New data on betekhtinite: Refinement of crystal structure and revision of chemical formula

Abstract

The crystal structure of betekhtinite from Dzhezkazgan copper ore deposit, Kazakhstan, has been refined to R1 = 0.047 for 1321 unique observed reflections. The mineral is orthorhombic, Immm, a = 3.9047(6), b = 14.796(2), c = 22.731(3) Å, and V = 1313.3(3) Å3. Structure refinement revealed five additional partially occupied Cu sites compared to the previous structural study. The structure contains one Pb and thirteen Cu sites. The coordination of the Pb site is sevenfold. Coordination geometries of the Cu sites are variable: The Cu1, Cu2, Cu3 Cu6, Cu7, Cu8, and Cu9 sites are tetrahedrally coordinated, whereas Cu4, Cu5, Cu10, Cu11, and Cu13 have a triangular coordination. The Cu12 site is coordinated by two S atoms to form a CuS2 dumbell. The crystal structure of betekhtinite is based upon complex Pb-Cu sulfide rods running parallel to the a axis. The rods have a rhombus-like cross sections with lateral dimensions of ca. 11 x 16 Å2. The core of the rod is composed from the CuS4 tetrahedra and may be considered a module extracted from the archetype structure of fluorite, CaF2. The tetrahedral columns are further incrustated by the Cu4S3 and Cu5S3 triangles and Pb atoms to form the [Pb2Cu16S15] rods, which are linked to each other along the b axis via S6 atoms. The low-occupied Cu sites are located in between the rods. The structural formula determined on the basis of the crystal-structure refinement can be written as Pb2Cu22.18Fe1.04S15, which is in good agreement with the chemical analyses of betekhtinite and disagrees notably with the formula Pb2(Cu,Fe)21S15 suggested by Dornberger-Schiff and Höhne. The general crystal chemical formula of betekhtinite can be written as Pb2(Cu,Fe)22-24S15. Information-based structural complexity parameters for betekhtinite are: Ig = 3.696 bits/atom and IG,total = 144.131 bits/cell. Decomposition of betekhtinite into a mixture of galena (PbS; Ig = 1.000 bits/atom; IG,total = 2.000 bits/cell) and chalcocite (Cu2S; IG = 1.500 bits/atom; IG,total = 12.000 bits/cell) at temperatures above 150 °C is associated with the loss of structural complexity and the rise of configurational entropy of the system.