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  • 1
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 739-746 
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of a crystal of natural melilite from San Venanzo, Umbria (Italy) of the general formula X2T1(T2)2O7, where X = Ca0.945Sr0.005Na0.04K0.01, T1 = Mg0.92Al0.08 and T2 = Si0.99Al0.01, has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P\bar 421m:p4mg, cell parameters a = 7.860 (1), c = 5.024 (1) Å, modulation vectors q1 = 0.2815 (3)(a* + b*), q2 = 0.2815 (3)(−a* + b*). The data collection was performed on a KumaCCD diffractometer. The structure was refined from 7606 reflections to final R = 0.0481. A special modification of the refinement program Jana2000 was necessary to take into account overlapping of satellite reflections m × n = ±1, which could not be properly separated in the integration procedure. The final model includes modulations of the atomic positions as well as modulations of the thermal parameters. The latter are induced by strong differences in the neighbourhood of the actual modulated positions. The occupational modulation was neither significant for X nor for T1 sites and the sites were supposed to be occupied only by Ca and Mg, respectively. As a consequence of the Ca and O positional modulations six-, seven- and eightfold Ca coordination occur throughout the structure and the thermal ellipsoid changes its shape correspondingly. The positional modulation of the atoms causes variations in the interatomic distances which, however, do not affect bond-valence sums considerably, but induce flattening and rotation in T1 and T2 tetrahedra, respectively.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2016-06-13
    Description: We designed a plate impact shock recovery experiment to simulate the starting materials and shock conditions associated with the only known natural quasicrystals, in the Khatyrka meteorite. At the boundaries among CuAl5, (Mg0.75Fe2+0.25)2SiO4 olivine, and the stainless steel chamber walls, the recovered specimen contains numerous micron-scale grains of a quasicrystalline phase displaying face-centered icosahedral symmetry and low phason strain. The compositional range of the icosahedral phase is Al68–73Fe11–16Cu10–12Cr1–4Ni1–2 and extends toward higher Al/(Cu+Fe) and Fe/Cu ratios than those reported for natural icosahedrite or for any previously known synthetic quasicrystal in the Al-Cu-Fe system. The shock-induced synthesis demonstrated in this experiment reinforces the evidence that natural quasicrystals formed during a shock event but leaves open the question of whether this synthesis pathway is attributable to the expanded thermodynamic stability range of the quasicrystalline phase at high pressure, to a favorable kinetic pathway that exists under shock conditions, or to both thermodynamic and kinetic factors.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 3
    Publication Date: 2020-08-12
    Description: The recent discovery in high-pressure experiments of compounds stable to 24–26 GPa with Fe4O5, Fe5O6, Fe7O9, and Fe9O11stoichiometry has raised questions about their existence within the Earth’s mantle. Incorporating both ferric and ferrous iron in their structures, these oxides if present within the Earth could also provide insight into diamond-forming processes at depth in the planet. Here we report the discovery of metallic particles, dominantly of FeNi (Fe0.71Ni0.24Cu0.05), in close spatial relation with nearly pure magnetite grains from a so-called superdeep diamond from the Earth’s mantle. The microstructural relation of magnetite within a ferropericlase (Mg0.60Fe0.40)O matrix suggests exsolution of the former. Taking into account the bulk chemistry reconstructed from the FeNi(Cu) alloy, we propose that it formed by decomposition of a complex metalMoxide (M4O5) with a stoichiometry of (Fe3+2.15Fe2+1.59Ni2+0.17Cu+0.04)Σ=3.95O5. We further suggest a possible link between this phase and variably oxidized ferropericlase that is commonly trapped in superdeep diamond. The observation of FeNi(Cu) metal in relation to magnetite exsolved from ferropericlase is interpreted as arising from a multistage process that starts from diamond encapsulation of ferropericlase followed by decompression and cooling under oxidized conditions, leading to the formation of complex oxides such as Fe4O5that subsequently decompose at shallowerP-Tconditions.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 4
    Publication Date: 2019-08-18
    Electronic ISSN: 2053-2733
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 5
    Publication Date: 2018
