A crystal structural study of gabrielsonite from Långban, Sweden

Långban is a worldwide famous mineralogical locality, almost unique for its mineral diversity and the peculiarities of the occurring minerals. The largest number of rare minerals occur in late-stage Pb–Mn–As–Sb-bearing fissures, typically oxychlorides, arsenates and arsenites. The great species diversity is due to the unusual, almost unique co-presence of Mn2+, Mn3+, Sb5+, As3+, As5+, Pb2+, Ba2+ and Be2+. Gabrielsonite was here established as a new species by Moore (1967), with chemical formula PbFe(AsO4)(OH), space group P21ma, a = 7.86, b = 5.98, c = 8.62 Å. During the SYNTHESYS SE-TAF-5983 research visit to the Naturhistoriska riksmuseet, gabrielsonite crystals were extracted from a museum specimen (NRM#19250353), with the aim of investigating its crystal structure. Single crystal intensity data were collected at Elettra synchrotron facility, XRD1 beamline, with wavelength 0.59043 Å, resulting in 2490 unique reflections, with Rint = 0.0796. The gabrielsonite structure was solved in space group Pmc21, with a = 6.006(1), b = 8.661(2), c = 7.909(2) Å, and refined up to R1 = 0.0344, wR2 = 0.1140, S = 1.042. Two independent Pb cation sites, with half occupancy, are found in gabrielsonite structure, showing distorted four coordination, with bond lengths ranging from 2.203(5) to 2.606(7) Å. The two independent As sites have both a half partial occupancy, and present the typical trigonal pyramidal coordination, with bond lengths ranging from 1.785(6) to 1.814(5) Å. Fe shows octahedral coordination, with bond lengths ranging from 1.936(5) to 2.118(6) Å. About the anionic part, two of the four independent oxygen sites show half partial occupancy, while the two remaining anionic sites are fully occupied; a bond valence analyses allowed to identify one of these sites as a hydroxyl. The crystal structure of gabrielsonite is made up by chains of edge sharing Fe octahedra (FOC) running along a, whereas Pb1 and As1 polyhedra form through corner sharing “layers” (PAL) parallel to (010). As2 and Pb2 polyhedra link by corner sharing to form chains (PAC) running along a, with Pb and As polyhedra regularly alternating along the chain. The structure of gabrielsonite can be described through a regular alternation along b of slabs hosting PAC, FOC, and PAL structural elements. Alternatively, by analogy with the minerals of the adelite-descloizite group, it can be described through a “framework” made up by FOC linked by corner sharing to AsO3 groups, with Pb atoms hosted in the framework channels running along a. Present structural analyses indicate for gabrielsonite the new chemical formula PbFeAsO3(OH), to be confirmed through appropriate EPMA, EXAFS, FTIR and Raman studies