Site-selective As–P substitution and hydrogen bonding in the crystal structure of philipsburgite, Cu₅Zn((As,P)O₄)₂(OH)₆·H₂O

Philipsburgite, Cu₅Zn((As,P)O₄)₂(OH)₆·H₂O, from the Middle Pit, Gold Hill Mine, Tooele Co., Utah, USA, was studied by single-crystal X-ray diffraction and scanning electron microscopy. The empirical formula of the studied sample is (Cu_4.69Zn_1.23)(As_0.86P_0.18O₄)₂(OH)_5.61·H₂O, which agrees well with the previous reports on the mineral. Philipsburgite is monoclinic, P2₁/c, a = 12.385(6), b = 9.261(4), c = 10.770(5) Å, β = 97.10(1)^o, V = 1225.7(9) ų (at 100 K), and Z = 4. The crystal structure was refined to R₁ = 0.046 for 2563 unique observed reflections with |F_o| ≥ 4σ_F. The crystal structure of philipsburgite is isotypic to that of kipushite and can be considered as a complex three-dimensional framework consisting of two types of layers stacked parallel to the a-axis. The A-type layer is formed by the edge-sharing Jahn–Teller-distorted Cuφ₆ octahedra [φ = O²⁻, (OH)⁻, H₂O]. Two adjacent octahedral layers are linked via (As2O₄) tetrahedra. The B-type layer is built by corner-sharing (ZnO₄) and (As1O₄) tetrahedra and is formed by the four- and eight-membered tetrahedral rings. The A:B ratio of the A and B layers is equal to 2:1. The hydrogen bonding network in philipsburgite is rather complex and consists of two- and three-center hydrogen bonds. The As1 site accommodates ca. 18% of P and is a preferable position for the P substitution in philipsburgite. The observed selectivity of the As1 site for P may indicate that, for the intermediate compositions with the P:As ratios close to 1:1, there is a fully ordered species with P prevalent at the As1 site and As prevalent at the As2 site. The intermediate composition would, therefore, be Cu₅Zn(AsO₄)(PO₄)(OH)₆·H₂O and such a mineral can be considered as a separate species, according to the rules of the International Mineralogical Association (IMA). Philipsburgite should be considered as structurally complex with the Shannon information contents of 4.954 bits/atom and 614.320 bits/cell. The obvious reason for the structural complexity of the mineral is its modularity, i.e., the presence of two structurally distinct modules, the octahedral–tetrahedral (A) and tetrahedral (B) layers.