In order to clarify the symmetry problem along the stannite -kësterite join [Cu 2 FeSnS 4 -Cu 2 ZnSnS 4 ], a structural study of synthetic Cu 2 Fe 1-x Zn x SnS 4 single crystals was performed (x = 0, 0.2, 0.5, 0.7, 0.8 and 1, respectively). The metal distribution among the tetrahedral cavities was determined by refining different models in both the I4 and I42m space groups. The best agreement was obtained in I42m, even for the Zn-rich members of the series. However, two different mechanisms of incorporation take place along the stannite-kësterite join. For pure stannite and zincian stannite (x = 0, 0.2, 0.5), the 2a position (0,0,0) is mainly occupied by (Fe,Zn), whereas Cu is the dominant species at 4d (0,½,¼). For ferroan kësterite and pure kësterite (x = 0.7, 0.8, 1), the 2a position is fully occupied by Cu, whereas (Zn,Fe) and the remaining Cu are disordered at 4d. On the basis of the structural results, pure Me-S bond-distances are proposed for Fe, Cu, Zn in both 2a and 4d sites, and the metal distribution among the tetrahedral sites is obtained accordingly. For x ≥ 0.7, the Me-S distance found for the atom located at 2a closely approaches that found for the atom located at 4d, thus producing a more regular framework. Accordingly, distortion parameters and 2 of the S(Me 3 Sn) tetrahedron decrease with increasing Zn. This feature, in turn, is the reason for the pseudocubic symmetry of the lattice observed in the Zn-rich region (2a close to the c parameter). The unit-cell volume linearly increases with increasing Zn, thus confirming the mainly covalent character of the bonds in these compounds. The previously noted inversion of slope in the unit-cell parameters at x = 0.7 corresponds to the point of the series wherein Cu becomes predominant at the 2a site. The proposed model accounts for the structural and geometrical variations observed along the stannite-kësterite series, even if no change of space group is assumed.
The discovery of a natural quasicrystal, icosahedrite (Al 63 Cu 24 Fe 13 ), accompanied by khatyrkite (CuAl 2 ) and cupalite (CuAl) in the CV3 carbonaceous chondrite Khatyrka has posed a mystery as to what extraterrestrial processes led to the formation and preservation of these metal alloys. Here we present a range of evidence, including the discovery of high-pressure phases never observed before in a CV3 chondrite, indicating that an impact shock generated a heterogeneous distribution of pressures and temperatures in which some portions reached at least 5 GPa and 1,200°C. The conditions were sufficient to melt Al-Cu-bearing minerals, which then rapidly solidified into icosahedrite and other Al-Cu metal phases. The meteorite also contains heretofore unobserved phases of iron-nickel and iron sulphide with substantial amounts of Al and Cu. The presence of these phases in Khatyrka provides further proof that the Al-Cu alloys are natural products of unusual processes that occurred in the early solar system.
We present evidence that a rock sample found in the Koryak Mountains in Russia and containing icosahedrite, an icosahedral quasicrystalline phase with composition Al 63 Cu 24 Fe 13 , is part of a meteorite, likely formed in the early solar system about 4.5 Gya. The quasicrystal grains are intergrown with diopside, forsterite, stishovite, and additional metallic phases [khatyrkite (CuAl 2 ), cupalite (CuAl), and β-phase (AlCuFe)]. This assemblage, in turn, is enclosed in a white rind consisting of diopside, hedenbergite, spinel (MgAl 2 O 4 ), nepheline, and forsterite. Particularly notable is a grain of stishovite (from the interior), a tetragonal polymorph of silica that only occurs at ultrahigh pressures (≥10 Gpa), that contains an inclusion of quasicrystal. An extraterrestrial origin is inferred from secondary ion mass spectrometry 18 O∕ 16 O and 17 O∕ 16 O measurements of the pyroxene and olivine intergrown with the metal that show them to have isotopic compositions unlike any terrestrial minerals and instead overlap those of anhydrous phases in carbonaceous chondrite meteorites. The spinel from the white rind has an isotopic composition suggesting that it was part of a calciumaluminum-rich inclusion similar to those found in CV3 chondrites. The mechanism that produced this exotic assemblage is not yet understood. The assemblage (metallic copper-aluminum alloy) is extremely reduced, and the close association of aluminum (high temperature refractory lithophile) with copper (low temperature chalcophile) is unexpected. Nevertheless, our evidence indicates that quasicrystals can form naturally under astrophysical conditions and remain stable over cosmic timescales, giving unique insights on their existence in nature and stability. I cosahedrite, a naturally occurring quasicrystal, with composition Al 63 Cu 24 Fe 13 and exhibiting Bragg peak diffraction with icosahedral symmetry, was recently discovered in a rock sample found in the Museo di Storia Naturale of the Università degli Studi di Firenze (catalog number 46407/G), labeled as khatyrkite and identified as coming from the Khatyrka ultramafic zone in the Koryak Mountains in the Chukotka Autonomous Okrug of Far Eastern Russia (1, 2). Investigations over the past year have determined that the sample studied here, with size 0.27 × 0.30× 0.32 cm, is one of two rocks containing khatyrkite (CuAl 2 ) and cupalite (CuAl) to have been found in a claybed at the Listvenovyi stream in 1979 by V. V. Kryachko, the other being the holotype sample (3) deposited in the St. Petersburg Mining Institute. The holotype is an aggregate of metallic crystals, approximately 1 mm across, with no sign of surrounding host rock-forming minerals visible on the exterior. The interior has not been examined to date. By contrast, the sample studied here shows a remarkably complex set of assemblages (Fig. 1, Fig. 2), with a core containing quasicrystalline icosahedrite, crystalline metallic phases khatyrkite, cupalite, and β-phase (AlCuFe), diopside (Fig. S1), and forsterite (Fig. S2), as well ...
