Journal of Applied Geochemistry

Investigations on X-ray crystallography and mineral chemistry have been carried out on microlite and chevkinite from Nellore Schist Belt (NSB) and the results are presented. The crystallographic parameters of the investigated microlite are: unit cell dimension (a_o) = 10.3537 Å and unit cell volume (V) = 1109.93 ų. Microlite contains high Ta₂O₅ (59.37%) and notable amounts of Nb₂O₅ (13.30%), CaO (13.43%), TiO₂ (2.10%) and Fe₂O₃(2.83%) with high Fe₂O₃/Fe₂O₃ ratio (∼4). It is radioactive (U₃O₈ 0.54% and ThO₂ 0.37%). Chondrite-normalised rare-earth element (REE) plot of the microlite reveals enrichment of light REE (LREE) over heavy REE (HREE) with pronounced negative Eu-anomaly (Eu/Eu* = 0.11).

The calculated unit cell constants of the (metamict) chevkinite are: a_o = 13.4251 Å, b_o = 5.7232 Å and c_o = 11.0645 Å, with a V of 836.868 ų and beta angle of 100.1354°. It is characterized by variable and high Fe₂O₃ (7.94–47.00%), FeO (4.13–4.31%), SiO₂ (4.50–22.63%), CaO (5.37–8.56%), Al₂O₃ (4.50–13.36%), TiO₂ (0.55–2.50%), TaO₂ (13.20–15.64%), Nb₂O₅ (2.82–3.17%) and MnO (1.75–5.68%) contents, with notable amounts of Na₂O (0.43–0.69%) and K₂O (0.10–0.13%). It is also radioactive (U₃O₈ 0.300.35% and ThO₂ 0.40–1.12%). Like microlite, chevkinite also reveals a preponderance of LREE (∑ 41625-127418 ppm) relative to HREE(∑ 1210–2654 ppm).

Very high values of (La/Lu)_N (15.49–720.35), (La/Yb)_N (15.80–935.11) and Ce/Yb)_N (14.15–795.91) ratios in microlite and chevkinite reflect pronounced fractionation of LREE over HREE. Interestingly, appreciable fractionation among HREE is also brought out by the high (Gd/Lu) _N (1.86–57.81) and (Gd/Yb)_N (2.27–75.05) ratios. Available data reveals that the melt from which the investigated Rare Metal and Rare Earth (RMRE)-bearing minerals had formed was fractionated to certain extent. However, due to sporadic nature of such fractionated pegmatite melt, RMRE mineral abundances in pegmatites of the area are limited.