Columbite-tantalite is an important accessory mineral in the Harding granitic pegmatite, a subhorizontal, zoned, complex spodumene-type pegmatite emplaced within the Proterozoic Vadito Group in northern New Mexico. Columbite-tantalite is most abundant in the beryl (wall) zones of the pegmatite, with lesser quantities in the interior units. Average compositions of 25 samples plotted in the columbite quadrilateral form a distinct trend halfway along the MnNb 2 O 6 -MnTa 2 O 6 join, defined by 0.25<Ta/(Ta+Nb)<0.57 and 0.91<Mn/(Mn+Fe)<0.99. Internal 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 lithologic units. The average Mn/(Mn+Fe) value increases only slightly from 0.95 in the beryl zones to 0.98 in the interior units. These variations are accompanied by minor decreases in the average amounts of W, Ti, and U. The structural state of columbite-tantalite from the Harding pegmatite is bimodal. Samples from the beryl zone are highly disordered 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 indicate that most partially ordered samples represent a simple transition between disordered and ordered distributions of cations; for example, there are no stacking faults or unusual superstructures. However, one highly disordered specimen may contain micrometer-scale domains and nanometer-scale intergrowths of completely disordered (f = Q = 0) and partially 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 minor elements. The average U content of most samples is <0.5 wt.% UO 2 , thus restricting the accumulated radiation damage to a low level. Some radiation damage is documented by HRTEM in local regions of the sample containing 0.4 to 0.8 wt.% UO 2 .

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