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Bindi L.,University of Florence | Topa D.,Natural History Museum Wien | Keutsch F.N.,Harvard University
Periodico di Mineralogia | Year: 2015

The intriguing recovery of an extremely Cu-rich cupropearceite crystal (Cu = 7.62 a.p.f.u.) from Tsumeb, Namibia, allowed us to study the effect of the entry of Cu in the positions usually occupied by Ag in the B module layer in the structure of the minerals belonging to the pearceite-polybasite group. Electron microprobe analysis yields the formula [Cu6(As1.89Fe0.01) Σ=1.90S7.05][(Ag8.36Cu0.62)Σ=8.98CuS4.06]. Lattice parameters are: A = 7.144(1), c = 11.745(2) Å, V = 519.1(2) Å3. The structure was refined in the space group P3- m1 up to R = 0.0278 using 489 observed reflections [2σ(I) level]. The entry of these elevated amounts of Cu in the structure does not induce a structural change and shows that pearceite is able to accommodate higher contents of Cu than previously thought. This allows hypothesis of the existence of an ideal [Cu6As2S7][Cu9CuS4] end member in nature. Such a proposed Ag-free mineral would show the highest structural disorder so far observed for a pearceite structure and, consequently, the highest ionic conductivity and could find applications in silver photography as sensitizer or in optics and microelectronics as rewritable storage medium.

Makovicky E.,Copenhagen University | Topa D.,Natural History Museum Wien
Mineralogical Magazine | Year: 2014

The current state of the lillianite homologous series is presented, with its two branches - the lillianite branch of the predominantly Pb-Bi-Ag sulfosalts and the andorite branch of predominantly Pb-Sb-Ag sulfosalts. Both the natural and synthetic members are discussed, especially from the structural and compositional point of view and the related, chemically distinct and structurally more complicated members of the series are described. A number of new published, or hitherto unpublished observations is given, together with fairly exhaustive tables of data. Relationships between the complex structures of different andorite species and principal structural features of oversubstituted As-Sb and Bi-Sb species are discussed. Contrary to many studies of this subject, synthetic phases form an integral part of the paper. It is concluded that research in this long-known homologous series still supplies new interesting phases, especially in the fields of synthetic products and in its andorite branch. © 2014 The Mineralogical Society.

Makovicky E.,Copenhagen University | Topa D.,Natural History Museum Wien | Tajeddin H.,Tarbiat Modares University | Putz H.,Friedl ZT GmbH | Zagler G.,Konig Ludwig Strasse 20
Canadian Mineralogist | Year: 2013

Ferdowsiite is a member of a mineral association subsidiary to the principal metamorphic minerals of the Barika ore deposit in NW Iran. The empirical formula of ferdowsiite (based on 32 apfu, 16Me + 16S) is Ag7.97Pb0.08 Sb4.75As3.15Bi0.01S16.04. The crystal-structure formula is Ag8(Sb5.49As2.51)∑8S16; the simplified formula is Ag8(Sb5As3)∑8S16. The slightly plumbian variety of ferdowsiite has the idealized formula Pb2xAg4-xSb3-xAsS8 with x =∼0.2. Ferdowsiite is monoclinic, space group P21/n, a 8.677(2) Å, b 5.799(1) Å, c 13.839(3) Å, β 96.175(4)°, V 692.28(10) Å3, Z = 1 for the formula Ag8(Sb5As3)∑8S16. Calculated powder X-ray data are listed; main diffraction lines are [dcalc (Irel) (hkl)]: 3.225 (96)(113); 3.205 (100)(2¯12); 2.8995 (78)(020); 2.7559 (90)(301); 2.7073 (79)(1¯05); 1.9401 (22)(206); 1.9226 (22)(4¯04). Plumbian ferdowsiite has the same space group, P21/n. It has a 8.746(7) Å, b 5.805(4) Å, c 13.929(10) Å, β 96.236(11)°, unit cell volume 703.05 Å3. The mineral is greyish black, opaque, with dark grey streak and metallic luster. It is brittle with irregular fracture; no cleavage or parting was observed. Calculated density is 5.3 g/cm3. In reflected light ferdowsiite is greyish white. Internal reflections are not observable. Pleochroism is distinct, white to grey. Bireflectance is distinct; reflectance values are [λ(nm), Rmin,Rmax]: 470, 34.9, 37.3; 546, 33.4, 35.9; 589, 32.4, 35.1; 650, 30.7, 33.5. Anisotropism is distinct and rotation tints vary between dark brown and bluish grey. Ferdowsiite is associated with large corroded crystals of arsenquatrandorite and is partly replaced by guettardite. It also corrodes smithite. Ferdowsiite is often associated with tetrahedrite/tennantite and forms worm-like symplectites with it. This symplectite is partly replaced on the surface by guettardite. Ferdowsiite is one more member of the group of ABX2-type sulfosalts of silver, combining antimony and arsenic, with a crystal structure different from other members of this group. The closest crystallographic and structural affinity is to the ternary sulfosalt diaphorite.

