The mineral bastnasite (or bastnaesite) is one of a family of three carbonate-fluoride minerals, which includes bastnasite-(Ce) with a formula of (Ce, La)CO3F, bastnasite-(La) with a formula of (La, Ce)CO3F, and bastnasite-(Y) with a formula of (Y, Ce)CO3F. Some of the bastnasites contain OH− instead of F− and receive the name of hydroxylbastnasite. Most bastnasite is bastnasite-(Ce), and cerium is by far the most common of the rare earths in this class of minerals. Bastnasite and the phosphate mineral monazite are the two largest sources of cerium and other rare-earth elements.
Bastnasite was first described by the Swedish chemist Wilhelm Hisinger in 1838. It is named for the Bastnas mine near Riddarhyttan, Vastmanland, Sweden.
Bastnasite also occurs as very high-quality specimens at the Zagi Mountains, Pakistan.
Bastnasite occurs in alkali granite and syenite and in associated pegmatites. It also occurs in carbonatites and in associated fenites and other metasomatites.
Bastnasite 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 bastnasite-(Ce) with a more accurate formula of (Ce, La)CO3F. There is also bastnasite-(La) with a formula of (La, Ce)CO3F. And finally there is bastnasite-(Y) with a formula of (Y, Ce)CO3F. There is little difference in the three in terms of physical properties and most bastnasite is bastnasite-(Ce). Cerium in most natural bastnasites usually dominates the others. Bastnasite and the phosphate mineral monazite are the two largest sources of cerium, an important industrial metal.
Bastnasite 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 bastnasite. In fact, the two have been shown to alter back and forth with the addition or loss of CaCO3 in natural environments.
Bastnasite forms a series with the minerals hydroxylbastnasite-(Ce) [(Ce,La)CO3(OH,F)] and hydroxylbastnasite-(Nd). The three are members of a substitution series that involves the possible substitution of fluoride (F−) ions with hydroxyl (OH−) ions.
Bastnasite gets its name from its type locality, the Bastnas Mine, Riddarhyttan, Vastmanland, Sweden. Ore from the Bastnas Mine led to the discovery of several new minerals and chemical elements by Swedish scientists such as Jons Jakob Berzelius, Wilhelm Hisinger and Carl Gustav Mosander. Among these are the chemical elements cerium, which was described by Hisinger in 1803, and lanthanum in 1839. Hisinger, who was also the owner of the Bastnas mine, chose to name one of the new minerals bastnasit when it was first described by him in 1838.
Although a scarce mineral and never in great concentrations, it is one of the more common rare-earth carbonates. Bastnasite 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, bastnasite is the leading ore mineral. Some bastnasite 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. In 1949, the huge carbonatite-hosted bastnasite 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 needed by the color television industry, to provide the red phosphor, 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. The europium content was at least double that of a typical monazite. Mountain Pass bastnasite was the world's major source of lanthanides from the 1960s to the 1980s. Thereafter, China became an increasingly important rare earth supply. Chinese deposits of bastnasite 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 bastnasite occurs in association with monazite (plus enough magnetite to sustain one of the largest steel mills in China), and unlike carbonatite bastnasites, is relatively closer to monazite lanthanide compositions, with the exception of its generous 0.2% content of europium. At Mountain Pass, bastnasite ore was finely ground, and subjected to flotation to separate the bulk of the bastnasite from the accompanying barite, calcite, and dolomite. Marketable products include each of the major intermediates of the ore dressing process: flotation concentrate, acid-washed flotation concentrate, calcined acid washed bastnasite, and finally a cerium concentrate, which was the insoluble residue left after the calcined bastnasite had been leached with hydrochloric acid. The lanthanides that dissolved as a result of the acid treatment were subjected to solvent extraction, to capture the europium, and purify the other individual components of the ore. A further product included a lanthanide mix, depleted of much of the cerium, and essentially all of samarium and heavier lanthanides. The calcination of bastnasite had driven off the carbon dioxide content, leaving an oxide-fluoride, in which the cerium content had become oxidized to the less basic quadrivalent state. However, the high temperature of the calcination gave less-reactive oxide, and the use of hydrochloric acid, which can cause reduction of quadrivalent cerium, led to an incomplete separation of cerium and the trivalent lanthanides. By contrast, in China, processing of bastnasite, after concentration, starts with heating with sulfuric acid. Bastnasite ore is typically used to produce rare-earth metals. The following steps and process flow diagram detail the rare-earth-metal extraction process from the ore.