Folvikite, Sb (super 5+) Mn (super 3+) (Mg,Mn (super 2+) ) (sub 10) O (sub 8) (BO (sub 3) ) (sub 4) , a new oxyborate mineral from the Kitteln Mine, Nordmark ore district, Varmland, Sweden; description and crystal structure

Folvikite, Sb (super 5+) Mn (super 3+) (Mg,Mn (super 2+) ) (sub 10) O (sub 8) (BO (sub 3) ) (sub 4) , is a new oxyborate mineral from the Kitteln mine, Varmland, 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/cm (super 3) . Folvikite is biaxial with indeterminate optic sign due to pervasive twinning. The optic axial angle is 68.9(4) degrees . 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) Aa, beta = 94.399(9) degrees , V = 364.22(16) Aa (super 3) and Z = 1. Chemical analysis by electron microprobe gave Sb (sub 2) O (sub 5) 18.15, MgO 24.11, MnO 29.73, Mn (sub 2) O (sub 3) 11.62, Al (sub 2) O (sub 3) 0.27, Fe (sub 2) O (sub 3) 0.45, B (sub 2) O (sub 3) 15.27, sum 99.60 wt.%. The B (sub 2) O (sub 3) content was assigned as B = 4 apfu and the Mn (sub 2) O (sub 3) / (MnO + Mn (sub 2) O (sub 3) ) ratio was determined from the crystal structure. The empirical formula was normalized on the basis of 20 anions pfu: (Sb (super 5+) (sub 1.02) Mn (super 3+) (sub 1.34) Al (sub 0.05) Fe (super 3+) (sub 0.05) Mg (sub 5.46) Mn (super 2+) (sub 3.820.26) ) (sub Sigma 12) O (sub 8) (BO (sub 3) ) (sub 4) . A simplified formula may be written as Sb (super 5+) Mn (super 3+) (Mg,Mn (super 2+) ) (sub 10) O (sub 8) (BO (sub 3) ) (sub 4) with Z = 1. The crystal structure was solved by direct methods and refined to an R (sub 1) index of 4.1%. Folvikite is a member of the (3 Aa) zigzag wallpaper-borate structures in which chains of edge-sharing octahedra extend along the c axis and are cross-linked by BO (sub 3) groups. There are five X sites partly occupied by Mn (super 2+) > Mg, one octahedrally coordinated M-site occupied by Sb (super 5+) > Mg, two M sites occupied by Mg > or = Mn > Sb (super 5+) , two M sites occupied by Mn (super 3+) > Mn (super 2+) , two M sites occupied by Mg > Mn (super 2+) , 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.