The crystal structure of mereheadite (monoclinic, Cm, a = 17.372(1), b = 27.9419(19), c = 10.6661(6) Å, β = 93.152(5)o, V = 5169.6(5) Å3) has been solved by direct methods and refined to R1 = 0.058 for 6279 unique observed reflections. The structure consists of alternating Pb-O/OH blocks and Pb-Cl sheets oriented parallel to the (201) plane and belongs to the 1:1 type of lead oxide halides with PbO blocks. It contains 30 symmetrically independent Pb positions, 28 of which belong to the PbO blocks, whilst two positions (Pb12 and Pb16) are located within the tetragonal sheets of the Cl- anions. Mereheadite is thus the first naturally occurring lead oxychloride mineral with inter-layer Pb ions. The coordination configurations of the Pb atoms of the PbO blocks are distorted versions of the square antiprism. In one half of the coordination hemisphere, they are coordinated by hard O2- and OH- anions whose number varies from three to four, whereas the other coordination hemisphere invariably consists of four soft Cl- anions located at the vertices of a distorted square. The Pb12 and Pb16 atoms in between the PbO blocks have an almost planar square coordination of four Cl- anions. These PbCl4 squares are complemented by triangular TO3 groups (T = B, C) so that a sevenfold coordination is achieved. The Pb-O/OH block in mereheadite can be obtained from the ideal PbO block by the following list of procedures: (1) removal of some PbO4 groups that results in the formation of square-shaped vacancies; (2) insertion of TO3 groups into these vacancies; (3) removal of some Pb atoms (that correspond to the Pb1A and Pb2A sites), thus transforming coordination of associated O sites from tetrahedral OPb4 to triangular OHPb3; and (4) replacement of two O2- anions by one OH- anion with twofold coordination; this results in formation of the 1 × 2 elongated rectangular vacancy. The structural formula that can be derived on the basis of the results of single-crystal structure determination is Pb47O24(OH)13Cl25(BO3)2(CO3). Welch et al. (1998) proposed the formula Pb2O(OH)Cl for mereheadite, which assumes that neither borate nor carbonate is an essential constituent of mereheadite and their presence in the mineral is due to disordered replacements of Cl- anions. However, our study demonstrates that this is not the case, as BO3 and CO3 groups have well-defined structural positions confined in the vacancies of the Pb-O/OH blocks and are therefore essential constituents. Our results also show that mereheadite is not a polymorph of blixite, but is in fact related to symesite. Symesite thus becomes the baseline member of a group of structurally-related minerals.

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