Peterandresenite, Mn₄Nb₆O₁₉.14H₂O, a new mineral containing the Lindqvist ion from a syenite pegmatite of the Larvik Plutonic Complex, southern Norway

Peterandresenite (IMA2012-084), ideally Mn₄Nb₆O₁₉.14H₂O, is the first naturally occurring hexaniobate. It was found at the AS Granit larvikite quarry in Tvedalen, Larvik, Vestfold, Norway, by private collector Peter Andresen, after whom the mineral is named. The mineral was found on fracture surfaces and in tiny vugs in the centre of a miaskitic pegmatite dike. It occurs as equidimensional, transparent to translucent orange crystals up to 1 mm with a pale orange streak and a vitreous to resinous lustre. The Mohs hardness is 2-2.5 and the mineral is brittle with uneven fracture and no cleavage. D_(calc.) = 3.05 g/cm³ and D_(meas.) = g/cm³. Peterandresenite is biaxial (-) and the refractive indices (white light) are: α = 1.760(5), β = 1.795(5) and γ = 1.800(5); 2V_(meas.) = 43(2) and 2V_(calc.) = 40.7 . The mineral exhibits strong dispersion (r>v) and is pleochroic with X (colourless) < Z(pale orange) << Y (medium orange). The optical orientation is: X ∼ c, Y ∼ a*, Z = b. The empirical formula based on electron microprobe analyses is (Mn_3.92Ca_0.05Na_0.03)_Σ4.00(Nb_5.71Mn_0.13Fe_0.12S i_0.03)_Σ5.99O_18.57.14H₂O. The five strongest reflections in the X-ray diffraction pattern [d_obs. in Aa (I) (hkl)] are: 9.8977 (82) (001), 7.7104 (42) (110), 7.4689 (39) (20-1), 7.1026 (63) (11-1) and 2.9260 (100) (42-2). The mineral is monoclinic, C2/m, with a = 15.329(1), b = 9.4121(5), c = 11.2832(9) Aa, β = 118.650(4) , V = 1428.6(2) Aa³ and Z = 2. Peterandresenite has a novel structure consisting of six edge-sharing Nb-octahedra forming a super octahedron known as a Lindqvist ion. One Mn²⁺ octahedron interconnects three Lindqvist ions to form a two-dimensional layer perpendicular to the c-axis. A second Mn²⁺ octahedron bridging with the Lindqvist ion protrudes into the adjacent layer along the c-axis and creates a three-dimensional structure via hydrogen bonds.