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Gjerdingenite-Mn from Norway — a new mineral species in the labuntsovite group

¹ Geologisk Museum, Universitetet i Oslo, Postboks 1172 Blindern, N-0318 Oslo, Norway
² Institute of Problems of Chemical Physics, Chernogolovka, Moscow Region 142432, Russia
³ Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstrasse 14, A-1090 Wien, Austria
⁴ Alpha-Geophysik in Sachsen, Neudorfer Strasse 18, D-09629 Burkersdorf, Germany
⁵ NPO "Regenerator", Skladochnaya 6, Moscow 127018, Russia
⁶ Moscow State University, Vorobievy Gory, Moscow 119899, Russia

^* E-mail: gunnar.raade{at}nhm.uio.no

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].6H ₂O. 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)°, V = 1414.3(6) ų, 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 [d_obs 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 = 1.670(2), β = 1.685(2), = 1.775(5); 2V_meas = 52(8)°, 2V_calc = 46(5)°. The axial dispersion is weak, r < v; the orientation is Z = a, Y = b. Mohs hardness is about 5; D_calc = 2.93 g/cm³. The pseudo-orthorhombic 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.

Key-words: gjerdingenite-Mn, new mineral, labuntsovite group, chemical analysis, crystal structure, Norway.

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