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

Abstract

Pyroaurite [Mg₆Fe₂³⁺ (OH)₁₆][(CO₃)(H₂O)] from the Kovdor Pluton on the Kola Peninsula, Russia, and the Långban deposit in Filipstad, Värmland, Sweden were studied with single crystal and powder X-ray diffraction, an electron microprobe, and Raman spectroscopy. Both samples are rhombohedral, space group R3̅m, a = 3.126(3), c = 23.52(2) Å (Kovdor), and a = 3.1007(9), c = 23.34(1) (Långban). 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 (Långban). 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).