COMPOSITION AND STRUCTURAL STATE OF COLUMBITE-TANTALITE FROM THE HARDING PEGMATITE, TAOS COUNTY, NEW-MEXICO

Columbite - tantalite is an important accessory mineral in the Harding granitic pegmatite, a subhorizontal, zoned, complex spodumene-type pe gmatite emplaced within the Proterozoic Vadito Group in northern New M exico. Columbite - tantalite is most abundant in the beryl (wall) zone s of the pegmatite, with lesser quantities in the interior units. Aver age compositions of 25 samples plotted in the columbite quadrilateral form a distinct trend halfway along the MnNb2O6- MnTa2O6 join, defined by 0.25 < Ta/(Ta + Nb) < 0.57 and 0.91 < Mn/(Mn + Fe) < 0.99. Interna l fractionation is evident in the Ta/(Ta + Nb) value which, on average , increases from 0.38 in the beryl zones to 0.49 in the interior litho logic units. The average Mn/(Mn + Fe) value increases only slightly fr om 0.95 in the beryl zones to 0.98 in the interior units. These variat ions are accompanied by minor decreases in the average amounts of W, T i, and U, The structural state of columbite - tantalite from the Hardi ng pegmatite is bimodal. Samples from the beryl zone are highly disord ered to partially ordered (f between 0.11 and 0.62, Q between 0.16 and 0.66), but samples from interior units are highly ordered (f between 0.97 and 1.02, Q between 0.87 and 0.93). The XRD and TEM results indic ate that most partially ordered samples represent a simple transition between disordered and ordered distributions of cations; for example, there are no stacking faults ol unusual superstructures. However, one highly disordered specimen may contain micrometer-scale domains and na nometer-scale intergrowths of completely disordered (f = Q = 0) and pa rtially ordered (f = Q = 0.2) columbite. Possible factors causing the observed trend in structural state include a decrease in cooling rate by as much as two to three orders of magnitude and the presence of min or elements. The average U content of most samples is < 0.5 wt.% UO2, thus restricting the accumulated radiation damage to a low level. Some radiation damage is documented by HRTEM in local regions of the sampl e containing 0.4 to 0.8 wt.% UO2.