Heftetjernite, a new scandium mineral from the Heftetjern pegmatite, Tørdal, Norway

Authors: Kolitsch, Uwe; Kristiansen, Roy; Raade, Gunnar; Tillmanns, Ekkehart

Source: European Journal of Mineralogy, Volume 22, Number 2, March 2010 , pp. 309-316(8)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

Heftetjernite, ideally ScTaO₄, is a new scandium mineral from the Heftetjern pegmatite, Tørdal, Telemark, Norway. In the type specimen, it occurs as minute, elongate tabular, very dark brown crystals in a single small void in albite. Other associated minerals are fluorite, muscovite, altered milarite, a metamict, dark greyish brown mineral of the pyrochlore-microlite group, and an unidentified, orange-brown, tabular, nearly X-ray amorphous Ti-Y-Ta-Nb-mineral. Electron-microprobe analysis yielded the empirical formula (Sc_0.64Sn_0.13Mn_0.12Fe_0.08Ti_0.06)_Σ1.03(Ta_0.69Nb_0.30)_Σ0.99O₄ which clearly demonstrates the charge-balanced substitution scheme 2Sc³⁺ = (Sn,Ti)⁴⁺ + (Mn,Fe)²⁺. The mineral crystallises in the wolframite structure type, with space group P2/c and a = 4.784(1), b = 5.693(1), c = 5.120(1) Å, β = 91.15(3) , V = 139.42(5) Å³ (Z = 2). A synthetic equivalent is known. Strongest lines in the calculated X-ray powder diffraction pattern of heftetjernite are [d in Å (I) (hkl)]: 3.000 (100) (11-1), 2.9570 (97) (111), 3.662 (53) (110), 2.4877 (34) (02-1), 4.783 (33) (100), 3.807 (32) (01-1). The crystal structure was refined to R(F) = 1.39 % from single-crystal X-ray diffraction data (293 K). It is based on two types of edge-sharing, distorted octahedra occupied predominantly by Sc and Ta, respectively.

Heftetjernite is translucent to transparent, with a dark brownish (with a reddish hue) streak and adamantine lustre. It is brittle, has a perfect {010} cleavage, irregular fracture and a Mohs hardness estimated to be around 4.5 by comparison to ferberite; D_x = 6.44 g/cm³ (from crystal-structure analysis). Optically, the mineral is biaxial with an unknown optical sign, weakly pleochroic (yellowish brown with a reddish tint to reddish brown), with no observable dispersion. A mean refractive index of 2.23 was calculated from the Gladstone-Dale relationship using the X-ray density.

Heftetjernite is named after its type locality. The mineral is compared with synthetic ScTaO₄, ScNbO₄, iwashiroite-(Y) and formanite-(Y) (both nominally YTaO₄), and some comments are made on the relation to Sc-bearing ixiolite.

Keywords: HEFTETJERNITE; NEW MINERAL; SCANDIUM TANTALATE; CRYSTAL STRUCTURE; CHEMICAL ANALYSIS

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2010/0022-1987

Publication date: 2010-03-01

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