Description and recognition of potassic-richterite, an amphibole supergroup mineral from the Pajsberg ore field, Värmland, Sweden

Potassic-richterite, ideally A K B (NaCa) C Mg 5 T Si 8 O 22 W (OH) 2 , is recognized as a valid member of the amphibole supergroup (IMA-CNMNC 2017–102). Type material is from the Pajsberg Mn-Fe ore field, Filipstad, Värmland, Sweden, where the mineral occurs in a Mn-rich skarn, closely associated with mainly phlogopite, jacobsite and tephroite. The megascopic colour is straw yellow to grayish brown and the luster vitreous. The nearly anhedral crystals, up to 4 mm in length, are pale yellow (non-pleochroic) in thin section and optically biaxial (−), with α = 1.615(5), β = 1.625(5), γ = 1.635(5). The calculated density is 3.07 g·cm −1 . VHN 100 is in the range 610–946. Cleavage is perfect along {110}. EPMA analysis in combination with Mössbauer and infrared spectroscopy yields the empirical formula (K 0.61 Na 0.30 Pb 0.02 ) ∑0.93 (Na 1.14 Ca 0.79 Mn 0.07 ) ∑2 (Mg 4.31 Mn 0.47 Fe 3+ 0.20 ) ∑5 (Si 7.95 Al 0.04 Fe 3+ 0.01 ) ∑8 O 22 (OH 1.82 F 0.18 ) ∑2 for a fragment used for collection of single-crystal X-ray diffraction data. The infra-red spectra show absorption bands at 3672 cm −1 and 3736 cm −1 for the α direction. The crystal structure was refined in space group C 2/ m to R 1 = 3.6% [ I > 2σ( I )], with resulting cell parameters a = 9.9977(3) Å, b = 18.0409(4) Å, c = 5.2794(2) Å, γ = 104.465(4)°, V = 922.05(5) Å 3 and Z = 2. The A and M (4) sites split into A ( m ) (K + ), A (2/ m ) (Na + ), A (2) (Pb 2+ ), and M (4′) (Mn 2+ ) subsites, respectively. The remaining Mn 2+ is strongly ordered at the octahedrally coordinated M (2) site, possibly together with most of Fe 3+ . The skarn bearing potassic-richterite formed at peak metamorphism, under conditions of low SiO 2 and Al 2 O 3 activities and relatively high oxygen fugacities.