Major Mineral Ores
The fluorocarbonate mineral, Bastnäsite (Ce, La, Y)CO3F, is the most productive global mineral source for rare earth elements. This mineral tends to contain abundant light rare earth elements (LREE) and very low proportions of heavy ones, and tends to be specifically high in cerium, lanthanum, yttrium, and neodymium.
As with most ores, the real mineral environment is which bastnäsite is recovered is far more complex than the simplified chemical formula of the single mineral. Dozens of REE fluorocarbonate minerals are known. Various common substitutions in the chemistry of bastnäsite yield a series of related minerals that may be found together in bastnäsitic ores. Three variations in nomenclature are used to describe a few common ranges in the metal portion of the solid solution series. These are bastnäsite-(Ce), bastnäsite-(Y), and bastnäsite-(La).
Related minerals may also form from substitution of the fluorine or carbonate anions. These include parisite, and various hydroxylbastnäsites, among others.
Bastnäsite ores have been found in a variety of igneous contexts, ranging from carbonatites, granites, and pegmatites, as well as in hydrothermal and bauxite deposits.
Monazite, a rare earth phosphate, is the second most common mineral used as a rare earth ore. Like bastnäsite, the other primary REE ore, a variable naming system is used to express the primary elemental composition of monazite ores. The 4 terms, monazite-Ce, monazite-La, monazite-Nd, and monazite-Pr, each reflect varying abundances of rare earths, but never reflect the exclusive presence of only one element. Monazite, like bastnäsite and the other common ores, contains more LREEs than HREEs, and always contains a mix of various rare earths. Monazite is typically associated with somewhat higher ratios of heavy rare earths than are found in bastnäsite ore deposits.
Monazite is a very dense mineral. As a result, it collects in placer sands that result from sorting, by gravity, of the products of the weathering of the exposed igneous (primarily pegmatite) rock masses in which it originally formed. In addition to these sandy sources, the mineral is also mined in place from several locations.
Due to the ability of thorium, a radioactive element, to substitute for the rare earths in the monazite structure, radioactive byproducts are a challenge in some monazite mining locations. These byproducts, including the thorium daughter product, uranium, may become mineable coproducts in extreme cases. Not all monazite mineral sources contain significant percentages of thorium, however, and these challeges are dealt with (or taken advantage of) on a case by case basis.
Xenotime is the 3rd most important rare earth element ore, after monazite and bastnäsite. Of the 3 common REE ores, xenotime typically contains the highest ratios of heavy rare earth elements.
The generalized chemical description of xenotime is yttrium phosphate (YPO4). The yttrium is easily substituted by several of the heavy rare earth elements, dysprosium, ytterbium, erbium, and gadolinium, followed by lesser quantities of terbium, holmium, thulium, and lutetium, as well as by uranium and thorium. Uranium and thorium will not universally be present in significant quantites in xenotime ores. Uranium and thorium present either a mineable coproduct or a nuisance, depending entirely upon mine context, quantity, and location.
Xenotime is a definative member of the group of HREE or Yttrium group REE ores. These ores contain heavy REEs in abundances not typically seen among the bastnäsite and monazite ores, which can be described as the contrasting Cerium group, or primarily LREE ores. These two clusterings of mutually substituting elements are referred to as 'Cerium' and 'Yttrium' groups after their dominant (most common) members.
Xenotime is related to monazite. The two are very similar phosphates. The first, monazite, is built primarily around the element, cerium, which is subatituted readily by the various elements among the first half of the lanthanides, meaning the light rare earth elements, or LREE. The second mineral, xenotime, is built primarily around yttrium, which is readily substituted by the various HREE, the heavier second half of the lanthanide range of elements.
These two ores can be found together in the same area, and represent a continuum of mineral formation based upon change in temperature and pressure. At lower temperatures and pressures, monazite will form, and at higher temperatures and pressures, xenotime will form. When the crystal structure of the phosphate mineral changes, reflecting its formation temp./pressure, one or the other of the two rare earth element groups gets excluded from the crystal latice. This effect can be used measure the formation temperature and pressure ranges within a monazite/xenotime ore formation or region. The relationship between monazite and xenotime was studied and clarified by Gratz and Heinrich, 1997. (See links.)
The phosphate portion of xenotime and monazite constitute significant mineable coproducts in these ores.
Minor Mineral Ores
Long et al., 2010, pp 5-11
Bauer et al., 2010