The crystal structure of kintoreite, PbFe3(PO4)2(OH,H2O)6

The crystal structure of kintoreite, PbFe₃(PO₄)₂(OH,H₂O)₆, has been refined. The mineral is rhombohedral, R3m with a = 7.3310(7), c = 16.885(2) Å, Z = 3; the structure has been refined to R = 3.0% and R_w = 3.0% using 183 observed reflections [I > 2σ(I)]. Kintoreite has the alunite-type structure which consists of sheets of corner-sharing Fe(O,OH)₆ octahedra parallel to (001). The sheets are composed of clusters of three cornerlinked octahedra which are tilted so that the three apical O atoms form the base of the XO₄ tetrahedra. The clusters of octahedra are linked to similar groups by corner-sharing to form six membered rings. The Pb cations occupy the cavities between pairs of octahedral sheets and are surrounded by six oxygen atoms from the tetrahedra and six oxygen atoms from the octahedra to form a very distorted icosahedron. The mean bond lengths for the various coordination polyhedra are X-O 1.55 Å, (X= P, As, S); Fe-(O, OH) 2.01 Å; Pb-O 2.84 Å. The composition of the crystal used in the refinement was PbFe₃(PO₄)_1.3(AsO₄) _0.4(SO₄)_0.3(OH,H₂O)₆. The XO₄ anions are disordered, as in beudantite, rather than being ordered, as they are claimed to be in corkite.