Hjalmarite, a new Na–Mn member of the amphibole supergroup, from Mn skarn in the Långban deposit, Värmland, Sweden

Hjalmarite, ideally ^ANa^B(NaMn)^CMg₅^TSi₈O₂₂^W(OH)₂, is a new root-name member of the amphibole supergroup, discovered in skarn from the Långban Fe–Mn–(Ba–As–Pb–Sb–Be–B) deposit, Filipstad, Värmland, Sweden (IMA-CNMNC 2017-070). It occurs closely associated with mainly rhodonite and quartz. It is grayish white with vitreous luster and non-fluorescent. The crystals are up to 5 mm in length and display splintery fracture and perfect cleavage on {110}. Hjalmarite is colorless (non-pleochroic) in thin section and optically biaxial (–), with α = 1.620(5), β = 1.630(5), γ = 1.640(5). The calculated density is 3.12 Mg/m³. Average VHN₁₀₀ is 782, corresponding to ca. 5½ Mohs. An empirical formula, derived from electron-microprobe analyses in combination with crystal-structure refinements, is (Na_0.84K_0.16)_Σ1(Na_1.01Mn_0.55Ca_0.43Sr_0.01)_Σ2(Mg_3.83Mn_1.16Al_0.01)_Σ5(Si_7.99Al_0.01)_Σ8O₂₂(OH_1.92F_0.08)_Σ2. An infra-red spectrum of hjalmarite shows distinct absorption bands at 3673 cm⁻¹ and 3731 cm⁻¹ polarized in the α direction. The eight strongest Bragg peaks in the powder X-ray diffraction pattern are [d (Å), I (%), (hkl)]: 3.164, 100, (310); 2.837, 50, (330); 8.50, 44, (110); 3.302; 40, (240); 1.670, 34, (461); 1.448, 32, (⁠$6¯$61); 2.727, 30, (151); 2.183, 18 (261). Single-crystal X-ray diffraction data were collected at 298 K and 180 K. The crystal structure at ambient temperature was refined in space group C2/m to R1 = 2.6% [I > 2σ(I)], with observed unit-cell parameters a = 9.9113(3), b = 18.1361(4), c = 5.2831(5) Å, β = 103.658(5)° and V = 922.80(9) ų. The A and M(4) sites split into A(m) (K⁺, Na⁺), A(2) (Na⁺), and M(4’) (Mn²⁺) subsites, respectively. Among the octahedrally coordinated C group cations, Mn²⁺ orders strongly at the M(2) site. No significant violation of C2/m symmetry or change in the structure topology was detected at low temperature (R1 = 2.1%). The hjalmarite-bearing skarn formed at peak regional metamorphism, T ≥ 600 °C, at conditions of high SiO₂ activity and relatively low oxygen fugacity. The mineral name honors the Swedish geologist and mineralogist S.A. Hjalmar Sjögren (1856–1922).