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Bindi L.,University of Florence | Bindi L.,CNR Institute of Geosciences and Earth Resources | Pinch W.W.,19 Stonebridge Lane
Canadian Mineralogist | Year: 2014

The crystal structure of the mineral cameronite, a rare copper-silver telluride, was solved using intensity data collected from a crystal of the holotype material from the Good Hope mine, Vulcan, Colorado (USA). The study revealed that the structure is monoclinic, space group C2/c, with cell parameters: a 17.906(1), b 17.927(1), c 21.230(2) Å, β 98.081(8)°, and V 6747.2(8) Å3. The refinement of an anisotropic model led to an R index of 0.0216 for 9688 independent reflections. Nineteen Te sites and 14 metal (M) sites occur in the crystal structure of cameronite. Six M sites host Cu with variable amounts of Ag, whereas the remaining eight M sites host Cu only. All of the atoms are in tetrahedral coordination and complex MTe4, M(Te3M), M(Te2M2), TeM4, Te(M3Te), Te(M2Te2), Te(MTe3), and TeTe4 crystal-chemical environments are formed, as is typical of many intermetallic compounds. The short bond distances observed mainly among the Te atoms are discussed in relation to other copper and silver tellurides and pure metals. Electron microprobe analyses of the crystal used for the structural study led to the formula Cu7.08Ag1.06Fe0.01S0.01Te9.84, on the basis of 18 atoms. On the basis of information gained from the structural and chemical characterization, the crystal-chemical formula was revised, yielding Cu5-x(Cu,Ag)3+xTe10 (Z 14) with x = 0.43 instead of Cu7AgTe10 (Z 16) as previously reported. Source

Hawthorne F.C.,University of Manitoba | Abdu Y.A.,University of Manitoba | Ball N.A.,University of Manitoba | Pinch W.W.,19 Stonebridge Lane
American Mineralogist | Year: 2013

Carlfrancisite, Mn2+ 3(Mn2+,Mg,Fe 3+,Al)42(As3+O3)2(As 5+O4)4[(Si,As5+)O4] 6[(As5+,Si)O4]2(OH)42, is a new mineral from the Kombat mine, Otavi Valley, Namibia, and occurs as curved platy aggregates ∼2 cm across on a matrix of Mn arsenates and oxides. It is yellowy orange to pale yellow with a very pale-yellow streak, translucent with a vitreous to opalescent luster, and does not fluoresce under ultraviolet light. Cleavage is micaceous on {001}, and no parting or twinning was observed. Mohs hardness is 3, and carlfrancisite is brittle with a hackly fracture. The calculated density is 3.620 g/cm3. Optical properties were measured with a Bloss spindle stage for the wavelength 590 nm using a gel filter. The indices of refraction are e = 1.756, ω = 1.758, and it is non-pleochroic. Carlfrancisite is trigonal, space group R3̄c, a = 8.2238(2), c = 205.113(6) Å, V = 12 013.5(4) Å3, Z = 6, c:a = 1:24.941. The seven strongest lines in the X-ray powder-diffraction pattern are as follows: d (Å), I, (hkl): 2.826, 100, (2̄ 2 44); 2.371, 88, (2̄ 3 40, 1̄ 3 41); 1.552, 84, (1̄ 5 0); 2.676, 63, (2̄ 3 7); 3.243, 54, (0 1 56, 1̄ 2 39); 4.107, 48, (1̄ 2 0); 2.918, 47, (0 2 40). Chemical analysis by electron microprobe and crystal-structure refinement gave As 2O5 13.07, As2O3 3.18, P 2O5 0.50, V2O5 0.74, SiO2 8.96, Al2O3 0.78, FeO 0.22, MnO 53.25, MgO 9.37, H 2O(calc) 8.42, sum 98.49 wt%. The H2O content and the valence states of As were determined by crystal-structure analysis. The empirical formula is Mn2+ 33.55Mg10.39Fe 2+ 0.14Al0.68 As3+ 1.44(Si6.67P0.32V5+ 0.37As5+ 5.08)O54 (OH)42 on the basis of 96 anions with (OH) = 42 apfu. The structure of carlfrancisite is closely related to that of mcgovernite and turtmannite. Source

Tait K.T.,Royal Ontario Museum | Yang H.,University of Arizona | Downs R.T.,University of Arizona | Li C.,University of Arizona | Pinch W.W.,19 Stonebridge Lane
American Mineralogist | Year: 2010