The porphyry-type Pagoni Rachi Mo-Cu-Te-Ag-Au prospect, in northern Greece, is a porphyry-epithermal system hosted by an Oligocene dacite porphyry and quartz-feldspar porphyry dikes. The rare mineral rheniite (ReS 2 ) and molybdenite, with very high contents of Re (up to 4.7 wt% Re), occur in quartz veins along with Fe-Cu sulfides, Pb-, Sn-, and Cl-bearing oxides, hematite, ilmenite and tellurides of Bi; these veins are spatially related to sericitic and transitional sericitic-sodic-potassic alteration. Earlier-formed gold-bearing quartz and magnetite veins with sodic-potassic-calcic alteration and late precious-metal telluride-rich carbonate-quartz veins with argillic alteration contain only minor amounts of molybdenite. The composition of rheniite ranges from almost pure, stoichiometric rheniite with low Mo content to rheniite with up to 5.99 wt% Mo. Petrographic, scanning electron microscope, and structural studies suggest that the high Re content of molybdenite from Pagoni Rachi is the result of the isovalent Re-for-Mo substitution in molybdenite. This fact is corroborated by the progressive shortening of the Mo-S mean bond-distance (from 2.414 Å in pure molybdenite to 2.355 Å), as well as by the isotropic decrease of the unit-cell values with the increase of the Re:Mo ratio. The structural analysis of four molybdenite crystals, with the highest Re content ever reported in nature, demonstrates that they crystallize as the 2H polytype and not the 3R polytype, as previously hypothesized, thus suggesting that Re concentration does not correlate with a specific polytype. The fluid inclusions in quartz in the Re-bearing molybdenite-rheniite veins at Pagoni Rachi show that they homogenize to either the liquid or vapor phases (354 to 428°C) or by halite dissolution at 317 to 585°C, which equates to salinities of 40 to 59 wt% NaCl equiv. Rheniite and molybdenite likely precipitated as temperatures fell below 400°C during phase separation under relatively oxidizing conditions, at elevated chlorine activity, and from relatively acid hydrothermal solutions. However, the presence of Pb oxides, Sn-bearing minerals and tellurides § E-mail address: firstname.lastname@example.org 1014 thE CanaDian MinERalogist is compelling evidence that rheniite and Re-rich molybdenite may have formed directly from the vapor as sublimates, in a manner similar to the way they are deposited at the Kudriavy volcano, Kurile Islands.Keywords: porphyry-epithermal deposit, rheniite, Re-rich molybdenite, polytype, phase separation, tellurides, sublimates, Pagoni Rachi, Greece.soMMaiRE L'indice minéralisé en Mo-Cu-Te-Ag-Au de Pagoni Rachi, de type porphyre, situé dans le nord de la Grèce, est un système épithermal encaissé par un porphyre dacitique oligocène et des filons de porphyre quartzofeldspathique. Nous y avons trouvé la rhéniïte (ReS 2 ), espèce rare, et la molybdénite très enrichie en Re (jusqu'à 4.7% Re, poids), dans des veines de quartz avec des sulfures de Fe-Cu, des oxydes contenant Pb, Sn, et Cl, l'hématite, l'ilménite et des tellurure...
Crystal data for natural and synthetic arsenic sulfides are reported and discussed. Most of them [a-and b-dimorphite, realgar, b-As 4 S 4 phase, pararealgar, Kutoglu's As 4 S 4 (II) phase, alacranite, uzonite, orthorhombic As 4 S 5 phase] have a crystal structure consisting of a packing of cage-like, covalently bonded As 4 S n (n ¼ 3, 4 and 5) molecules held together by weak interactions of van der Waals character. Their structures are compared in terms of molecular packing and molecular parameters. The layered structural arrangement of orpiment, As 2 S 3 , is described and the effects of the incorporation of Se replacing for S is discussed. The structures of wakabayashilite and getchellite, which contain mixed (As, Sb) coordination polyhedra, are also described to outline the geometric effects of the Sb ! As substitution. The results of recent studies dealing with the effects of the exposure of realgar or other arsenic sulfides to visible light are reported and discussed. Their interest in the study of arsenical pigments and their preservation in artwork is outlined with some examples of application.
We report the first occurrence of a natural quasicrystal with decagonal symmetry. The quasicrystal, with composition Al71Ni24Fe5, was discovered in the Khatyrka meteorite, a recently described CV3 carbonaceous chondrite. Icosahedrite, Al63Cu24Fe13, the first natural quasicrystal to be identified, was found in the same meteorite. The new quasicrystal was found associated with steinhardtite (Al38Ni32Fe30), Fe-poor steinhardtite (Al50Ni40Fe10), Al-bearing trevorite (NiFe2O4) and Al-bearing taenite (FeNi). Laboratory studies of decagonal Al71Ni24Fe5 have shown that it is stable over a narrow range of temperatures, 1120 K to 1200 K at standard pressure, providing support for our earlier conclusion that the Khatyrka meteorite reached heterogeneous high temperatures [1100 < T(K) ≤ 1500] and then rapidly cooled after being heated during an impact-induced shock that occurred in outer space 4.5 Gya. The occurrences of metallic Al alloyed with Cu, Ni, and Fe raises new questions regarding conditions that can be achieved in the early solar nebula.
scite is a Brooklyn-based startup that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
334 Leonard St
Brooklyn, NY 11211
Copyright © 2023 scite Inc. All rights reserved.
Made with 💙 for researchers