Bastnäsite (mineral group)

Although a scarce mineral and never in great concentrations, it is one of the more common rare-earth carbonates. Bastnäsite has been found in karst bauxite deposits in Hungary, Greece and the Balkans region. Also found in carbonatites, a rare carbonate igneous intrusive rock, at the Fen Complex, Norway; Bayan Obo, Mongolia; Kangankunde, Malawi; Kizilcaoren, Turkey and the Mountain Pass rare earth mine in California, US. At Mountain Pass, bastnäsite is the leading ore mineral. Some bastnäsite has been found in the unusual granites of the Langesundsfjord area, Norway; Kola Peninsula, Russia; Mont Saint-Hilaire mines, Ontario, and Thor Lake deposits, Northwest Territories, Canada. Hydrothermal sources have also been reported. The formation of hydroxylbastnasite (NdCO3OH) can also occur via the crystallization of a rare-earth bearing amorphous precursor. With increasing temperature, the habit of NdCO3OH crystals changes progressively to more complex spherulitic or dendritic morphologies. The development of these crystal morphologies has been suggested to be controlled by the level at which supersaturation is reached in the aqueous solution during the breakdown of the amorphous precursor. At higher temperature (e.g., 220 °C) and after rapid heating (e.g. < 1 h) the amorphous precursor breaks down rapidly and the fast supersaturation promotes spherulitic growth. At a lower temperature (e.g., 165 °C) and slow heating (100 min) the supersaturation levels are approached more slowly than required for spherulitic growth, and thus more regular triangular pyramidal shapes form.

Bastnäsite has cerium, lanthanum and yttrium in its generalized formula but officially the mineral is divided into three minerals based on the predominant rare-earth element. There is bastnäsite-(Ce) with a more accurate formula of (Ce, La)CO3F. There is also bastnäsite-(La) with a formula of (La, Ce)CO3F. And finally there is bastnäsite-(Y) with a formula of (Y, Ce)CO3F. There is little difference in the three in terms of physical properties and most bastnäsite is bastnäsite-(Ce). Cerium in most natural bastnäsites usually dominates the others. Bastnäsite and the phosphate mineral monazite are the two largest sources of cerium, an important industrial metal. Bastnäsite is closely related to the mineral series parisite. The two are both rare-earth fluorocarbonates, but parisite's formula of Ca(Ce, La, Nd)2(CO3)3F2 contains calcium (and a small amount of neodymium) and a different ratio of constituent ions. Parisite could be viewed as a formula unit of calcite (CaCO3) added to two formula units of bastnäsite. In fact, the two have been shown to alter back and forth with the addition or loss of CaCO3 in natural environments. Bastnäsite forms a series with the minerals hydroxylbastnäsite-(Ce) [(Ce,La)CO3(OH,F)] and hydroxylbastnäsite-(Nd). The three are members of a substitution series that involves the possible substitution of fluoride (F) ions with hydroxyl (OH) ions.

In 1949, the huge carbonatite-hosted bastnäsite deposit was discovered at Mountain Pass, San Bernardino County, California. This discovery alerted geologists to the existence of a whole new class of rare earth deposit: the rare earth containing carbonatite. Other examples were soon recognized, particularly in Africa and China. The exploitation of this deposit began in the mid-1960s after it had been purchased by Molycorp (Molybdenum Corporation of America). The lanthanide composition of the ore included 0.1% europium oxide, which was sorely needed by the burgeoning color television industry, to provide the red phosphor, so as to maximize picture brightness. The composition of the lanthanides was about 49% cerium, 33% lanthanum, 12% neodymium, and 5% praseodymium, with some samarium and gadolinium, or distinctly more lanthanum and less neodymium and heavies as compared to commercial monazite. However, the europium content was at least double that of a typical monazite. Mountain Pass bastnäsite was the world's major source of lanthanides from the 1960s to the 1980s. Thereafter, China became increasingly important to world rare earth supply. Chinese deposits of bastnäsite include several in Sichuan Province, and the massive deposit at Bayan Obo, Inner Mongolia, which had been discovered early in the 20th century, but not exploited until much later. Bayan Obo is currently (2008) providing the majority of the world's lanthanides. Bayan Obo bastnäsite occurs in association with monazite (plus enough magnetite to sustain one of the largest steel mills in China), and unlike carbonatite bastnäsites, is relatively closer to monazite lanthanide compositions, with the exception of its generous 0.2% content of europium.