Refinement of the mangan-neptunite crystal structure

The crystal structure of mangan-neptunite, a manganese analogue of neptunite, has been refined in two space groups (Cc and C2/c). The mineral is monoclinic, with the correct space group Cc; the unit-cell dimensions are: a = 16.4821(6), b = 12.5195(4), c = 10.0292(3) Å, β = 115.474(1)°, and V = 1868.31 ų. The crystal structure has been refined to R ₁ = 0.0307 (wR ₂ = 0.0901) on the basis of 4892 observed reflections with |F _hkl | ≥ 4σ|F _hkl |. The most plausible acentric model is caused by the Ti- and (Fe, Mn, Mg)-ordering in the structure. Ti-octahedrons are strongly distorted and consist of short bond Ti-O (1.7 Å), one long bond (2.2 Å), and four equal bonds (2.0 Å). Fe-octahedrons are regularly shaped, with all Fe-O bonds being approximately identical.     

Refinement of the crystal structure of wöhlerite

Summary The crystal structure of wöhlerite (monoclinicP2₁,a=10.823(3,b=10.244(3),c=7.290(2) Å, =109.00(4)^o) was refined using 2343 independent diffractions toR ₁=0.019 andR ₂=0.025.The refinement confirms the main features of the structure model proposed byShibayeva andBelov (1962), namely the walls of large cation polyhedra interconnected as in cuspidine, with Si₂O₇ groups linked to the polyhedra. At difference from the structure proposed by the quoted authors, both the Si₂O₇ groups are clinging to edges of calcium polyhedra.The cation distribution in the polyhedral walls and the location of fluorine anions are clarified and discussed, resulting in the following crystal chemical formula Na₄(Ca_7.48Mn_0.20Fe_0.32)Zr₂(Nb_1.6Ti_0.4) (Si₂O₇)₄O₄F₂(O_1.6F_0.4).

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Verfeinerung der Kristallstruktur des Wöhlerits

Zusammenfassung Die Kristallstruktur des Wöhlerits (monoklinP2₁,a ₀=10,823(3),b ₀=10,244(3),c ₀=7,290(2) Å, =109,00(4)^o) wurde mit 2343 unabhängigen Reflexen aufR ₁=0,019 undR ₂=0,025 verfeinert.Die Verfeinerung bestätigt im wesentlichen Züge des vonShibayeva undBelov (1962) vorgeschlagenen Strukturmodells, nämlich Bänder aus den Polyedern um die großen Kationen, die — wie im Cuspidin — durch an den Polyedern hängenden Si₂O₇-Gruppen verknüpft sind.Die Kationenverteilung in den Polyederbändern und die Lokalisation der Fluor-Anionen wurden aufgeklärt und werden diskutiert; es ergibt sich folgende kristallchemische Formel Na₄(Ca_7,48 Mn_0,20Fe_0,32)(Zr₂(Nb_1,6 Ti_0,4) (Si₂O₇)₄O₄F₂(O_1,6F_0,4).

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Refinement of the crystal structure of låvenite

Summary The crystal structure of låvenite (monoclinicP2 ₁ /a,a=10.83(1),b=9.98(1),c=7.174(5) Å, =108.1(1)^o), was refined using 1743 independent reflections toR=0.032 andR _w=0.036. The refined structure is in agreement with the structure model proposed bySimonov andBelov (1960), characterized by the presence of walls of large cation polyhedra. These walls are four columns large, linked together by both direct connection as well as by Si₂O₇ groups. The cation distribution inside the wall is explained and compared with that found in other phase of the cuspidine group. The crystal chemical formula resulting from structural and chemical analyses on låvenite from Langesund (Norway) is (Na_6.50Ca_1.47Y_0.03) (Ca_0.93 Mn_1.26Fe _0.87 ²⁺ Ti_0.65Zr_0.18Mg_0.05) (Zr_3.42Nb_0.58) (Si₂O₇)₄O_3.82F_3.98.

