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. (28th August 2018)

• Record Type: Journal Article
• Authors: Cooper, Mark A.; Raade, Gunnar; Ball, Neil A.; Abdu, Yassir A.; Hawthorne, Frank C.; Rowe, Ralph
• Is Part Of: Mineralogical magazine. Volume 82:Number 4(2018)
• Page Start: 821
• Page End: 836
• Publication Date: 2018-08-28
• Subjects: folvikite, -- new mineral, -- chemical analysis, -- crystal structure, -- infrared spectrum, -- Kitteln mine, -- Värmland, -- Sweden; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/minmag.2017.081.059
• Languages: English
• ISSNs: 0026-461X

Abstract

ABSTRACT: Folvikite, Sb 5+ Mn 3+ (Mg, Mn 2+ )10 O8 (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/cm 3 . 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 P 2, a = 5.3767(10), b = 6.2108(10), c = 10.9389(18) Å, β = 94.399(9)°, V = 364.22(16) Å 3 and Z = 1. Chemical analysis by electron microprobe gave Sb2 O5 18.15, MgO 24.11, MnO 29.73, Mn2 O3 11.62, Al2 O3 0.27, Fe2 O3 0.45, B2 O3 15.27, sum 99.60 wt.%. The B2 O3 content was assigned as B = 4 apfu and the Mn2 O3 / (MnO + Mn2 O3 ) ratio was determined from the crystal structure. The empirical formula was normalized on the basis of 20 anions pfu: (Sb 5+ 1.02 Mn 3+ 1.34 Al0.05 Fe 3+ 0.05 Mg5.46 Mn 2+ 3.82 □0.26 )Σ12 O8 (BO3 )4 . A simplified formula may be written as Sb 5+ Mn 3+ (Mg, Mn 2+ )10 O8 (BO3 )4 with Z = 1. The crystal structure was solved by direct methods and refined to an R 1 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 Mn 2+ > Mg, one octahedrally coordinated M -site occupied by Sb 5+ > Mg, two M sites occupied by Mg ≥ Mn > Sb 5+, two M sites occupied by Mn 3+ > Mn 2+, two M sites occupied by Mg > Mn 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.