Original paper

Gjerdingenite-Mn from Norway a new mineral species in the labuntsovite group: descriptive data and crystal structure

Raade, Gunnar; Chukanov, Nikita V.; Kolitsch, Uwe; Möckel, Steffen; Zadov, Aleksandr E.; Pekov, Igor V.

Abstract

Gjerdingenite-Mn occurs as orange-yellow to brownish prisms up to 1 mm long in miarolitic cavities of a sodic granite ('ekerite') at Gjerdingselva, Lunner, Oppland, Oslo Region, Norway. The simplified formula is (K, Na)2(Mn, Fe) [(Nb, Ti)4(Si4O12)2(O, OH)4]·6H2O. It is the Mn-dominant analogue of gjerdingenite-Fe, the Ti analogue of kuzmenkoite-Mn and a dimorph of organovaite-Mn. The mineral is monoclinic, C2/m, with a = 14.563(3), b = 13.961(3), c = 7.851(2) Å,β = 117.62(3)º, V = 1414.3(6) Å3, Z = 2. The crystal structure was refined to R(F) = 0.079 on the basis of 1207 observed reflections and is compared to the structure of gjerdingenite-Fe. The strongest five reflections of the X-ray powder-diffraction pattern [dobs in Å (I) (hkl)] are: 6.96 (100) (020, 001), 4.94 (80) (021), 3.22 (90) (42-1, 400, 40-2), 3.10 (80) (041, 022) and 2.510 (40) ( 44-1, 401, 40-3, 042). The mineral is optically biaxial (+) with α = 1.670(2), β = 1.685(2), γ = 1.775(5); 2Vmeas = 52(8)º, 2Vcalc = 46(5)º. The axial dispersion is weak, r < v; the orientation is z = a, Y = b. Mohs hardness is about 5; Dcalc = 2.93 g/cm3. The pseudoorthorhombic crystals are twinned on {001}, elongate along [010] and show the forms {001}, {100}, {201} and {021}. Some general aspects on the formation of labuntsovite-group minerals are discussed.

Keywords

gjerdingenite-mnnew minerallabuntsovite groupchemical analysiscrystal structurenorway