Crystal Chemistry of Pyroaurite from the Kovdor Pluton, Kola Peninsula, Russia, and the Långban Fe–Mn deposit, Värmland, Sweden

Pyroaurite [Mg6Fe23+ (OH)16][(CO3)(H2O)] from the Kovdor Pluton on the Kola Peninsula, Russia, and the Langban deposit in Filipstad, Varmland, Sweden were studied with single crystal and powder X-ray diffraction, an electron microprobe, and Raman spectroscopy. Both samples are rhombohedral, space group R3m, a = 3.126(3), c = 23.52(2) A (Kovdor), and a = 3.1007(9), c = 23.34(1) (Langban). The powder XRD revealed only the 3R polytype. The ratio of di- and trivalent cations M2+: M3+ was determined as ~3.1–3.2 (Kovdor) and ~3.0 (Langban). The Raman spectroscopy of the Kovdor sample verified hydroxyl groups and/or water molecules in the mineral (absorption bands in the region of 3600–3500 cm–1) and carbonate groups (absorption bands in the region of 1346–1058 cm–1). Based on the data obtained, the studied samples should be identified as pyroaurite-3R (hydrotalcite group).