| Abstrakt: |
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.) = 3.10(1) g/cm3. Peterandresenite is biaxial (-) and the refractive indices (white light) are: a = 1.760(5), ß = 1.795(5) and ? = 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 (Mn3.92Ca0.05Na0.03)4.00(Nb5.71Mn0.13Fe0.12Si0.03)5.99O18.57·14H2O. The five strongest reflections in the X-ray diffraction pattern [d obs.in Å (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) Å, ß = 118.650(4)°, V = 1428.6(2) Å3 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. |
