Abstract

The crystal structure of eclarite, first determined by Kupčík in 1984, has been refined on material from the scheelite deposit at Felbertal, Salzburg Province, Austria, to an R1 value of 4.5% on the basis of 4141 unique reflections [Fo > 4σ(Fo)]. The chemical formula of material examined, based on 20.85 large cations (Z = 4) is Cu0.76Fe0.45Ag0.12Pb8.14Bi12.59S27.91; its unit-cell parameters are a 4.0307(1), b 22.7011(6), and c 54.615(1) Å, space group Pmcn. All the principal features of Kupčík’s structure were confirmed. Three cation sites appear split into Pb and Bi subsites, and the octahedral site of the “cosalite-like” fragment is a partially occupied Bi site, flanked by three-fold coordinated, partly occupied Cu1 and Cu2 sites. The tetrahedral (Cu,Fe) site is split into a triangular position and a tetrahedral position. Replacement of Fe by Cu at the tetrahedral site and replacement of Bi by Cu at the octahedral site are positively correlated. They result in a general formula of eclarite equal to Cu1.5nFe1−n Pb9–1.25nBi12+nS28 in which n is the molar proportion of the copper-based fully substituted ideal end-member Cu1.5Pb7.75Bi13.0S28, and (1–n), that of the Fe-based ideal end-member FePb9Bi12S28. We propose a new interpretation of the crystal structure of eclarite, as a patchwork of large galenobismutite-like regions joined by interstitial elements.

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