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 (Sc0.64Sn0.13Mn0.12Fe0.08Ti0.06)∑1.03(Ta0.69Nb0.30)∑0.99O4 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; Dx = 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.

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