Crystal Chemistry of Pyroaurite from the Kovdor Pluton, Kola Peninsula, Russia, and the Langban Fe-Mn deposit, Varmland, Sweden

Pyroaurite [Mg6Fe2 (3+) (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 R3I...m, a = 3.126(3), c = 23.52(2) (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 M (2+): M (3+) was determined as similar to 3.1-3.2 (Kovdor) and similar to 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).