The role of the minor substitutions in the crystal structure of natural challacolloite, K2Pb2Cl5, and hephaistosite, TlPb2Cl5, from Vulcano (Aeolian Archipelago, Italy)

Crystals of challacolloite, KPb(2)Cl(5), and hephaistosite, TlPb(2)Cl(5), from volcanic sublimates formed on the crater rim of the "La Fossa Crater" at Vulcano, Aeolian Archipelago, Italy, were investigated. Chemical compositions were (K(0.93)Tl(0. 02))(Sigma=0.95) Pb(2.04) (Cl(4.90)Br(0.11))(Sigma=5.01) and Tl(0.94)Pb(2.01)(Cl(4.91)Br(0.14) )(Sigma=5.05), respectively. Single crystal X-ray measurements showed monoclinic symmetry for both phases, space group P2(1)/c, with the following unit-cell parameters. a = 8.8989(4), b = 7.9717(5), c = 12.5624(8) angstrom, beta = 90.022(4)degrees, V = 891.2(1) angstrom(3), Z = 4 (challacolloite) and a = 9.0026(6), b = 7.9723(6), c = 12.5693(9) angstrom, beta = 90.046 (4)degrees, V = 902.1(1) angstrom(3), Z = 4 (hephaistosite). The structure refinements converge to R = 3.99% and R = 3.86%, respectively. The effects of Br <-> Cl and K <-> Tl substitutions on the structure of these natural compounds have been discussed.