging from 0.12 to 1.45 wt.%), discovered among high-temperature fumarole incrustations at "La Fossa" crater, Island of Vulcano, Italy, has been chemically and structurally investigated. The mineral is commonly associated with bismuthinite, cannizzarite, lillianite, heyrovskýite and galena, and rarely with kirkiite, vurroite and other less well characterized Pb(Bi)-sulfochlorides. Electron-microprobe investigations show that Cl-bearing galenobismutite from Volcano has a wide compositional field (Pb:Bi atomic ratio ranging from 0.52 to 0.72). The statistical analysis of chemical data shows correlations between the main elements and chlorine, positive Pb versus Cl and negative Bi versus Cl, according to the coupled heterovalent substitution scheme: Pb2+ + Cl- ⇌ Bi3+ + S2-. The compositional variation of Cl-bearing galenobismutite can be represented by the general empirical formula Pb1+xBi2-x (S4-x-yClxSey), with 0 4σ(Fo)] and 1.49% for 376 reflections [Fo > 4σ(Fo)], respectively. The structure of Cl-bearing galenobismutite is topologically identical to that of Cl-free galenobismutite. The excess of Pb observed in Cl-bearing galenobismutite is incorporated in the trigonal prismatic M2gb and M3gb sites. No Pb-for-Bi substitution was observed at the octahedral position M1gb. We do not find evidence of a preferential distribution of Cl among the anion positions
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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