Tantalum (Ta) and niobium (Nb) are high field strength elements that have vital high-tech application and a growing demand. A major geological source for these metals is highly fractionated tin (Sn) systems associated with cassiterite mineralization in granites and pegmatites formed through extended fractionation and liquidus undercooling mechanisms. In this study preliminary results of an experimental determination of the partitioning coefficients of Sn, Ta and Nb between cassiterite and silicate melt (Dcass/melt) will be presented to answer factors controlling the growth of such cassiterite's. We have investigated the effects of temperature and volatile concentration and type (F-, Cl- and OH-) on Dcass/melt for Sn, Ta and Nb using a Pt capsule with alternating layers of Ta+Nb bearing silicate melt modified from Linnen (1998), RuO2 (to buffer oxygen fugacity) and SnO2. Experiments were run at 1.0 GPa and temperatures from 1100 to 1300°C, at 50°C intervals. Initial results from EPMA analysis and BSE imaging have shown the growth of euhedral cassiterite grains, exsolution of water and chlorine, and coupled substitution of Ta and Nb with Al into the cassiterite lattice based on the equation:

Al3+ + (Ta/Nb)5+ = 2 Sn4+

Preliminary results have also shown that the cassiterite grains incorporate more Ta than Nb with approximately 4 wt.% Ta2O5 and 2.5 wt.% Nb2O5 as well as a drop in Dcass/meltNb in chloride rich melts.