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Abstract
Fredrikssonite is a new member of the pinakiolite group with ideal formula Mg2Mn3+(BO3)O2. It is orthorhombic, space group Pbam or Pba2 with a = 9.18(1), b = 12.555(6), c = 2.954(2) Å, Z = 4. The strongest reflections in the X-ray powder diffraction pattern are (d, I, hkl): 5.1680120; 2.590100240; 2.48690201; 2.20130250; 2.01350321, 430; 1.51340521, 171. Microprobe analysis yielded Al2O3 1.9, Fe2O3 5.4, Mn2O3 35.5, MgO 40.3, B2O3 17.9 (estimated by ion microprobe), sum = 101.0%. The chemical formula, calculated on the basis of total cations = 3, yields: Mg1.93(Mn3+ 0.87Fe3+ 0.13Al0.07(BO3)0.99O2.05. Fredrikssonite is reddish brown, slightly transparent, but nearly opaque in one direction. The hardness (Mohs) is approximately 6; the density is 3.84(5) (meas.), 3.80(calc.) g/cm3. The luster is vitreous. Cleavage was observed only on small fragments; its orientation is unknown. Optically, fredrikssonite is biaxial, positive, α = 1.82(2), β = < 1.86, γ = ≈ 1.99; pleochroism is strong with X = golden brown and Z = dark reddish brown to black; absorption: Z > X; dispersion is strong, r > v; Z = c. Fredrikssonite is found associated with calcite, adelite, brucite and hausmannite at the Långban Mine, Värmland, Sweden. In a second assemblage, fredrikssonite is associated with clinohumite, calcite, and jacobsite. An unnamed borate from Nordmark, Sweden, has the chemical formula (Mg1.19Mn0.81)σ2.00(Mn0.73 3+Sb0.15Fe0.11 3+Al0.01)σ1.00(BO3)O2. It occurs as broken, thin, bladed, black crystals in calcite.