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Verfeinerung der Kristallstruktur des Låvenits

Zusammenfassung Die Kristallstruktur des Låvenits (monoklinP2 ₁ /a,a=10,83(1),b=9,98(1),c=7,174(5) Å. =108,1(1)^o) wurde mit 1743 unabhängigen Reflexen aufR=0,032 undR _w=0,036 verfeinert.Die Verfeinerung bestätigt in wesentlichen Zügen das vonSimonov undBelov (1960) vorgeschlagene Strukturmodell, nämlich Bänder aus den Polyedern um die großen Kationen, die sowohl direkt als auch durch an den Polyedern hängende Si₂O₇-Gruppen miteinander verknüpft sind.Die Kationenverteilung in den Polyederbändern wurde aufgeklärt; es ergibt sich folgende kristallchemische Formel: (Na_6,50Ca_1,47Y_0,03) (Ca_0.93Mn_1,26Fe _0,87 ²⁺ Ti_0,65Zr_0,18Mg_0,05) (Zr_3,42Nb_0,58) (Si₂O₇)₄O_3,82F_3,98.

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With 3 Figures     

Lunar ilmenite (Refinement of the crystal structure)

The crystal structure of ilmenites from Mare Tranquillitatis and terrestrial basaltic rocks have been refined from X-ray data using least squares methods. The two structures are virtuallly identical both in structural parameters and relative degree of order. Occupation of Fe and Ti sites is highly ordered. The metal ions have low apparent charges (near 0.3) and formal oxidation states of Fe (II) and Ti (IV).     

Refinement of the crystal structure of bonshtedtite, Na3Fe(PO4)(CO3)

The crystal structure of bonshtedtite, Na₃Fe(PO₄)(CO₃) (monoclinic, P2₁/m, a = 5.137(4), b = 6.644(4), c = 8.908(6) Å, β = 90.554(14)°, V = 304.0(4) ų, Z = 2) has been refined to R ₁ = 0.041 on the basis of 1314 unique reflections. The structure is similar to that of other minerals of the bradleyite group. It is based on the [Fe(PO₄)(CO₃)]^3? layers oriented parallel to (001). The layers are formed by corner-sharing PO₄ tetrahedra and FeO₄(CO₃) complexes, where FeO₆ tetrahedra and CO₃ triangles are edge-shared. The topology of the octa-tetrahedral layer in bonshtedtite is similar to that of the autunite-group minerals, but it differs from the latter in terms of local topological properties.     

Refinement of the crystal structure of fornacite using the rietveld method

The crystal structure of fornacite Pb₂(Cu,Fe)[CrO₄(As,P)O₄OH] from the Berezovskii deposit (Central Urals, Russia) was refined by X-ray powder diffraction data using the Rietveld method. Fornacite is monoclinic, space group P2₁/c, the unit cell dimensions are a = 8.09015(12), b = 5.90913(9), c = 17.4839(2) Å, β = 109.99(2), V = 785.5(3) ų, and Z = 4. The structure was refined in the isotropic approximation of the atomic displacement parameters up to R _p = 0.0516, R _wp = 0.0692, R _B = 0.0229, and R _F = 0.0200. The fornacite structure is similar to that of minerals of the brackebuschite-group and consists of heteropolyhedral chains, built by the columns of edge-sharing Cu²⁺O₆ octahedra connected with isolated Cr⁶⁺O₄ and As⁵⁺O₄ tetrahedra. The chains are linked by ninefold Pb²⁺ polyhedra.     

