Doctor of Philosophy
Linnen, Robert L.
Flemming, Roberta L.
Niobium and tantalum are critical metals that are necessary for many modern technologies such as smartphones, computers, cars, etc. Ore minerals of niobium and tantalum are typically associated with pegmatites and include columbite, tantalite, wodginite, titanowodginite, microlite and pyrochlore. Solubility and crystallization mechanisms of columbite-(Mn) and tantalite-(Mn) have been extensively studied in haplogranitic melts, with little research into other ore minerals. A new method of synthesis has been developed enabling synthesis of columbite-(Mn), tantalite-(Mn), hafnon, zircon, and titanowodginite for use in experiments at temperatures ≤ 850 °C and 200 MPa, conditions attainable by cold seal pressure vessels.
Solubilities of wodginite, titanowodginite, microlite and pyrochlore are compared to those for columbite-(Mn) and tantalite-(Mn) in a flux-rich haplogranitic melt of alumina saturation index (ASI) 1.0 at 700 – 850 °C and 200 MPa. The effect of melt composition on solubilities of wodginite, titanowodginite, and microlite compared to tantalite-(Mn) is also investigated in highly fluxed haplogranitic melts of ASI 1.0, 1.10, and 1.24, at 700 °C and 800 °C and 200 MPa. The log solubility product (logKsp) for tantalite-(Mn) is highest (-2.32 mol2/kg2) followed by columbite-(Mn) (-2.68 mol2/kg2), and pyrochlore (-3.71 mol3/kg3) titanowodginite (-3.73 mol3/kg3), wodginite (-3.77 mol3/kg3), and microlite (-3.78 mol3/kg3) are almost identical within error at 750 °C. However, the tantalum mineral-melt partition coefficient solubilities of wodginite, titanowodginite, and tantalite-(Mn) are identical within error; microlite is different because it contains a major melt cation, sodium, and columbite-(Mn) is less soluble than pyrochlore. Wodginite, titanowodginite, and pyrochlore have similar temperature and compositional dependences to columbite-(Mn) and tantalite-(Mn), as described in previous studies, and microlite solubility conversely increases with ASI.
Experiments crystallizing columbite-(Mn) through the interaction of a manganese-rich hydrothermal fluid with a niobium-rich pegmatite melt have demonstrated that fluid-melt interactions are a plausible mechanism for the crystallization of niobium and tantalum ore minerals. These experiments show that saturation can be reached from lower, more reasonable niobium melt concentrations than previous experiments and from higher concentrations of manganese in a hydrothermal fluid. This is significant because natural pegmatites contain evidence of hydrothermal processes during rare metal crystallization stages.
McNeil, Alysha G., "Crystallization processes and solubility of columbite-(Mn), tantalite-(Mn), microlite, pyrochlore, wodginite and titanowodginite in highly fluxed haplogranitic melts" (2018). Electronic Thesis and Dissertation Repository. 5261.