Francisite, Cu₃ Bi(SeO₃)₂O₂ Cl, a new mineral from Iron Monarch, South Australia; description and crystal structure

Francisite is a new copper bismuth oxy-chloro selenite from Iron Monarch, South Australia. The new mineral occurs as radiating clusters of bright green bladed crystals up to 0.25 mm in length. The crystals are elongated along [010], and the principal forms are {100}, {011}, and {101}. Associated with francisite are barite, chlorargyrite, muscovite, native bismuth, naumannite, djurleite, and a number of poorly characterized selenides of Bi, Ag, and Cu. Francisite appears to have formed as a result of hydrothermal alteration of the selenide and sulfide minerals.

Electron microprobe analysis yielded CuO 32.62, Bi₂O₃ 35.75, SeO₂ 27.23, Cl 4.11, less O = Cl, 0.93, total 98.78 wt%; the simplified formula is close to Cu₃Bi(SeO₃)₂O₂Cl. The mineral is transparent and has a pale green streak. The estimated Mohs hardness is 3–4, and D_calc = 5.42 gm/cm³ Single-crystal studies gave an orthorhombic cell with a = 6.354(1), b = 9.630(1), c = 7.220(2) Å, V = 441.80(2) ų, space group Pmmn and Z = 2. The crystal structure was solved by Patterson methods and refined to R = 0.0443 and wR = 0.0395, using a set of 734 reflections, of which 434 were considered to be observed [I >3σ(I)]. The structure consists of an infinite three-dimensional framework of cations and O, containing eight-coordinate Bi³⁺, square-planar Cu²⁺, and three-coordinate Se⁴⁺. The Cl ions and the Se lone pair electrons occupy tunnels parallel to [001].

The strongest lines in the X-ray powder pattern are (d_obs, I_obs, hkl) 5.31 (60) (110); 3.39 (100) (121); 2.866 (80) (130); 2.652 (70) (220); 2.491 (60) (221); 1.695 (35) (242); 1.588 (60) (332). The name is for Glyn Francis of Iron Knob, South Australia.