Serpierite polytypoids from Zvezdel, Bulgaria, and Lavrion, Greece

Structural characteristics of serpierite samples from Zvezdel, Bulgaria, and Lavrion, Greece, are reported. The thermal behaviour of serpierite from Lavrion is discussed. The chemical composition of the studied samples is analysed by energy-dispersive spectroscopy (EDS) and confirmed by single-crystal structure refinements. The obtained chemical formulas correspond well to that of serpierite with Cu:Zn ratio varying between 2.9 and 5.6. The sample from Zvezdel, with composition Ca[Cu 3.3 Zn 0.7 (OH) 6 (SO 4 ) 2 •3H 2 O, crystallizes in the monoclinic crystal system, with space group I 2 and unit-cell parameters a = 18.418(3), b = 6.220(1), c = 12.091(2) Å, β = 90.78(1)˚, whereas the one from Lavrion Ca[Cu 2.8 Zn 1.2 (OH) 6 (SO 4 ) 2 ]•3H 2 O, shows similar unit-cell parameters a = 18.394(1), b = 6.256(1), c = 12.097(1) Å, β = 90.92(1)˚, but higher I 2 /m space-group symmetry. Both studied crystals exhibit serpierite structure topology, but different stacking sequence of the octahedral layers. While in previously studied serpierite of Sabelli and Zanazzi (Acta Cryst B24:1214-1221, 1968 ) there are two layers per unit cell, in currently studied samples there is only one. As a consequence, their unit-cell volumes are half than that of the first structurally characterized serpierite specimen with SG C 2/ c and unit-cell parameters a = 22.186(2), b = 6.250(2), c = 21.853(2) Å, β = 113.36(1)˚. Taking into account the structural peculiarities of the studied samples they are considered as serpierite polytypoids.