The jordanite homologous series has the general formula Pb4N-2Me3+ 6S4N+7 (Makovicky et al., 2006). The minerals belonging to this series are characterized by the stacking, along b, of trigonal prismatic layers with N-octahedra-thick layers. The N = 4 homologue is represented by the most common members of this series, jordanite, ideally Pb14As6S23, and geocronite, Pb14Sb6S23, whereas the N = 3 member, kirkiite, Pb10(As3Bi3)S19, is exceptional. These structural differences are reflected in the unit-cell parameters. Both N = 3 and N = 4 homologues are characterized by two short unit-cell axes of ≈ 8.5 and 8.9 Å, whereas the third long axis increases from 26 to 32 Å from kirkiite to the jordanite-geocronite pair. One of the classic localities for the occurrence of geocronite is represented by the Pollone mine, Valdicastello Carducci (Apuan Alps, Tuscany, Italy), where it has been known since XIX Century (Kerndt, 1845). During our study of the baryte - pyrite - iron oxide ore deposits from southern Apuan Alps, historical as well as new specimens of geocronite from this locality were rexamined using single-crystal X-ray diffraction coupled with electron-microprobe analysis. The chemistry of the studied samples ranges between As-bearing geocronite, Pb14Sb3.8As2.2S23, and Sb-bearing jordanite, Pb14As3.4Sb2.6S23. The crystal structure of grains representative of the different compositions have been refined, allowing a better understanding of the pnictogen partitioning within the different structural sites of the jordanite-geocronite pair (Biagioni et al., 2016). Surprisingly, one of the studied specimens, together with the two short unit-cell parameters typical of the jordanite series, revealed for the third one a value of 29.4 Å, intermediate between those of the N = 3 and N = 4 homologues. Consequently, it was hypothesized that it could be the first N = 3.5 homologue. The crystal structure solution confirmed this hypothesis, allowing the definition of the new mineral marcobaldiite, ideally Pb12(Sb,As,Bi)6S21 (IMA 2015-109). It was found in a quartz vein as a black prismatic crystal associated with tennantite. Marcobaldiite is triclinic, P-1, with unit-cell parameters a = 8.9248(9), b = 29.414(3), c = 8.5301(8) Å, α = 98.336(5), β = 118.175(5), γ = 90.856(5)°, V = 1944.1(3) Å3, Z = 2. Electron microprobe analysis gave (in wt.%): Pb 64.05, Bi 4.24, Sb 9.10, As 4.51, S 17.24, total 99.14. It corresponds to the chemical formula (on the basis of 18 cations per formula unit) Pb11.98(Sb2.90As2.33Bi0.79)Σ6.02S20.80. The crystal structure of marcobaldiite (R1 = 0.0682) can be described as formed by the 1:1 alternation of two different layers of distorted octahedra, one three-octahedra thick (kirkiite-type slab) and the other fouroctahedra thick (jordanite-type slab). Layers are connected by one atomic layer of three bicapped triangular prismatic Pb atoms and one triangular pyramidal As.
Jordanite homologues from the Pollone mine (Apuan Alps, Italy): the geocronite-jordanite isotypic pair and the new N = 3.5 homologue marcobaldiite
BIAGIONI, CRISTIAN;PASERO, MARCO;
2016-01-01
Abstract
The jordanite homologous series has the general formula Pb4N-2Me3+ 6S4N+7 (Makovicky et al., 2006). The minerals belonging to this series are characterized by the stacking, along b, of trigonal prismatic layers with N-octahedra-thick layers. The N = 4 homologue is represented by the most common members of this series, jordanite, ideally Pb14As6S23, and geocronite, Pb14Sb6S23, whereas the N = 3 member, kirkiite, Pb10(As3Bi3)S19, is exceptional. These structural differences are reflected in the unit-cell parameters. Both N = 3 and N = 4 homologues are characterized by two short unit-cell axes of ≈ 8.5 and 8.9 Å, whereas the third long axis increases from 26 to 32 Å from kirkiite to the jordanite-geocronite pair. One of the classic localities for the occurrence of geocronite is represented by the Pollone mine, Valdicastello Carducci (Apuan Alps, Tuscany, Italy), where it has been known since XIX Century (Kerndt, 1845). During our study of the baryte - pyrite - iron oxide ore deposits from southern Apuan Alps, historical as well as new specimens of geocronite from this locality were rexamined using single-crystal X-ray diffraction coupled with electron-microprobe analysis. The chemistry of the studied samples ranges between As-bearing geocronite, Pb14Sb3.8As2.2S23, and Sb-bearing jordanite, Pb14As3.4Sb2.6S23. The crystal structure of grains representative of the different compositions have been refined, allowing a better understanding of the pnictogen partitioning within the different structural sites of the jordanite-geocronite pair (Biagioni et al., 2016). Surprisingly, one of the studied specimens, together with the two short unit-cell parameters typical of the jordanite series, revealed for the third one a value of 29.4 Å, intermediate between those of the N = 3 and N = 4 homologues. Consequently, it was hypothesized that it could be the first N = 3.5 homologue. The crystal structure solution confirmed this hypothesis, allowing the definition of the new mineral marcobaldiite, ideally Pb12(Sb,As,Bi)6S21 (IMA 2015-109). It was found in a quartz vein as a black prismatic crystal associated with tennantite. Marcobaldiite is triclinic, P-1, with unit-cell parameters a = 8.9248(9), b = 29.414(3), c = 8.5301(8) Å, α = 98.336(5), β = 118.175(5), γ = 90.856(5)°, V = 1944.1(3) Å3, Z = 2. Electron microprobe analysis gave (in wt.%): Pb 64.05, Bi 4.24, Sb 9.10, As 4.51, S 17.24, total 99.14. It corresponds to the chemical formula (on the basis of 18 cations per formula unit) Pb11.98(Sb2.90As2.33Bi0.79)Σ6.02S20.80. The crystal structure of marcobaldiite (R1 = 0.0682) can be described as formed by the 1:1 alternation of two different layers of distorted octahedra, one three-octahedra thick (kirkiite-type slab) and the other fouroctahedra thick (jordanite-type slab). Layers are connected by one atomic layer of three bicapped triangular prismatic Pb atoms and one triangular pyramidal As.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