Esperite from Franklin, New Jersey, was first described by Moore and Ribbe (1965) as monoclinic with a well-developed "superlattice" a = 2 × 8.814(2) Å, b = 8.270(3) Å, c = 2 × 15.26(1) Å, β ≈ 90°, space group P21/n (subcell), and the chemical formula PbCa3(ZnSiO4)4. They attributed "superlattice" reflections to the ordered distributions of Pb and Ca cations over four beryllonite-type subcells for esperite with the Ca:Pb ratio greater than 2:1. We examined two esperite fragments from the type sample using single-crystal X-ray diffraction, electron microprobe analysis, and Raman spectroscopy. Although both fragments have Ca:Pb ≈ 1.8, one exhibits the "superlattice" reflections as observed by Moore and Ribbe (1965), whereas the other does not. The sample without "superlattice" reflections has unit-cell parameters a = 8.7889(2), b = 8.2685(2), c = 15.254(3) Å, β = 90.050(1)°, V = 1108.49(4) Å3, and the chemical composition Pb1.00(Ca1.86Fe2+ 0.07Mn0.04Cr3+0.02) Σ=1.99(Zn1.00Si1.00O4)3. Its crystal structure was solved in space group P21/n (R1 = 0.022). Esperite is isostructural with beryllonite, NaBePO4, and its ideal chemical formula should, therefore, be revised to PbCa 2(ZnSiO4)3, Z = 4. The ZnO4 and SiO4 tetrahedra in esperite share corners to form an ordered framework, with Pb2+ occupying the nine-coordinated site in the large channels and Ca2+ occupying the two distinct octahedral sites in the small channels. The so-called "superlattice" reflections are attributed to triple twins, a trilling of ∼60° rotational twinning around the b axis, similar to those observed in many other beryllonite-type materials. A phase transformation from a high-temperature polymorph to the esperite structure is proposed to be responsible for the twinning formation. Source

Kampf A.R.,Natural History Museum of Los Angeles County | Mills S.J.,University of British Columbia | Pinch W.W.,19 Stonebridge Lane
Mineralogy and Petrology | Year: 2011

Plumboselite, ideally Pb3O2(SeO3), is a new selenite (IMA2010-028) from the Tsumeb mine, Namibia. It occurs as fibres on clausthalite and is also associated with smithsonite, mimetite and vaterite. Plumboselite occurs in subparallel to divergent clusters of thin, flattened, colourless fibres up to 0.3 mm in length, but not exceeding 5 μm in width and 2 μm in thickness. The fibres are elongated parallel to [001] and flattened on {010}, with {010} the only form observed. The crystals have a dull to adamantine lustre and a white streak. The tenacity is brittle and the Mohs hardness is estimated to be between 2 and 3. Plumboselite crystals are optically biaxial with parallel extinction and are length fast in all orientations. The Gladstone-Dale relationship predicts nav = 2.115. The high indices of refraction and small crystal size prevented the determination of other optical properties. The calculated density is 7.814 g/cm3. The empirical formula (based on 5 O atoms) is Pb2.92Ca0.01Se1.03O5. Plumboselite is orthorhombic, space group Cmc21, a = 10.5384(11), b = 10.7452(13), c = 5.7577(7) Å, V = 651.98(12) Å3 and Z = 4. The five strongest lines in the powder X-ray diffraction pattern are [dobs in Å/(I)/hkl]: 3.155/(100)/221; 1.956/(26)/042,402; 2.886/(22)/311,002; 1.713/(21)/223; 2.691/(17)/040. The crystal structure was solved from single-crystal X-ray diffraction data and refined to R1 = 0.0371 on the basis of 200 unique reflections with Fo < 4σF. The structure is based on double [O2Pb3] chains of edge-sharing oxo-centered [OPb4] tetrahedra along c, between which are sited SeO3 triangles. The two independent Pb2+ atoms and the Se4+ atom have sterochemically active lone electron pairs. © 2010 Springer-Verlag. Source

Bindi L.,University of Florence | Bindi L.,CNR Institute of Geosciences and Earth Resources | Downs R.T.,University of Arizona | Spry P.G.,Iowa State University | And 2 more authors.
Mineralogical Magazine | Year: 2012

The crystal structure and chemical composition of two samples of fettelite from the type locality, including a portion of the holotype material, was investigated to verify if a previously proposed revision of the chemical formula was applicable, and to study the role of cation substitution for Hg that would suggest new members of the fettelite family. The crystal structure of fettelite from the type locality was found to be equivalent to that reported previously for the Chilean occurrence, and consists of an alternation of two kinds of layers along c: layer A with general composition [Ag 6As 2S 7] 2- and layer B with general composition [Ag 10HgAs 2S 8] 2+. In this structure, the Ag atoms occur in various coordination configurations, varying from quasi-linear to quasi-tetrahedral, the AsS 3 groups form pyramids as are typically observed in sulfosalts, and Hg links two sulfur atoms in a linear coordination. The refined compositions for the crystals in this study, [Ag 6As 2S 7][Ag 10(Fe 0.53Hg 0.47)As 2S 8] (R100124) and [Ag 6As 2S 7][Ag 10(Hg 0.79Cu 0.21)As 2S 8] (R110042), clearly indicate that new mineral species related to fettelite are likely to be found in nature. © 2012 Mineralogical Society. Source

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