Lead-antimony sulfosalts from Tuscany (Italy); IV, Crystal structure of pillaite, Pb (sub 9) Sb (super 10) S (sub 23) ClO (sub 0.5) , an expanded monoclinic derivative of hexagonal Bi(Bi (sub 2) S (sub 3) ) (sub 9) I (sub 3) , from the zinkenite group

The crystal structure of pillaite, a recently described Pb-Sb oxy-chloro-sulfosalt from the Buca della Vena mine, Tuscany (Italy), was solved and refined to a residual R of 0.045 in the monoclinic system, with space group C2/m, and a = 49.49(1), b = 4.1259(8), c = 21.828(4) Aa, beta = 99.62(3) degrees , V = 4394(2) Aa (super 3) . The structural formula is Pb (sub 9.16) Sb (sub 9.84) S (sub 23) ClO (sub 0.5) , corresponding to the electron-microprobe one Cu (sub 0.10) Pb (sub 9.16) Sb (sub 9.84) S (sub 22.94) Cl (sub 1.06) O (sub 0.5) (ideally Pb (sub 9) Sb (sub 10) S (sub 23) ClO (sub 0.5) ), with Z = 4 and D (sub calc.) = 5.87. The crystal-structure is organized around pseudo-threefold axes where chlorine atoms are in a trigonal prismatic coordination with Pb atoms. An oxygen position, half-filled, is bonded to two Sb atoms, like in scainiite. Pairs of pseudo-trigonal rods parallel to the elongation b-direction are connected along [102] to form rod-layers. The pillaite structure is an expanded homologue of hexagonal Bi(Bi (sub 2) S (sub 3) ) (sub 9) I (sub 3) . Among lead sulfosalts, it belongs to the zinkenite group of cyclically twinned structures. Pillaite is the first example of a Pb-Sb sulfosalt where small amounts of two anion species simultaneously play a specific crystal chemical role, which is essential for the stabilization of the crystal structure: chlorine, like in dadsonite, and oxygen, like in scainiite.