1,721,087 research outputs found
Double-stranded dimetallic helicates: assembling– disassembling driven by the CuI /CuII redox change and the principle of homochiral recognition
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Crystal structure refinement of margarosanite PbCa2Si3O9 and relationship with walstromite BaCa2Si3O9
Full text linkX-ray single-crystal diffraction data of margarosanite from the Parker Shaft, North Mine Hill, Franklin, New Jersey were collected using a Bruker-AXS three-circle diffractometer equipped with the Smart-Apex CCD detector. Margarosanite is triclinic, P-1, with unit cell parameters: a = 6.7370(3) Å, b = 9.5295(4) Å, c = 6.6961(3) Å, alfa = 110.185(2)°, beta = 103.048(2)°, gamma = 83.037(2)°, V = 392.63(3) Å3, Z = 2. The new structure refinement performed on 9206 diffraction data in the 2θ range 4 < θ < 107° (R1 = 0.035) improves the crystallographic details of a crystal structure described in the seventies and not further studied up to now. The crystal structure of margarosanite is defined by two different structural layers parallel to (-1 0 1): a Si layer constituted by isolated 3-fold rings of SiO4 tetrahedra and a Ca layer made by dimers of CaO6 octahedra and chains of CaO8 polyhedra interconnected via edge-sharing to form a Ca-polyhedral sheet. Additional edge-sharing chains of distorted 8-fold coordinated Pb polyhedra are also present. The crystallographic comparison between margarosanite and walstromite, the Ba-bearing natural counterpart, has been carried out in order to emphasize that the two minerals are isostructural
Aschamalmite (Pb6Bi2S9): crystal structure and ordering scheme for Pb and Bi atoms
No full textThe first single-crystal structure refinement of aschamalmite (Pb6Bi2S9) from Susa Valley (Piedmont, Italy) is reported. The mineral is monoclinic, C2/m, a = 13.719(1) Å, b = 4.132(1) Å, c = 31.419(3) Å, β = 90.94(1)°, V = 1 780.8(4) Å3, Z = 4. The Pb6Bi2S9 compound crystallizes also in an orthorhombic form as heyrovskyite (Cmcm) and our study is focused on understanding the reason leading to a change in symmetry. The aschamalmite structure forms because of ordering between Pb and Bi on the margins of the two octahedral layers that are symmetrically equivalent in heyrovskyite. The two alternate set of octahedral slabs are not related by a crystallographic mirror plane and the symmetry decreases to monoclinic. The cation ordering couples opposite sequences of Pb and Bi octahedra at the margins of slabs. In particular, the succession [Me4A]Bi-[Me5A]Pb-[Me4A]Bi-[Me5A]Pb faced to the series [Me4B]Pb-[Me5B]Bi-[Me4B]Pb-[Me5B]Bi occurs in about 70% of the unit-cells of the crystal, while the contrary sequence ([Me4A]Pb-[Me5A]Bi-[Me4A]Pb-[Me5A]Bi faced to [Me4B]Bi-[Me5B]Pb-[Me4B]Bi-[Me5B]Pb) occurs in the remaining unit-cells. The marginal octahedra have ideal populations (a.p.f.u.): [Me4A]1.40Bi+0.60Pb, [Me4B]1.40Pb+0.60Bi, [Me5A]1.40Pb+0.60Bi, [Me5B]1.40Bi+0.60Pb, in agreement with our structure-refinement results.
The probable site populations for pure heyrovskyite have been proposed, as well as the reasons that prevent the formation of a completely ordered monoclinic phase
High resolution X-ray diffraction data of pirssonite from Searles Lake, San Bernardino County, California
No full textCrystal structure of pirssonit
Häggite from the Gambatesa mine, Liguria, Italy: a refinement of the crystal structure
No full textThe crystal structure and crystal-chemical features of häggite have been confirmed on the basis of new X-ray diffraction data collected on a small single crystal from Gambatesa mine (Italy). The crystal structure of häggite is monoclinic, C2/m space group, with a = 12.2203(21), b= 2.9991(5), c = 4.8334(9) Å; = 98.384(4), V = 175.25(5) Å3, Z = 2. The crystal structure contains one independent V octahedron populated by V3+ and V4+ and the final bond valence analysis confirms the crystal-chemical formula of natural häggite : [V](V3+V4+)2[O1]O2-[O2](OH)2[O3](O2-OH)2. Only the O1 atom site, shared between two octahedra, shows an anomalously low incident-bond valence sums (1.72 valence units). The possible hydrogen bond motif in presence of 3 OH groups imposes the use of half populated H sites, which cannot be observed in the final ΔF map. The refinement of the complete structural model converges to a final Robs of 0.039 for 244 reflection with IO>3σ(IO)
Aschamalmite (Pb6Bi2S9): crystal structure refinement and ordering scheme for Pb and Bi atoms
No full textStructure refinement of okanoganite-Y: comparison with vicanite-Ce and preliminary crystal-chemical results.
No full textCorindone. I cromofori nei cristalli policromi di Amboarohy, Ihosy, Madagascar. Risultati degli studi microanalitici
No full textThe crystal-chemistry of okanoganite-(Y): an example of (sims+empa+sref) multi-analytical approach to solve a complex problem.
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