Bastnäsite-(y)

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.