Mischmetal
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Mischmetal (from German: Mischmetall - "mixed metals") is an unintentional alloy of rare earth elements in various naturally occurring proportions. A typical composition includes approximately 50% cerium and 45% lanthanum. Its most common use is in the "flint" ignition device of many lighters and torches, although an alloy of only rare-earth elements would be too soft to give good sparks. For this purpose, it is blended into a harder material known as ferrocerium.
Mischmetal is an intermediate product in the preparation of virtually all rare earth elements. This is because such elements are nearly identical in most chemical processes, meaning that ordinary geological and extraction processes do not distinguish them. Highly specialized processes, such as those developed by Carl Auer von Welsbach, exploit subtle differences in solubilitiy to separate it into its constituent elements, with each step producing only an incremental change in composition. Such processes later informed Marie Curie in her search for new elements.
Mischmetal, cerium, and the more desirable rare earth elements are related in a way that is somewhat analogous to the relationship between natural uranium, depleted uranium, and enriched uranium.de:Mischmetall