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

Potassic-richterite, ideally ^AK^B(NaCa)^CMg₅^TSi₈O₂₂^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. VHN₁₀₀ 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_0.61Na_0.30Pb_0.02)_∑0.93(Na_1.14Ca_0.79Mn_0.07)_∑2(Mg_4.31Mn_0.47Fe³⁺_0.20)_∑5(Si_7.95Al_0.04Fe³⁺_0.01)_∑8O₂₂(OH_1.82F_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 C2/m to R1 = 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) ų and Z = 2. The A and M(4) sites split into A(m) (K⁺), A(2/m) (Na⁺), A(2) (Pb²⁺), and M(4′) (Mn²⁺) subsites, respectively. The remaining Mn²⁺ is strongly ordered at the octahedrally coordinated M(2) site, possibly together with most of Fe³⁺. The skarn bearing potassic-richterite formed at peak metamorphism, under conditions of low SiO₂ and Al₂O₃ activities and relatively high oxygen fugacities.