Ferri-taramite, a new member of the amphibole supergroup, from the Jakobsberg Mn–Fe deposit, Värmland, Sweden

Abstract. Ferri-taramite (IMA CNMNC 2021-046), ideally ANaB(CaNa)C(Mg3Fe23+)(Si6Al2)O22W(OH)2, occurs in skarn from the Jakobsberg manganese mine, Värmland, Sweden. Associated minerals are celsian, phlogopite, aegirine-augite, andradite, hancockite, melanotekite, microcline (var. hyalophane), calcite, baryte, prehnite, macedonite and oxyplumboroméite. Conditions of formation, close to peak metamorphism (at circa 650 ∘C 0.4 GPa), include silica undersaturation, a slightly peralkaline character relatively high oxygen fugacities. forms poikiloblastic crystals up 5 mm is dark brownish black with yellowish grey streak. The amphibole brittle an uneven splintery fracture. Cleavage parallel {110} good. Hardness (Mohs) ∼ 6, Dcalc=3.227(5) g cm−3. Holotype ferri-taramite has experimental unit formula A(Na0.79K0.16Pb0.01)Σ0.96B(Ca1.26Na0.72Mn0.022+)Σ2C(Mg2.66Mn0.582+Fe0.162+Zn0.02Fe1.263+ Al0.26Ti0.06)Σ5.00T(Al1.86Si6.14)Σ8O22W(OH)2, based on chemical analyses (EDS, laser-ablation ICP-MS) spectroscopic (Mössbauer, infrared) single-crystal X-ray diffraction data. mineral optically biaxial (–), α=1.670(5), β=1.680(5) γ=1.685(5) white light 2Vmeas=70(10)∘ 2Vcalc=70.2∘. distinctly pleochroic transmitted light, X pale yellow, Y brown, Z brown absorption Y>Z>X. eight strongest reflections powder pattern (d values (in Å), Irel, hkl) 8.44, 60, 110; 3.392, 25, 131; 3.281, 39, 240; 3.140, 100, 310; 2.816, 45, 330; 2.7104, 38, 151; 1.3654, 26, 461; 1.4451, 33, 6‾61. Refined unit-cell parameters data a=9.89596(13), b=18.015(2), c=5.32164(7) Å, β=105.003(13)∘ V=916.38(2) Å3 for Z=2. Refinement crystal structure yielded R=2.26 % 2722 Io>2σ(I). Mn2+ Fe2+ ions show preference M1 M3 octahedrally coordinated sites, whereas Fe3+ strongly ordered at M2. A-group cations, K Na, split over two subsites, A(m) A(2), respectively.