Experimental study of concentration dependence of Ta2O5 and Nb2O5 solubility in the alkaline and carbonate solutions at T=550oC, P=500 bar and low oxygen fugacity (Co-CoO buffer)

Abstract

This study is a continuation of long-term work to build a quantitative model of genesis of deposits of Ta, Nb and Li, connected with granites. Importance of the problem is the need to use the available experimental data to estimate quantitatively a possible role of hydrothermal transport and redeposition of tantalum and niobium under physical-chemical conditions typical for Ta and Nb deposits, generally connected with alkaline rocks. Earlier we investigated concentration and temperature dependence of the tantalum and niobium oxides solubility in the sodium alkaline and carbonate solutions in a wide range of temperature, compositions and concentrations of solutions at P=1000 bar [Kotova and Zaraisky, 2009; Zaraisky et al., 2009]. The choice of sodium specificity of the solutions is related to the fact, that Ta-Nb mineralization in lithium-fluoric granites (“apogranites”), alkaline granites, alkaline metasomatites, sienites and carbonates is closely associated with sodium metasomatism (albitization, ribektization, egirinization). In order to practically cover the entire range of conditions for the formation of Ta-Nb deposits the experimental studies of Та2О5 and Nb2O5 solubility were carried out in aqueous alkaline solutions at P=500 bar. The tantalum and niobium oxides- chemical reagents Та2О5 and Nb2O5, being the analogues of tantite and nioboxide, seldom occurring in nature, were used as initial material in the runs. The concentration dependence of the Та2О5 and Nb2O5 solubility at T=550 o C, P=500 bar in the solutions of Na2CO3 and NaOH with the concentration (0.01; 0.1; 0.5, 1.0 и 2.0 m) in the presence of oxygen buffer Co–CoO was studied. Run duration was 21 days. Experiments were performed on a hydrothermal line. The capsules and the container with buffer were sealed into cold-seal pressure vessel of Tuttle type with big working volume that gives possibility to isolate the capsules from the container with buffer. The same technique was used to study Та2О5 solubility in fluoride and chloride solutions. After the run, the quenched aqueous solutions and the solid products were separated using a centrifuge. The solutions were then analyzed using ICP/MS (Inductively Coupled Plasma Mass Spectrometry) and ICP/AES (Atomic Emission Spectroscopy) for Nb, Ta, Mn, and Fe and admixture elements Ti, W, and Sn,. The composition of the solid product was characterized using optical microscopy, X-ray diffraction, and electron microprobe analysis (Cam Scan MV 2300 (VEGA TS5130MM). Experimental data are shown in fig. 1, 2, 3 and 4. The Ta2O5 solubility isotherms was found to have a negative trend in all studied alkaline and carbonate solutions at P=1000 bar. The concentration dependence of the Та2О5 was found to be more complicated at P=500 bar. In NaOH solutions a pressure decrease from 1000 bar to 500 leads to a decrease of tantalum content by a factor of 1 between 0.01 and 0.05 m NaOH. However, the Ta2O5 solubility increases by a factor of 1 in aqueous solutions with NaOH concentrations of 1 m and greater (fig. 1). In the Na2CO3 solutions with decreasing pressure from 1000 bar to 500 the tantalum content decreases by a factor of 1 (fig. 2). Experimentally established fact of the weak solubility of tantalum oxide in aqueous alkaline and carbonate solutions is fundamentally important for understanding of genesis of Ta deposits. The experimental data show that, for the range of concentrations, temperatures and pressures considered in this study, the Ta2O5 solubility is very low with concentrations on the order of 10 -5 to 10 -7 m in aqueous alkaline and carbonate solutions. The investigations have shown that at such low concentrations it is difficult to speak about possibility of Ta hydrothermal transport by alkaline and carbonate solutions. In NaOH and Na2CO3 solutions oxide tantalum has incongruent solubility. As solid phases wonderfully cut crystals of Na-tantite Na2Ta4O11 with the hexaoctahedric form typical for pyrochloride are formed in 0.01 and 0.1m NaOH and