Rare earth elements, a group of chemically very similar elements, have been in the news a lot in recent years because of their importance in many high-tech applications. This deposit in Mountain Pass, California, was discovered in the late 1940s and supplied much of the world’s demand for decades. However, the applications of rare earths were not as widespread as they are today, and as their applications expanded, competition, primarily from China, also intensified. Meanwhile, production costs at the mountain pass also rose. By 2002 the mine was closed.
Mountain Pass deposits are located in an “igneous limestone” called carbonatite (pronounced “carbon-ootite”). Carbonate minerals are most often found in sedimentary rocks such as limestone, where they precipitate either directly from water or through organisms, whose debris is incorporated into the limestone. In contrast, carbonatite was originally a magma that was cooled from a molten state. Carbonatite deposits are relatively rare and often contain high concentrations of rare earth elements.
Currently, China accounts for the majority of the world’s rare earth mining and processing facilities. Efforts to restart the Mountain Pass mine began in the 2010s due to national defense concerns about dependence on foreign suppliers. After several false starts, the mine appears to be operating on a daily basis. This effort is a reflection of the Strategic Minerals Program, which continued into the 1960s, in which the U.S. government subsidized the production of materials deemed essential to national security.
As a side note, Mountain Pass carbonatite is slightly radioactive due to the presence of thorium, a naturally occurring radioactive element. In fact, this is what led to the discovery of the first deposits in 1949, at a time when exploration for uranium with Geiger counters had become equivalent to mining for gold in the mid-20th century. Even small amounts of radioactivity make safe extraction of rare earth elements difficult, as the waste tends to be radioactive and requires special handling.
