On the labyrinthine crystal-chemistry of boleite a Pb-Ag-Cu hydroxyhalide

The chemical composition and the crystal structure of boleite from the Amelia Mine (Boléo District, Mexico) were investigated by a series of chemical analytical techniques, and by single crystal X-ray (data collected at 293 K) and neutron diffraction (at 20 K). The concentrations of more than 60 elements were measured. The empirical mineralogical formula of boleite, based on the multi analytical approach used in this study, is: (K_0.390Ca_0.165Na_0.095Rb_0.075Cd_0.040Cs_0.035Tl_0.002)_Σ0.80Pb_26.05Ag_8.93Cu_23.91Cl_61.64(OH)_48.39, of which the simplified formula should be given as (K,Ca,Na,Rb,Cd,Cs)Pb₂₆Ag₉Cu₂₄Cl₆₂(OH)₄₈. However, as Cd can also be considered as a potential substituent for Ag or Cu, the simplified formula transforms to (K,Ca,Na,Rb,Cs)Pb₂₆Ag₉Cu₂₄Cl₆₂(OH)₄₈. This finding provides a more complex scenario with respect to the previous mineralogical formula reported in the literature, i.e., KPb₂₆Ag₉Cu₂₄Cl₆₂(OH)₄₈. Chemical data obtained in this study show no significant evidence of potential substituents of Pb, Ag and Cu; the concentration of REE, PGE and other industrially relevant elements is insignificant. Despite a lack of crystallographic evidence, chemical data appear to suggest that partial Cl⁻vs. OH⁻ substitution can occur. Other potential substituents of Cl⁻, such as F⁻, have not been detected at a significant level. The X-ray and neutron refinement confirms the previously reported general structural model of boleite, but consistently show that a substitutional disorder affects the K site, manifested by a large and unusual displacement parameter. The magnitude of the displacement parameter reflects static disorder, in the form of substitutional disorder, due to differences in the local bonding topology among K, Ca, Na, Rb, (Cd) and Cs statistically populating the same site. The H bonding network in the structure of boleite is now unambiguously described on the basis of the neutron structural model, with two energetically favorable bonds, both having a O_donor-H…Cl_acceptor configuration. The structure of boleite does not contain H₂O molecules, but, instead, only two crystallographically independent hydroxyl groups.