ENVIRONMENTAL GEOCHEMISTRY AND MINERALOGY OF TANTALUM AND NIOBIUM AROUND A LCT PEGMATITE DEPOSIT OF THE RUDNY ALTAI, EAST KAZAKHSTAN

Abstract

Tantalum (Ta) and Niobium (Nb) are critical metals required in many applications related to the energy transition. They are typically found in rare-metal pegmatites, where they occur in minerals of the tantalite – columbite series (Fe,Mn)(Ta,Nb)2O6. The geochemistry of Ta and Nb in high-temperature magmatic systems is well understood by igneous petrologists. In contrast, although they are not considered particularly toxic, very little is known about their behavior in low-temperature environments. During a visit to a LCT (lithium-cesium-tantalum) pegmatite in the Altai, a number of Li-, Cs-, and Ta, Nb-bearing minerals were sampled. The locality also offers the possibility to sample soils and stream sediments. A preliminary review of the bibliography of Ta and Nb is first presented. Then the diversity of Ta and Nb minerals found in the last International Mineralogical Association (IMA) list of accepted minerals discussed and some conclusions regarding the possibility of identifying new minerals that have not been described previously. The main objective of this research is to extend the knowledge on the low-temperature aqueous geochemistry of tantalum and niobium minerals. The objective can be broken down into several smaller objectives, which are classification of all the approved Ta- and Nb-bearing minerals, sample the sediments in Asubulak and make an XRF elemental analysis, and derive the thermodynamic properties of the tantalite-columbite minerals at low-temperature and pressure via polyhedral regression analysis method. The results of the XRF analysis and the thermodynamic calculations showed the tantalite-columbite minerals are hardly soluble, and the source of tantalum and niobium in the weathered and aqueous environment must from less stable lepidolite mineral candidate and the phase is ‘aged’ Ta2O5.