    Description: 〈div data-abstract-type="normal"〉〈p〉During a study of the ore minerals belonging to the recently discovered Shuangjianzishan Ag–Pb–Zn deposit in NE China, we have discovered exceptional selenium enrichment in canfieldite (up to 11.6 wt.% of Se). Incorporation of Se into canfieldite has been investigated by an integrated approach using field emission scanning electron microscopy, electron microprobe and single-crystal X-ray diffraction. Canfieldite has been identified as one of the dominant Ag-bearing ore minerals in the studied deposit, which occurs mostly in slate-hosted vein type Ag–Pb–Zn ore bodies. Selenium is either homogeneously or, remarkably, heterogeneously distributed in the different canfieldite fragments studied. Chemical variations of Se are mostly attributable to a series of retrograde reactions resulting in diverse decomposition and exsolution of primary phases during cooling, or alternatively, related to influxes of Se-rich fluids during the formation of canfieldite. To evaluate the effects of the Se-for-S substitution in the structure, a crystal of Se-rich canfieldite [Ag〈span〉7.98〈/span〉Sn〈span〉1.02〈/span〉(S〈span〉4.19〈/span〉Se〈span〉1.81〈/span〉)〈span〉Σ6.00〈/span〉] was investigated. The unit-cell parameters are: 〈span〉a〈/span〉 = 10.8145(8) Å and 〈span〉V〈/span〉 = 1264.8(3) Å〈span〉3〈/span〉. The structure was refined in the space group 〈span〉F〈/span〉〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190709073523038-0660:S0026461X18001585:S0026461X18001585_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉3〈span〉m〈/span〉 to 〈span〉R〈/span〉〈span〉1〈/span〉 = 0.0315 for 194 independent reflections, with 20 parameters. The crystal structure of Se-rich canfieldite was found to be topologically identical to that of pure canfieldite. If the short Ag–Ag contacts are ignored (due to the disorder), the two Ag atoms in the structure can be considered as three-fold (Ag1) and four-fold (Ag2) coordinated. Tin adopts a regular tetrahedral coordination. As in the case of Te-rich canfieldite, the refinement of the site-occupancy factor indicates that Se is disordered over the three anion positions.〈/p〉〈/div〉
    Print ISSN: 0026-461X
    Electronic ISSN: 1471-8022
    Topics: Geosciences
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  • 6
    Publication Date: 2012-02-01
    Description: In order to evaluate the effects of Te-for-S substitution in the minerals of the argyrodite group, the crystal structure and chemical composition of a crystal of Te-rich canfieldite from the Lengenbach quarry, Binntal, Switzerland, was investigated. The unit-cell parameters are a 11.0003(6) Å and V 1331.1(1) Å3. The structure was solved and refined in the space group F4̅3m to R1 = 0.0308 for 194 independent reflections and 21 parameters. Quantitative analysis led to the chemical formula Ag8.05(Sn1.03Ge0.01)∑1.04(S3.95Te1.95Se0.01)∑5.91, ideally Ag8Sn(S,Te)6. The crystal structure of Te-rich canfieldite was found to be topologically identical to that of putzite, (Cu4.7Ag3.3)GeS6. Neglecting the short Ag-Ag contacts (due to disorder), the two Ag atoms in the structure can be considered as three-fold (Ag1) and four-fold (Ag2) coordinated. Tin adopts a regular tetrahedral coordination. The refinement of the site-occupancy factor indicates Te to be disordered over the three anion positions (i.e., X1, X2, and X3), with a preference for the X1 site.
    Print ISSN: 0008-4476
    Topics: Geosciences
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  • 7
    Publication Date: 2012-04-01
    Description: Arsenopyrite from La Roche-Balue quarry (Loire-Atlantique department, France), with the stoichiometric composition FeAsS, has been studied by X-ray single-crystal diffraction and 57Fe Mössbauer spectroscopy. Its unit cell is a 5.7612(8), b 5.6841(7), c 5.7674(8) Å, β 111.721(8)°, and V 175.46(4) Å3 (Z = 4). Taking into account very fine ubiquitous twinning on {101}, its crystal structure has been refined in the space group P21/c on the basis of 758 unique reflections [Fo 〉 4σ(Fo)] to R1 = 0.0298. Within uncertainty limits, it indicates three unmixed Fe, As and S positions. The 57Fe Mössbauer spectrum of this arsenopyrite shows two broad absorption peaks that were fitted using the superposition of three doublets denoted as A, B and C, with parameters relative to area S, isomer shift δ, and quadrupole splitting ΔEQ, as follows: SA 82.2%, δA 0.24(1) mm/s, ΔEQA 1.12(2) mm/s; SB 8.5%, δB 0.25(1) mm/s, ΔEQB 0.69(2) mm/s; SC 9.3%, δC 0.26(1) mm/s, ΔEQC 1.49(2) mm/s. This set of values agrees well with the most recently published results for arsenopyrite samples with a composition close to stoichiometry. The three distinct Fe positions indicated by Mössbauer spectroscopy are not visible in the X-ray study. Whereas the main doublet A would seem to correspond to the FeAs3S3 octahedron of the ideal structure, the two minor doublets B and C, with a ΔEQ shift towards marcasite and löllingite, respectively, may be due to local disorder (e.g., twin contact walls) changing the octahedral coordination of Fe to unequal As:S ratios.
