A refinement of the structure of cubanite, CuFe₂S₃

Abstract

AuszugDie Kristallstruktur von Cubanit CuFe2S3 wurde auf Grund der Intensitäten von 1790 beobachteten Interferenzen bis zu R = 0,045 (und bis zu Ii = 0,096 für alle 3301 möglichen Interferenzen) verfeinert. Die Verfeinerung führte zu Atomabstands-und Bindungswinkel-Werten, die viel genauer als die bisher mitgeteilten sind. Die Deformation der Tetraeder um die Metallatome läßt sich durch Abstoßungskräfte zwischen den Eisenatomen in den Mittelpunkten zweier Tetraeder mit gemeinsamer Kante erklären. Diese Kante ist 2,804(1) Ä lang. Die Metall-Schwefel-Abstände sind: Cu-S = 2,276(1) bis 2,326(1) A, Fe-S = 2,258(1) bis 2,304(1) Ä. Die Mehrdeutigkeit der Bestimmung der Raumgruppe, Pcmn oder Pc2in, wurde untersucht und die frühere Wahl der zentrosymmetrischen Raumgruppe Pcmn als richtig bestätigt. AbstractThe crystal structure of cubanite, CuFe2S3, has been refined to an R value of 0.096 (all 3301 data) and 0.045 (1790 "observed" data only), from multiple sets of four-circle diffractometer x-ray intensity data, collected from a spherical crystal. The anisotropic full-matrix least-squares refinement has yielded a set of bond lengths and angles which are much more precise than those of previous workers. The distortions from tetrahedral symmetry about the metal atoms are consistent with a repulsive force between Fe atoms, 2.804(1) Ä apart, in adjacent edge-sharing Fe-S coordination tetrahedra. The Cu-S lengths range from 2.276(1) to 2.326(1) Ä, and the Fe-S bonds from 2.258(1) to 2.304(1) Ä. The ambiguity in the choice of space group between Pcmn and Pc2in has been investigated, and the earlier choice of the centric space group Pcmn is favoured as being correct.

An experimental study of the formation of cubanite (CuFe2S3) in primitive meteorites

“…The experimental phase space (Fig. ) is consistent with the expected ranges of pH, f O 2 , and temperature for the CI‐chondrite parent body: (1) Cubanite is stable below 210 °C (Fleet ; Cabri ; Szymanski ; Caye et al ); (2) The combination of cubanite and pyrrhotite indicates temperatures below ≤200 °C (Yund and Kullerud ; Craig and Kullerud ; Vaughan and Craig ); (3) Oxygen‐isotope modeling predicts temperatures between 50 and 150 °C during peak alteration (Clayton and Mayeda ); (4) Modeling of hydrous activity on a CI‐like parent body (Zolensky et al. ) and mineral equilibrium calculations (Dufresne and Anders ) point to temperature, pH, and Eh ranges of 20–150 °C, 7–10, and −0.2 to 0.8 V, respectively; and (5) Oxygen fugacity is estimated to be greater than 10 −55 to 10 −70 bar (Brearley and references therein).…”

“…Cubanite is the low‐temperature orthorhombic polymorph of CuFe 2 S 3 (Azaroff and Buerger ; Fleet ; McCammon et al ). It is not stable above 210 °C, and the transition it undergoes to isocubanite (the high‐temperature cubic polymorph of CuFe 2 S 3 ) at this temperature is irreversible; upon cooling below 210 °C, isocubanite breaks down to chalcopyrite (CuFeS 2 ) and pyrrhotite (Fleet ; Cabri ; Cabri et al ; Szymanski ; Putnis ; Kaneda et al ; Caye et al ; Pruseth et al ). The presence of cubanite in the CI‐chondrite collection suggests that it is the product of low‐temperature (<210 °C) aqueous processes.…”

Orbital-selective behavior in cubanite CuFe2S3

“…In orthorhombic cubanite Cu and Fe atoms are tetrahedrally coordinated by S atoms with two FeS 4 tetrahedra sharing their edges. This gives pairs of Fe ions with rather short distances between them [7][8][9][10][11] . distance of 2.81 Å, however, is too long to represent a chemical bond.…”

“…In this respect, ternary chalcogenide compound CuFe 2 S 3 (mineral cubanite) is a promising candidate for such study, since its electronic and magnetic ground state properties are far from being clear. CuFe 2 S 3 crystallizes in the orthorhombic structure with a single class of Fe [7][8][9][10][11] . It was argued that iron has a nominal valency of +2.5, due to a "rapid electron exchange" between Fe 2+ and Fe 3+ ions in Fe-Fe dimers 12,13 .…”

In situ thermo‐Raman spectroscopy and ab initio vibrational assignment calculations of cubanite CuFe2S3

“…Cu and Fe atoms are coordinated by four S atoms in almost perfect tetrahedral coordination, where some of them (S1 atoms) form ‐Fe‐S1‐Fe‐ fragments and the other part (S2 atoms) form ‐Cu‐S2‐Fe‐ fragments. Two FeS 4 tetrahedra have common edges, which indicates the pairs of Fe cations with relatively short distances between them 9,10,14–16 . Thus, CuS 4 and FeS 4 tetrahedra should be considered as strongly connected.…”