Kleberite, Fe~(3+)Ti_6O_(11)(OH)_5, a new ilmenite alteration product, from Konigshain, northeast Germany

摘要

Kleberite, ideally Fe~(3+)Ti_6O_(11)(OH)_5, is a new mineral (IMA 2012-023) from Tertiary sands at Konigshain, Saxony, northeast Germany. It is also found in heavy mineral sands from the Murray Basin, southeast Australia and at Kalimantan, Indonesia. It occurs as rounded anhedral to euhedral translucent grains, 0.04-0.3 mm across, which are generally red-brown, but grade to orange with decreasing iron content. Associated minerals include ilmenite, pseudorutile, 'leucoxene', tourmaline and spinel. The density measured by pycnometry is 3.28 g cm"3, which is lower than the calculated density of 3.91 g cm~(-1), due to intragrain porosity which is not penetrated by the immersion fluid. The intragrain pores, of median diameter 18 nm, are partially filled with impurity phases including kaolinite, diaspore and quartz. Kleberite grains have a uniaxial (-) character, but localized regions are weakly biaxial (-) with 2V close to zero. The mean refractive index, calculated from reflectance measurements, is 2.16(3). The mean empirical formula from electron-microprobe analyses of 15 Konigshain kleberite grains is Fe_(1.01)~(3+)Mg_(0.06)Ti_6O_(11.2)(OH)_(4.8)[Al_(0.59)Si_(0.31)P_(0.04)O_(1.60)-1.8H_2O], where the formula element in square brackets represents impurities in the pores. Kleberite forms over a compositional range with [Ti]/[Fe + Ti] atomic ratios from 0.8-0.9. It has monoclinic symmetry, P2,/c, with a = 7.537(1), b = 4.5795(4), c = 9.885(1) A, β = 131.02(1)°. The six strongest lines in the powder X-ray diffraction (XRD) pattern [listed as d in A (I)] are as follows: 1.676(100), 2.170(82), 2.466(27), 1.423(22), 3.933(8), 2.764(9). The structure was refined by the Rietveld method on powder XRD data to R_p = 6.3,R_(wp) = 8.1, R_B = 4.0. Kleberite is isostructural with tivanite; their structural formulae are [Ti_3~(4+)][Ti_3~(4+)Fe~(3+)]O_(11)(OH)_5 and [Ti_4~(4+)][V_4~(3+)]O_(12)(OH)_4, respectively. Kleberite has dominant Ti~(4+) in place of V3+ in the M(2) metal-atom site. The related mineral pseudorutile, [Ti_4][(Fe~(3+),Ti)_4](O,OH)_(16), with Fe~(3+) > Ti~(4+) has dominant Fe~(3+) in this site. Kleberite grains from different localities commonly contain residual MgO-rich ferrian ilmenite. The chemical and physical relationships between the ilmenite and coexisting kleberite are used to evaluate different alteration mechanisms involving selective leaching of divalent oxides from ilmenite and pseudomorphic replacement reactions, respectively.