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) 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)°, 2Vcalc = 46(5)°. 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 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.

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