Refinement of the crystal structure of papagoite,CaCuAlSi2O6(OH)3

Summary The crystal structure of papagoite, CaCuAlSi₂O₆(OH)₃, monoclinic,a = 2.926 (3),b = 11.496 (3),c = 4.696 (1) Å, = 100.81 (2)°, V = 685.4 (3) ų, space groupC 2/m,Z = 4, has been refined to anR index of 3.4% for 913 observed reflections measured with MoK X-radiation. The single unique Cu cation is surrounded by six anions in a pseudo-octahedral arrangement showing strong Jahn-Teller distortion. Edge-sharing rutile-like chains of pseudo-octahedrally coordinated Cu and Al extend in the Y direction, and are cross-linked into an octahedral sheet by Ca₂Ø₁₀ (O = unspecified ligand, O or OH) dimers. These sheets are linked by (Si₄O₁₂) rings to form a mixed tetrahedral-octahedral framework.

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Verfeinerung der Kristallstruktur von Papagoit, CaCuAlSi₂O₆(OH)₃

Zusammenfassung Die Kristallstruktur von Papagoit, CaCuAlSi₂O₆(OH)₃, monoklina = 12,926 (3),b = 11.496 (3),c = 4.696 (1) Å, = 100.81 (2)°,V = 685.4 (3) ų, RaumgruppeC2/m,Z = 4, wurde für 913 beobachtete, MoK-Röntgenstrahlung gemessene Reflexe aufR = 3,4% verfeinert. Das eine symmetrieunabhängige Kupfer-Kation ist pseudooktaedrisch von sechs Anionen mit einer starken Jahn-Teller-Verzerrung umgeben. Die pseudo-oktaedrisch koordinierten Cu- und Al-Atome werden über Kanten zu parallel zur y-Achse verlaufenden rutil-ähnlichen Ketten verbunden, die zusammen mit den Ca₂Ø₁₀-Gruppen (Ø = nicht spezifsierter Ligand, O bzw. OH) Oktaederschichten bilden. Diese Schichten werden durch (Si₄O₁₂)-Ringe zu einem aus Tetraedern und Oktaedern bestehenden Netzwerk verknüpft.

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Aktashite Cu6Hg3As4S12 from the Aktash deposit, Altai, Russia: Refinement and crystal chemical analysis of the structure

The composition and structure of aktashite from the Aktash deposit, Gorny Altai, Russia, have been studied by electron microprobe and X-ray structural analysis. On the basis of close compositions and crystal structures, the identity of aktashite from the Gal-Khaya and Aktash deposits has been demonstrated. Crystals of aktashite are of trigonal symmetry; the unit-cell dimensions are: a = 13.7500(4), c = 9.3600(3) Å, V = 532.54(8) ų, space group R3, Z = 3 for the composition of Cu₆Hg₃As₄S₁₂, R = 0.043. The structure of aktashite as a framework of vertex-shared HgS_4? and CuS_4? tetrahedrons of the same orientation is intimately related to the sphalerite-type structure. The earlier identified uncommon cluster group [As₄] has been verified and its parameters have been refined. It is shown that the structure may be represented as construction blocks (As₄S₁₂)_12? packed according to the law of the distorted cubic I-cell.     

Investigation of the crystal structure of newberyite,MgHPO4·3H2O,by single crystal neutron diffraction

Summary Elastic neutron diffraction data were collected from a single crystal of synthetic newberyite (=1.269 Å). The anisotropic least-squares refinement of 146 parameters against 1008 observations yieldedR=1.9%. The environment of water molecules, hydrogen bonding, especially of bifurcated geometry, and the thermal motion of hydrogen atoms are discussed.

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Untersuchung der Kristallstruktur von Newberyit, MgHPO₄·3H₂O, durch Einkristallneutronenbeugung

Zusammenfassung Gegen 1008 elastische Neutronenbeugungsreflexe eines sythetischen Newberyit Einkristalles wurden 146 freie Parameter bisR=1,9% anisotrop verfeinert. Die Bindungsgeometrie der Kristallwassermoleküle, die Wasserstoffbrückenbindungen, zumal Gabelungen, und die thermischen Schwingungsparameter der Protonen werden behandelt.

