Hinsdalite and plumbogummite, their atomic arrangements and disordered lead sites

The crystal structures of hinsdalite, PbAl₃[(P_0.69,S_0.31)O₄)]₂(OH,H₂O)₆, and of a plumbogummite containing a small amount of arsenic, PbAl₃[(P_0.95,As_0.05)O₄)]₂(OH,H₂O)₆, have been refined in space group R3m, with a = 7.029(4) and c = 16.789(4) Å, and a = 7.039(5) and c = 16.761(3) Å, respectively. The refinements, using 258 (plumbogummite: 297) observed reflections with I≥ 3σ (1), led to R =3.0 % (3.7 %) and R_W = 3.0 % (3.2 %). Both minerals have the beudantite/crandallite structure type with hinsdalite being characterised by disordered (P,S)O₄ tetrahedra with an average (P,S)-O distance of 1.52 Å. The Pb atoms in both minerals are displaced from the origin and are disordered about their sites, as in other Pb containing members of the alunite family (beudantite, kintoreite, and plumbojarosite). The disorder of the Pb atoms is confined to the (0001) plane; in hindsdalite, Pb is at (0.0312, 0.0312, 0.0), while in plumbogummite it is at (0.0409, 0.0409, 0.0). Pb-O distances average 2.79 Å in both minerals. The hydrogen-bonding networks are modelled with the help of bond-valence summations.