Makovicky E.,Copenhagen University | Topa D.,Natural History Museum Wien
Zeitschrift fur Kristallographie - Crystalline Materials | Year: 2014

Ferdowsiite is monoclinic, a=8.677(2) Å, b =5.799(1) Å, c=13.839(3) Å, β=96.175(4)°, unit cell volume 692.3 Å3. Space group P21/n. Refinement from single-crystal X-ray diffraction data reached R 1=0.028 for 626 Fo>4σ(Fo) and 0.032 for all 716 Fo used. Value of wR2 is 0.065. The crystal structure of ferdowsiite, approximately Ag8Sb4(As,Sb)4S16, contains four distinct cation and four different anion sites in the asymmetric unit, all in general positions. Besides two Ag sites and one Sb site, the crystal structure contains one mixed As-Sb coordination polyhedron (0.63 As and 0.37 Sb in the site). The Sb1 site has three short Sb-S bonds 2.503-2.645 Å. As and Sb in the mixed site were refined separately, with isotropic displacement coefficients. As has typical bond length values of 2.248-2.354 Å whereas Sb has 2.443-2.392 Å, i.e., the observed ligand positions are visibly influenced by the predominant arsenic. The crystal structure of ferdowsiite is a superstructure of a PbS like motif. The {100} planes of the PbS-like substructure are the (105), (301) and (010) planes in terms of the ferdowsiite lattice. The structure contains zig-zag chains of Sb1 connected via short Sb-S bonds and flanked by (Sb,As)S3 groups and Ag polyhedra. Groups of four SbS5-and (Sb,As)S5 coordination pyramids form an interconnected network with interspaces which accommodate both the lone electron pairs and the tetrahedrally coordinated Ag situated in coordination octahedra. Partial lead substitution takes place in the coordination polyhedra of Sb1 and Ag2. In the paper, the structure is compared with the other Ag(As,Sb,Bi)(S,Se)2 structures. The configu-rationally closest PbS-based homeotypic structure, however, is diaphorite, which is an ordered superstructure of the ferdowsiite arrangement, with a substantial presence of Pb, and without As.

Ageeva O.,University of Vienna | Ageeva O.,Russian Academy of Sciences | Habler G.,University of Vienna | Topa D.,Natural History Museum Wien | And 6 more authors.
American Journal of Science | Year: 2016

Shape and lattice orientation relations as well as chemical compositions of Fe-Ti-oxide micro-inclusions and plagioclase host crystals in rocks of a gabbroplagiogranite assemblage from the Mid-Atlantic ridge at 13°34' N were studied using electron back scatter diffraction, transmission electron microscopy and field-emission gun-electron microprobe analyzer. Several evolutionary stages of the micro-inclusionhost assemblages were discerned, starting with precipitation of Fe-Ti-oxides from a super-saturated plagioclase in otherwise unaltered gabbro, followed by transformation and re-crystallization of the micro-inclusions as well as chemical alteration of both inclusions and host during plagiogranite intrusion and subsequent hydrothermal alteration. A detailed sequence of petrogenetic processes could be reconstructed. Fe-Ti-oxide micro-inclusions are the main carriers of the paleo-magnetic record of these rocks, and understanding the transformations affecting Fe-Ti-oxide microinclusions in the highly dynamic mid-ocean ridge environment is crucial for interpreting paleo-magnetic data.

Makovicky E.,Copenhagen University | Topa D.,Natural History Museum Wien
Mineralogical Magazine | Year: 2013

Electron microprobe analysis of barikaite (Topa et al., 2013) indicates the chemical formula Ag2.90Tl0.04Pb9.31As 11.26Sb8.12S40.37. Barikaite is monoclinic, with a 8.533(1) Å, b 8.075(1) Å, c 24.828(2) Å, and β 99.077(1) ; unit-cell volume 1689.2 Å3 and the space-group setting is P 21/n. This compares well with the unit-cell parameters of rathite Pb10Tl0.9As17.9Sb 1.3Ag2S40 from the Lengenbach deposit with the same lattice setting. Barikaite is a member of sartorite homologous series (N = 4). The unit cell of barikaite contains eight cation sites and ten anion sites. Four of the cation sites have mixed occupancies - the split sites As2 Sb2, As3 Sb3, Ag5 As5 and the site Me 6 with three cations involved. Two of the lead sites, Pb1 and Pb2, display tricapped trigonal prismatic coordinations and alternate along the 8.53 Å a direction. They form zig-zag walls parallel to (001). There are three distinct [100] columns of alternating cations, As1 (As, Sb)2, Sb4 (As, Sb)3 and (As, Ag)5 (Pb, Sb)6 which together form trapezoidally configured single (013) layers. These layers aggregate into tightlybonded double layers, separated by lone electron pair micelles. In barikaite, the predominantly As-occupied and Sb-occupied sites are distributed in a chess-board-like scheme. © 2013 Mineralogical Society.

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