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The crystal structure of sarkinite,Mn2AsO4(OH)

Summary Sarkinite is a basic manganese arsenate, Mn₂AsO₄(OH). The lattice parameters are:a=12.779 (2) Å,b=13.596 (2) Å,c=10.208 (2) Å, =108°53 (6). Space groupP2₁/a,Z=16. The crystal structure has been solved by direct methods from three-dimensional X-ray diffractometer data and refined by least-squares methods toR=0.052 for 3519 independent reflections. The crystal structure is built up by a three-dimensional framework of MnO₄(OH)₂ octahedra, MnO₄(OH) trigonal bipyramids and AsO₄ tetrahedra, as found in wagnerite. Isotypy of sarkinite with triploidite is confirmed.

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Die Kristallstruktur des Sarkinits, Mn₂AsO₄(OH)

Zusammenfassung Die Kristallstruktur des basischen Manganarsenates Sarkinit, Mn₂AsO₄(OH), mit den Gitterkonstantena=12,779 (2) Å,b=13,596 (2) Å,c=10,208 (2) Å, =108°53 (6). RaumgruppeP2₁/a,Z=16, wurde mit dreidimensionalen Röntgendiffraktometermessungen durch direkte Methoden gelöst und nach dem kleinste-Quadrate-Verfahren verfeinert (R=0,052 für 3519 unabhängige Reflexe). Die Struktur besteht aus einem dreidimensionalen Gerüst aus MnO₄(OH)₂-Oktaedern, trigonalen Bipyramiden von MnO₄(OH) und AsO₄-Tetraedern wie in Wagnerit. Die Isotypie von Sarkinit mit Triploidit wurde bestätigt.

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The temperature dependence of the crystal structure of berlinite, a quartz-type form of AlPO4

The temperature dependence of atomic positions, mean-square displacements and probability density functions in a synthetic specimen of berlinite was analyzed using X-ray single crystal data measured at fourteen values of temperature. The characteristic features and the temperature dependence of the structure of berlinite were found to be quite similar to those of quartz, in detail. With increasing temperature, the equilibrium position of an atom, if expressed in fractional coordinates, appears to move steadily toward its β-position along a straight line, and finally attain the β-phase positions after abrupt displacements in a narrow temperature range around 583?°C. The temperature dependence of the displacements of atoms from the corresponding high-symmetry β-positions, in α-berlinite, is well fitted with a classical Landau type expression of first-order phase transitions. The highly anisotropic mean square displacements, 〈u ²〉, of O atoms increase with increasing temperature, especially in a narrow range just below the α–β transition, in the directions nearly perpendicular to the librating Al-O-P bonds, attaining a local maximum of 0.115–0.117 Ų just above the transition point. With varying temperature, the principal axes for the O atoms change their directions smoothly toward those of the high temperature phase, while the axis with the largest 〈u ²〉 of the Al or P thermal ellipsoid remains in 〈100〉. These diffraction results are interpreted in terms of displacive structure transition involving both the ordered α- and β-forms. The Al–O and P–O bond distances are nearly constant around 1.73 and 1.53 Å, respectively, through the experimental range of temperature.     

The crystal structure of scotlandite,PbSO3

Summary The crystal structure of scotlandite —a=4.505(2),b=5.333(2),c=6.405(6) Å, =106.24(3)^o; space groupP2₁/m; cell content 2 {PbSO₃} — was determined from singlecrystal X-ray diffractometer data. Scotlandite is isotypic with molybdomenite, PbSeO₃. Lead is coordinated to nine oxygen atoms with Pb-O_av=2.75 Å, and possibly further to one sulphur atom with Pb–S=3.46 Å. The average S–O distance in the pyramidal SO₃ group is 1.52 Å. The structural relationships to cerussite, PbCO₃, are discussed.

