Regarded by most people as a very exotic element, scandium is not all that rare. Its average abundance in Earth's crust is 22 ppm, which can be compared with 25 ppm for cobalt and 13 ppm for lead. Other familiar metals that we use, such as molybdenum, tin, tungsten, silver, and gold, are far below in crustal abundances. While these other metals tend to be concentrated in economically exploitable deposits, the problem with scandium is that it is dispersed in common rock-forming minerals. Accordingly, minerals with scandium as a main constituent are few and rare. This makes scandium an exciting element for the mineralogist and geochemist. Although today's total market is relatively small, an increasing demand for scandium calls for an increased effort to better understand the distribution of the element and to locate new deposits.
Only nine major scandium minerals are known so far (year of first description in parentheses): thortveitite (1911), bazzite (1915), kolbeckite (1926; not originally recognized as a scandium mineral), jervisite (1982), cascandite (1982), juonniite (1997), pretulite (1998), scandiobabingtonite (1998), and kristiansenite (2002). Kolbeckite, juonniite, and pretulite are phosphates; the rest are silicates. A few more new species are in the pipeline for formal description. Thortveitite, Sc2Si2O7, was once the world's most expensive mineral and was produced in small amounts from granite pegmatites in south Norway. Radioactive calcium isotopes were produced from it for use in medicine.
In minerals, the most common substitution mechanism involving scandium is the replacement of aluminum and trivalent iron. This is the main reason for the dispersal of scandium in the lithosphere and is explained by the valency of scandium (Sc3+) and similarities in ionic radii. Scandium can also substitute for yttrium and the heavy lanthanides (rare-earth elements), despite a larger difference in ionic radii. Incorporation of scandium in minerals containing magnesium, manganese, and divalent iron--or niobium, tantalum, tin, and zirconium--is explained by various coupled (heterovalent) substitutions; for example, Sc3+ + (Ti,Sn)4+ (Fe,Mn)2+ + (Nb,Ta)5+, Sc3+ + Na+ (Mg,Fe)2+ + Ca2+, Sc3+ + Ca2+ Sn4+ + Na+, and Sc3+ + P5+ Zr4+ + Si4+. No wonder scandium is a dispersed element! In common rocks, it is mainly present in ferromagnesian minerals like pyroxenes, amphiboles, micas, garnets, and epidote-group minerals.
Owing to its dispersed nature and resulting low concentration, scandium is produced exclusively as a by-product during processing of various ores and has also been recovered from mine tailings. The principal scandium-producing countries are China, Russia, Ukraine, and Kazakhstan. Future resources are known in the U.S., Australia, and Norway. Undiscovered scandium resources are probably very large.
|ROCK SOLID Thortveitite, Sc2Si2O7, such as these crystals from Norway, was once the world's most expensive mineral.
PHOTO BY TOM V. SEGALSTAD
Investigations of the use of scandium in aluminum alloys began in the Soviet Union in the 1970s, although such alloys were patented in the U.S. in 1971. During the 1980s, Russian scientists implemented scandium in several alloy systems. Some of the advantages of using scandium in aluminum alloys are to achieve grain refinement during casting and welding, increased strength from Al3Sc precipitates, increased resistance to recrystallization, and enhanced superplastic properties. It has been claimed that scandium provides the highest increment of strengthening per atomic percent of any of the other aluminum-alloying elements.
Principal uses for scandium are in high-strength aluminum alloys, high-intensity metal halide lamps, electronics, and laser research. A recently developed application is in welding wire, and future demand is expected to be in fuel cells. Approximately 15 different commercial Al-Sc alloys have been developed in Russia, and some of them are used for aerospace applications. In Europe and the U.S., scandium-containing alloys have been evaluated for use in structural parts in airplanes. The combination of high strength and light weight makes Al-Sc alloys suitable for a number of applications.
But the high price of scandium has so far restricted widespread use of such alloys in the Western world. Uses have been primarily in sports equipment like baseball and softball bats, bicycle frames, lacrosse sticks, and even in handgun frames and cylinders! With the current prices, an addition of typically 0.2 weight % Sc to an aluminum alloy will quadruple the price. However, should the price fall by 90% or more, scandium will be in immediate demand for large-scale production of several alloy types. Of course, such a development is dependent on stable, long-term supplies. I am convinced that scandium's heyday is ahead of us.
Gunnar Raade is a senior curator of minerals at the Geological Museum, University of Oslo, Norway. He has been involved in the description of 13 new mineral species, among them one new scandium mineral (kristiansenite).
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