Crystal-chemical study of ecdemite from Harstigen, a new natural member of the layered lead oxyhalides group Natale PERCHIAZZI 1,* , Ulf HÅLENIUS 2 , Pietro VIGNOLA 3 and Nicola DEMITRI 4 1 Dipartimento Di Scienze Della Terra, Università Di Pisa, Via Santa Maria 53, 56126 Pisa, Italy *Corresponding author, e-mail: natale.perchiazzi@unipi.it 2 Department of Geosciences, Swedish Museum of Natural History, Box 50007, 0405 Stockholm, Sweden 3 CNR – Istituto Per La Dinamica Dei Processi Ambientali, Via Botticelli 23, 20131 Milano, Italy 4 Elettra – Sincrotrone Trieste S.C.P.A., S.S. 14 km 163,5 in Area Science Park – Basovizza, 34149 Trieste, Italy Abstract: Ecdemite from Harstigen, Värmland, Central Sweden, was studied through Raman and FTIR spectroscopy, electron- microprobe techniques, synchrotron powder and single-crystal diffraction. The ideal mineral formula proposed by Palache, Pb 6 Cl 4 As 2 O 7 , is confirmed. In contrast to previous suggestions, however, the present study demonstrates that the mineral is monoclinic, with space-group type P2 1 and unit-cell parameters a = 10.828(4) Å, b = 10.826(2) Å, c = 6.970(1) Å, b = 113.26°(2). The crystal-structural study of ecdemite shows that the mineral can be considered as made up of regularly alternating Cl layers and litharge-like defective layers, also hosting As cations, with the representative formula {Pb 2 [Pb 4 O](AsO 3 ) 2 } 4+ , in a 1:1 ratio. These features indicate that ecdemite can be considered as a new member of the layered lead oxyhalides group. Key-words: ecdemite; Harstigen; Sweden; arsenic; lead; layered oxychlorides; FTIR; Raman spectroscopy; synchrotron single-crystal study; synchrotron X-ray powder diffraction pattern. 1. Introduction Arsenite minerals are quite uncommon in nature, and they are among the most interesting phases of the worldwide famous Långban-type localities in Sweden. Nearly 300 mineral species are known for the group of neighbouring mines at Harstigen, Jakobsberg, Långban, Mangruvan, Nordmark and Sjögruvan, located within ~20 km of each other near the S–E border of the Swedish province of Värmland. Among these, 30 mineral species are unique to these localities and comprise several lead minerals such as oxychlorides (two species), arsenates (four species) and arsenites (eight species). Among lead arsenites, the crystal structures of freedite (Pertlik, 1987), finnemanite (Effenberger & Pertlik, 1979), gabrielsonite (Perchiazzi et al. , 2018), paulmooreite (Araki et al., 1980), trigonite (Pertlik, 1978) were already deter- mined, but the crystal structures of rouseite, heliophyllite and ecdemite have remained unknown. The present study of ecdemite was performed, together with the crystal-structure study of gabrielsonite (Perchiazzi et al., 2018), within the frame of the SYNTHESYS funded visit SE-TAF-5983 to Naturhistoriska riksmuseet, Stockholm, with the aim of providing further insights into the crystal chemistry of the Pb–As minerals found at the Långban-type deposits. Nordenskiöld (1877) defined ecdemite as a new species, assigning the mineral to the tetragonal system on the basis of morphological studies, with chemical data pointing to the ideal formula Pb 7 As 2 O 8 Cl 4 . Moreover, he mentioned the presence of an orthorhombic mineral containing the same elements as ecdemite but not in a sufficient amount to fully define its chemical composition. It was subsequently defined as the new species heliophyllite by Flink (1888). Hamberg (1889) investigated ecdemite or heliophyllite from Harstigen, finding in every sample intergrowths of uniaxial and biaxial lamellae, proposing for them the common formula Pb 13 As 4 O 15 Cl 8 . On the basis of Nordenskiöld’s (1877) data, Palache et al. (1951) proposed the chemical formula Pb 6 Cl 4 As 2 O 7 for ecdemite. Sillén & Melander (1941) examined ecdemite and nadorite, PbSbClO 2 , from Långban as part of their investigations on bismuth oxyhalides with layered structure. They performed X-ray Laue, Weissenberg and precession studies on a cleavage plate of ecdemite from Harstigen. The authors report for ecdemite a P4/mmm tetragonal symmetry with a = 10.8, c = 25.6 Å, without any space-group assignment. They commented that “However plates out of this sample proved under the polarizing microscope to be biaxial. ” Sillén & Melander (1941) performed X-ray powder diffraction and optical studies on ecdemite samples from Jakobsberg and Harstigen. The authors reported that X-ray powder data Eur. J. Mineral. 2019, 31, 609–617 Published online 13 April 2019 https://doi.org/10.1127/ejm/2019/0031-2853 0935-1221/19/0031-2853 $ 4.05 Ó 2019 E. Schweizerbart’sche Verlagsbuchhandlung, 70176 Stuttgart, Germany Downloaded from https://pubs.geoscienceworld.org/eurjmin/article-pdf/31/3/609/4792382/ejm_31_3_0609_0617_perchiazzi_2853_online.pdf by E. Schweizerbart'sche user on 15 July 2019