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Die Kristallstruktur des Scotlandits, PbSO₃

Zusammenfassung Die Kristallstruktur des Scotlandits —a=4,505(2),b=5,333(2),c=6,405(6) Å, =106,24(3)^o; RaumgruppeP2₁/m; Zellinhalt 2 {PbSO₃} — wurde aus Einkristall-Röntgendiffraktometerdaten bestimmt. Scotlandit ist mit Molybdomenit, PbSeO₃, isotyp. Blei wird von neun Sauerstoffatomen mit Pb–O_av=2,75 Å und möglicherweise zusätzlich von einem Schwefelatom mit Pb–S=3,46 Å koordiniert. Der durchschnittliche S–O-Abstand in der pyramidalen SO₃-Gruppe mißt 1,52 Å. Die strukturellen Beziehungen zu Cerussit, PbCO₃, werden diskutiert.

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Garavellite, FeSbBiS4, from the Caspari mine, North Rhine-Westphalia, Germany: composition, physical properties and determination of the crystal structure

Summary Garavellite, FeSbBiS₄, was found in a sample of the mineralogical collection of the Natural History Museum of the University of Florence. The sample is from the Cu–Fe deposit of Caspari, Saverland, North Rhine-Westphalia, Germany. Garavellite occurs as very rare, elongated prismatic crystals up to 100 μm in length, spatially associated with large berthierite crystals, bismuthinite, chalcopyrite, and siderite. It does not contain inclusions of or intergrowths with other minerals. Macroscopically garavellite is grey in colour and shows a grey-black streak. The Vickers hardness (VHN₅₀) is 206 kg/mm². In plane-polarized incident light garavellite is grey in colour, with distinct bireflectance. Reflectance percentages for R_min and R_max are 33.8, 41.8 (471.1 nm), 33.3, 40.9 (548.3 nm), 32.7, 39.5 (586.6 nm), and 32.4, 38.8 (652.3 nm), respectively. Garavellite is orthorhombic, space group Pnam, with the following unit-cell parameters: a = 11.413(1) ?, b = 14.164(1) ?, c = 3.759(1) ?, V = 607.7(2) ?³, and Z = 4. Electron microprobe analyses give the chemical formula Fe_0.94Cu_0.01As_0.01Sb_1.02Bi_0.99S_4.03. The crystal structure has been solved and refined to R = 2.38%. It consists of FeS₆ octahedra forming edge-sharing chains parallel to [001] with the Sb³⁺ and Bi³⁺ cations inserted between the chains. The crystal-chemical relationships with berthierite as well as the different lone-pair stereochemical activities of antimony and bismuth in the two structures are discussed.     

Refinement and comparison of the crystal structures of a dolomite and of an Fe-rich ankerite

Summary The crystal structures of a dolomite and of an iron rich ankerite were refined from three-dimensional X-ray data by least squares methods toR0.03. The angle of rotation of the CO₃ group is 6.35(5)^o in dolomite but only 5.28(5)^o in ankerite, a consequence of the different sizes of the (Mg, Fe)O₆ octahedra. The carbonate group deviates very slightly from planarity in both minerals. The Ca–O distances are somewhat larger in both minerals than in calcite; the Mg–O distance in dolomite, on the contrary, is somewhat smaller than in magnesite.

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Verfeinerung und Vergleich der Kristallstrukturen eines Dolomits und eines Fe-reichen Ankerits

Zusammenfassung Die Kristallstrukturen eines Dolomits und eines eisenreichen Ankerits wurden aus dreidimensionalen Röntgendaten mittels der Methode der kleinsten Quadrate aufR0,03 verfeinert. Der Verdrehungswinkel der CO₃-Gruppe ist im Dolomit 6,35(5)^o, im Ankerit nur 5,28(5)^o; das ist eine Folge der unterschiedlichen Größen der (Mg, Fe)O₆-Oktaeder. Die Gestalt der Karbonatgruppe weicht in beiden Mineralen ganz leicht von planar ab. Die Ca–O-Abstände sind in beiden Mineralen etwas größer als im Calcit, der Mg–O-Abstand ist hingegen im Dolomit etwas kleiner als im Magnesit.

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With 1 Figure