Thermal expansion of francisite, [Cu3BiO2](SeO3)(2)Cl, and its interpretation based on oxocentered copper-bismuth tetrahedra

The thermal expansion of francisite, [Cu3BiO2](SeO3)(2)Cl, a rare copper(II )-bismuth(III) oxide chloride selenite, was studied by high-temperature X-r ay powder diffraction over the temperature range 293-773 K. The mineral is stable up to 748 K at which temperature it decomposes to [Bi2O2]Se, CuO and , probably, [BiO][CuSe]. The thermal expansion has an anisotropic character (alpha(a) = 9.0, alpha(b) = 4.7, alpha(c) = 17.0 . 10(-6) K-1), which is d etermined by the distribution of the bonds between additional oxygen atoms (O-a) and metal atoms (A). These bonds form two-dimensional systems in whic h each O-a atom is tetrahedrally coordinated by one Bi and three Cu atoms. Thus the structure can be described in terms of oxocentered metal atom tetr ahedra consisting of [(O2Cu3Bi)-O-a] layers of ((OCu3Bi)-O-a) tetrahedra, ( SeO3) groups and Cl anions. This description gives the most reasonable view point to explain the anisotropic thermal expansion character of francisite.