The crystal chemical role of Zn in alunite-type minerals: Structure refinements for kintoreite and zincian kintoreite

Kintoreite, Pb[Fe.sub.3][H.sub.0.94][[[(P[O.sub.4]).sub.0.97] [(S[O.sub.4]).sub.0.03].sub.2][(OH).sub.6], and zincian kintoreite, Pb[Zn.sub.0.3][Fe.sub.3][H.sub.0.24][[[(P[O.sub.4]).sub.0.54] [(S[O.sub.4]).sub.0.08][(As[O.sub.4]).sub.038]].sub.2][(OH).sub.6], have rhombohedral symmetry, space group R[bar.3]m, with hexagonal cell parameters a = 7.2963(5) [Angstrom], c = 16.8491(5) [Angstrom], and a = 7.3789(3) [Angstrom], c = 16.8552(7) [Angstrom], respectively. The structures have been refined using single-crystal X-ray data to [R.sub.1] = 0.030 for 374 observed reflections and [R.sub.1] = 0.035 for 399 observed reflections, respectively. The structures of both minerals comprise rhombohedral stacking of (001) composite layers of corner-shared octahedra and tetrahedra with Pb atoms occupying icosahedral sites between the layers, as in the alunite-type structure. The corner-connected octahedra form three-membered and six-membered rings as in hexagonal tungsten bronzes. The structure of zincian kintoreite differs from other alunite-type structures in having partial occupation, by Zn, of new sites within the six-membered rings in the octahedral layers. The Zn is displaced to an off-center position in the hexagonal ring, where it assumes fivefold trigonal-bipyramidal coordination, to three of the hydroxyl anions forming the ring, and to the apical O anions of the X[O.sub.4] tetrahedra on opposite sides of the ring. The different structural modes of Zn incorporation into S[O.sub.4]-dominant and (P, As)[O.sub.4]-dominant members of [A.sup.2+][B.sup.3+.sub.3] [(X[O.sub.4]).sub.2][(OH).sub.6] alunite-type minerals are discussed in terms of the different charge-compensation mechanisms involved. Keywords: Zincian kintoreite, crystal structure, alunite-type structure, single-crystal study