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) Å, β = 99.62(3)0, V = 4394(2) Å3. The structural formula is Pb9.16Sb9.84S23ClO0.5, corresponding to the electron-microprobe one Cu0.10Pb9.16Sb9.84S22.94Cl1.06O 0.5 (ideally Pb9Sb10S23ClO0.5), with Z = 4 and Dcale. = 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 [102̄] to form rod-layers. The pillaite structure is an expanded homologue of hexagonal Bi(Bi2S3)9I3. 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.

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