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

Hjalmarite, ideally ANaB(NaMn)CMg5TSi8O22W(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 (Na0.84K0.16)Σ1(Na1.01Mn0.55Ca0.43Sr0.01)Σ2(Mg3.83Mn1.16Al0.01)Σ5(Si7.99Al0.01)Σ8O22(OH1.92F0.08)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, (⁠-661); 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) Å3. 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).