Crystallochemical studies on davidite from Bichun, Jaipur District, Rajasthan, India

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Marco E. Ciriotti
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Crystallochemical studies on davidite from Bichun, Jaipur District, Rajasthan, India

Messaggio da Marco E. Ciriotti » dom 04 mar, 2018 11:37

▪ Singh, Y., Saxena, A., Bhatt, A.K., Viswanathan, R., Shaji, T.S., Nanda, L.K. (2018): Crystallochemical studies on davidite from Bichun, Jaipur District, Rajasthan, India. Journal of Earth System Science, 127, 4.

Crystallochemical data on metamict davidite from albitites and albitised rocks from the Bichun area (Jaipur district, Rajasthan, India) of Banded Gneissic Complex (BGC) are provided. Davidite occurs as euhedral, subhedral to anhedral crystals in the form of disseminated grains and also as fracture filled veins. The crystals of davidite are up to 8 cm in length and 6 cm in width. The powder X-ray diffraction (XRD) pattern of the heat-treated davidite (at 900∘C) reveals well-defined reflections of crystallographic planes. The calculated unit-cell parameters of the heat treated davidite are: a0=b0=10.3556\AA and c0=20.9067\AA , with unit-cell volume (V)=1941.6385\AA 3; and α=β=90∘ and γ=120∘, which are in agreement with the values of davidite standard. Geochemical data reveals that the investigated davidite contains 51.5–52.6% TiO2, 14.8–15.1% Fe2O3, 9.8–10.2% FeO, 6.97–7.12% U3O8, 6.72–6.92% RE2O3, 3.85–3.61% K2O, 0.9–1.4% Al2O3, and 0.8–1.2% SiO2. The calculated structural formulae of the two davidite crystals are: D-1: K0.0044/0.004Ba0.0044/0.005Ca0.20/0.20Na0.012/0.012Mn0.053/0.053Mg0.14/0.14Pb0.0076/0.008Fe2.675/2.675Fe1.59/1.59Y0.1175/0.118P0.053/0.053Nb0.008/0.008Sn0.001/0.001Zr0.033/0.033U0.468/0.468Th0.009/0.009REE0.6829/0.683)6.05/6.05(Ti12.15/12.15Fe1.9022/1.903Si0.372/0.372Al0.517/0.517Cr0.018/0.018Co0.009/0.009Ni0.027/0.027)15/15O36/36(OH0.319/0.319[]1.681/1.681)2/2 and D-2: (K0.004/0.004Ba0.005/0.005Ca0.20/0.20Na0.012/0.012Mn0.05/0.05Mg0.094/0.094Pb0.007/0.007Fe2.58/2.58Fe1.71/1.71Y0.112/0.112P0.106/0.106Nb0.006/0.006Sn0.001/0.001Zr0.03/0.03U0.48/0.48Th0.009/0.009REE0.665/0.665)6.088/6.088(Ti12.48/12.48Fe1.87/1.87Si0.249/0.249Al0.334/0.334Cr0.019/0.019Co0.008/0.008Ni0.04/0.04)15/15O36/36(OH0.098/0.098[]1.90/1.90)2/2. The calculated structural formulae are not fully stoichiometric, which could be due to metamict nature of davidite. The characteristic feature of distribution pattern of REE in davidite is unusually high concentration of LREE and HREE and substantially low content of MREE. It may be due to the occupation of REEs in two distinct crystallographic sites in davidite structure, i.e., M(1) and M(O) sites. Chondrite-normalised plot of davidite reveals a pronounced negative Eu-anomaly (Eu/Eu∗=0.30−0.39), which suggests extremely fractionated nature of the metasomatising fluids from which davidite had crystallized. Metamict davidite-bearing U ores not only from Rajasthan, but also from other parts of India are likely to yield very high U leachability, thereby making them attractive sources of U, which otherwise are ignored by mineral engineers as uneconomic U ores.
Marco E. Ciriotti

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