Chemical aspects of cassiterite: Yellowknife pegmatite field

Journal of Pegmatology VOLUME 1, NUMBER 1
Chemical aspects of cassiterite: Yellowknife pegmatite field, Northwest Territories, Canada Michael A, Wise, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560, U.S.A.

Cassiterite is common in many rare-element pegmatites of the Yellowknife pegmatite field, particularly those which contain spodumene. It is usually subordinate to columbite-tantalite or ferrotapiolite, although in some pegmatites it exists as the dominant Nb,Ta,Sn phase. Cassiterite occurs primarily as massive anhedral grains, rarely showing crystal forms, or as veinlets in quartz + muscovite + plagioclase assemblages. It is black and is easily distinguishable from other Nb- and Ta-oxide minerals by its light brown streak. Cassiterite may contain inclusions of columbite-tantalite, ferrotapiolite, quartz and feldspars. In polished sections, cassiterite has been observed as anhedral grains in columbite-tantalite, ferrotapiolite an ixolite or as epitaxial overgrowths on ferrotapiolite. The chemistry of cassiterite, ideally SnO₂, in granitic pegmatites is often characterized by the presence of significant quantities of Fe, Ta, Nb and Mn. Cassiterite from the Yellowknife pegmatite field frequently contains elevated concentrations of these elements, which are incorporated into the cassiterite structure by substituting for Sn. Overall, as much as 16 wt% of Fe, Ta, Nb, Ti and Mn oxides substitutes for Sn. Typically, the Yellowknife cassiterites show considerable accumulations of Fe and Ta with only minor to trace concentrations of Nb, Ti and Mn. Calculated Fe₂O₃ contents range from 0.0 to 3.6% whereas FeO varies from 0.0 to 2.3%. The Ta₂O₅ content of most Yellowknife cassiterite falls within the range 0 to 2%, although cassiterites with Ta₂O₅ in excess of 5% do occur locally. The Ti content of most cassiterite, including those of pegmatitic origin, is typically low (< 0.1% TiO₂). Yellowknife cassiterite show little to no accumulations of Ti, although local enrichment up to 0.6% TiO₂ have been observed.	Limited substitution of Sn by Fe²⁺, Fe³⁺, Nb and Ta is common in the Yellowknife cassiterites. Tantalum is more likely to occur in cassiterite than Nb, which may be a reflection of the extreme fractionation achieved by the pegmatite at the time of cassiterite crystallization. Although cassiterite compositions vary widely in their Ta/(Ta+Nb) ratios, they always show Ta>Nb. Thus, the chemistry of the Yellowknife cassiterites may be viewed as solid solutions of SnO₂ and a "tapiolite component" (Fe>Mn)(Ta>Nb)₂O₆ as compositions tend to plot close to the Sn-(Fe,Mn)-(Ta,Nb)₂ join. The form of Fe in cassiterite has been the subject of much debate. As indicated by Mossbauer studies, it has been shown that Fe in cassiterite can occur both as Fe²⁺ and Fe³⁺ . For the Yellowknife data, most cassiterite plot along the FeTa₂O₆-FeTaO₄ join, suggesting extensive Fe²⁺-Fe³⁺ solid solution. The incorporation of Fe²⁺ into the cassiterite seems to occur more easily than Fe³⁺.