An Experimental Study on the Formation of Scandium-Rich Rocks

Scandium is used in the production of high-strength aluminum alloys and solid-oxide fuel cells, but has a high price due to its limited global supply. Scandium is primarily produced as a byproduct of titanium, uranium, and REE mining. Elevated scandium concentrations can also be found in ferrodiorite, laterite, pegmatite, Climax-type, and Sn-W deposits, where it is hosted in minerals such as clinopyroxene, amphibole, apatite, cassiterite, and ilmenorutile. There are also several rare minerals with scandium as a primary constituent, such as thortveitite and bazzite. We suggest that the variety of scandium-bearing rocks and minerals may be linked by A-type magmatism, such that mafic scandium-bearing rocks could represent early fractionates of A-type magmas.

In order to constrain the relationship between mafic and felsic scandium-bearing rocks, experiments have been designed to fractionally crystallize a basaltic melt approximating the scandium-rich Kiviniemi ferrodiorite (49.7% SiO₂ and 260 ppm Sc₂O₃). Experiments are performed in MHC cold-seal pressure vessels with charges comprising a melt (glass) and water within a thin-walled gold capsule. Experimental charges are heated to temperatures of 937 to 965°C for 1-2 hours, cooled and held at 880 to 907°C for up to 48 hours, and then rapidly quenched. Run products variably include glass, plagioclase, olivine, pyroxene, biotite, iron-titanium oxides, and aqueous solution. Preliminary mineral/melt partition coefficients are: D^Ol/Melt = 3.0 ± 0.6, D^Plag/Melt = 0.6 ± 0.1, D^Pyx/Melt= 6.9 ± 0.9, and D^Bt/Melt = 3.5 ± 0.6. Run product glass compositions range from 53 to 69% SiO₂ and both scandium and iron concentrations decrease with increasing SiO₂ concentration. Thus, increasing fractionation should result in a scandium-poor melt. To constrain the behavior of scandium in highly fractionated melts, experiments at temperatures down to 800°C will be conducted wherein the starting melt is of the same composition as run product melts of previous, higher temperature experiments. The experiments conducted in this study will provide constraints on the behavior of scandium during fractionation and attempt to link mafic and felsic scandium-rich rocks.