    Print ISSN: 0008-4476
    Topics: Geosciences
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  • 8
    Publication Date: 2012-07-01
    Description: Manganoquadratite, ideally AgMnAsS3, is a new mineral from the Uchucchacua polymetallic deposit, Oyon district, Catajambo, Lima Department, Peru. It occurs as dark gray, anhedral to subhedral grains up 0.5 mm across, closely associated with alabandite, Mn-rich calcite, Mn-rich sphalerite, proustite, pyrite, pyrrhotite, tennantite, argentotennantite, stannite, and other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S. Manganoquadratite is opaque with a metallic luster and possesses a reddish-brown streak. It is brittle, the Vickers microhardness (VHN10) is 81 kg/mm2 (range 75–96) (corresponding Mohs hardness of 2–2½). The calculated density is 4.680 g/cm3 (on the basis of the empirical formula). In plane-polarized reflected light, manganoquadratite is moderately bireflectant and very weakly pleochroic from dark gray to a blue gray. Internal reflections are absent. Between crossed polars, the mineral is anisotropic, without characteristic rotation tints. Reflectance percentages (Rmin and Rmax) for the four standard COM wavelengths are 29.5, 31.8 (471.1 nm), 28.1, 30.5 (548.3 nm), 27.3, 29.3 (586.6 nm), and 26.0, 28.2 (652.3 nm), respectively.Manganoquadratite is tetragonal, space group P4322, with unit-cell parameters: a = 5.4496(5), c = 32.949(1) Å, V = 978.5(1) Å3, c:a = 6.046, Z = 8. The structure, refined to R1 = 0.0863 for 907 reflections with Fo 〉 4σ(Fo), consists of a stacking along [001] of alabandite-like Mn2S2 layers connected to each to other by a couple of AgAsS2 sheets where As3+ forms typical AsS3 groups, whereas Ag+ cations are fivefold coordinated. The six strongest lines in the observed X-ray powder-diffraction pattern [d in Å (I/I0) (hkl)] are: 3.14 (60) (116), 2.739 (50) (0 0 12), 2.710 (100) (200), 1.927(70) (2 0 12 + 220), 1.645 (25) (3 0 16), and 1.573 (20) (22 12).Electron microprobe analyses gave the chemical formula (on the basis of six atoms) (Ag0.95Cu0.05)∑=1.00 (Mn0.96Pb0.04)∑=1.00(As0.87Sb0.14)∑=1.01S2.99, leading to the simplified formula AgMnAsS3.The name was chosen to indicate the close analogy of the formula and unit-cell dimensions with quadratite, Ag(Cd,Pb)(As,Sb)S3. The new mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA 2011-008.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 9
    Publication Date: 2011-02-01
    Description: The crystal structure and chemical composition of a crystal of Na2MgSi5O12 garnet synthesized in the model system Mg3Al2Si3O12-Na2MgSi5O12 at 17.5 GPa and 1700 {degrees}C have been investigated. Quantitative analysis leads to the following formula: Na1.98Mg1.00Si5.01O12. Na2MgSi5O12 garnet was found to be tetragonal, space group I41/acd, with lattice parameters a = 11.3966(6), c = 11.3369(5) A, V = 1472.5(1) A3. The structure was refined to R = 5.13% using 771 independent reflections. Sodium and Mg are disordered at the X sites (with a mean bond distance of 2.308 A for both the sites), whereas Si is ordered at both the Y (mean: 1.793 A) and Z sites (means: 1.630 and 1.624 A). Na-bearing majoritic garnet may be an important potential sodium concentrator in the lower parts of the upper mantle and transition zone. The successful synthesis of the Na2MgSi5O12 end-member and its structural characterization is of key importance because the study of its thermodynamic constants combined with the data of computer modeling provides new constraints on thermobarometry of majorite garnet assemblages.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 10
    Publication Date: 2011-05-01
    Description: The mineral fettelite, [Ag6As2S7][Ag10HgAs2S8], has been recently structurally characterized. On the whole, the structure can be described as a regular succession of two module layers stacked along the c-axis: a first module layer (labeled A) with composition [Ag6As2S7]2- and a second module layer (labeled B) with composition [Ag10HgAs2S8]2+. Here we report an integrated high-temperature single-crystal X-ray diffraction (HT-SCXRD), differential scanning calorimetry (DSC), and complex impedance spectroscopy (CIS) study on a sample of fettelite from Chanarcillo, Copiapo Province, Chile. DSC and conductivity measurements pointed out that fettelite shows a ionic-transition at about 380 K. HT-SCXRD experiments confirmed the phase transition toward a disordered phase having a trigonal symmetry with the a and b unit-cell parameters halved. In the HT-structure, the disorder is located in the B layer where the Ag-Hg cations are found in various sites corresponding to the most pronounced probability density function locations of diffusion-like paths. This indicates that at least two polytypes could exist for fettelite, the ordered, monoclinic RT-structure (space group C2), and a fast ion conducting, trigonal, disordered HT-form (space group P[IMG]f1.gif" ALT="Formula" BORDER="0"〉m1) with a and b parameters halved. The two unit-cell types (corresponding to two different polytypes) could be also found in nature. Slightly different chemical compositions for different fettelite samples (e.g., different Ag/Hg ratios) could play a crucial role as driving forces for different unit-cell stabilizations.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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