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Kintoreite, PbFe 3 H 0.94 [(PO 4 ) 0.97 (SO 4 ) 0.03 ] 2 (OH) 6 , and zincian kintoreite, PbZn 0.3 Fe 3 H 0.24 [(PO 4 ) 0.54  (SO 4 ) 0.08 (AsO 4 ) 0.38 ] 2 (OH) 6 , have rhombohedral symmetry, space group R3m, with hexagonal cell parameters a = 7.2963(5) Å, c = 16.8491(5) Å, and a = 7.3789(3) Å, c = 16.8552(7) Å, respectively. The structures have been refined using single-crystal X-ray data to R 1  = 0.030 for 374 observed reflections and R 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 cornerconnected 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 XO 4  tetrahedra on opposite sides of the ring. The different structural modes of Zn incorporation into SO 4 -dominant and (P,As)O 4 -dominant members of A 2+ B 3  3+ (XO 4 ) 2 (OH) 6  alunite-type minerals are discussed in terms of the different charge-compensation mechanisms involved.

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