Folvikite, Sb5+Mn3+(Mg,Mn2+)10O8(BO3)4, a new oxyborate mineral from the Kitteln mine, Nordmark ore district, Värmland, Sweden: description and crystal structure

Abstract

ABSTRACTFolvikite, Sb5+Mn3+(Mg,Mn2+)10O8(BO3)4, is a new oxyborate mineral from the Kitteln mine, Värmland, Sweden, where it occurs as a primary skarn mineral embedded in calcite. It forms striated prismatic crystals up to 0.3 mm, and is black to dark reddish-brown with submetallic lustre and a reddish-brown streak. It is brittle, has a Mohs hardness of 6, and the calculated density is 4.14 g/cm3. Folvikite is biaxial with indeterminate optic sign due to pervasive twinning. The optic axial angle is 68.9(4)°. Refractive indices were not measured; the calculated mean refractive index is 1.85. Strong pleochroism was observed in plane-polarized light: AB = brown (intermediate), OB = dark brown (maximum) and ON = honey brown (minimum). Folvikite is monoclinic, space group P2, a = 5.3767(10), b = 6.2108(10), c = 10.9389(18) Å, β = 94.399(9)°, V = 364.22(16) Å3 and Z = 1. Chemical analysis by electron microprobe gave Sb2O5 18.15, MgO 24.11, MnO 29.73, Mn2O3 11.62, Al2O3 0.27, Fe2O3 0.45, B2O3 15.27, sum 99.60 wt.%. The B2O3 content was assigned as B = 4 apfu and the Mn2O3 / (MnO + Mn2O3) ratio was determined from the crystal structure. The empirical formula was normalized on the basis of 20 anions pfu: (Sb5+1.02Mn3+1.34Al0.05Fe3+0.05Mg5.46Mn2+3.82□0.26)Σ12O8(BO3)4. A simplified formula may be written as Sb5+Mn3+(Mg,Mn2+)10O8(BO3)4 with Z = 1. The crystal structure was solved by direct methods and refined to an R1 index of 4.1%. Folvikite is a member of the (3 Å) zigzag wallpaper-borate structures in which chains of edge-sharing octahedra extend along the c axis and are cross-linked by BO3 groups. There are five X sites partly occupied by Mn2+ > Mg, one octahedrally coordinated M-site occupied by Sb5+ > Mg, two M sites occupied by Mg ≥ Mn > Sb5+, two M sites occupied by Mn3+ > Mn2+, two M sites occupied by Mg > Mn2+, and one M-site occupied by Mg > □; plus two [3]-coordinated B sites occupied by B. As with the other zigzag borates, the polyhedra are arranged in F-walls, C-walls and S-columns.