Rare-earth bearing minerals

Mineral matrix is formed chiefly by La or Ce . The last one is widely met as luminescence impurity center, but in rare-earth bearing minerals it is subjected to concentration quenching because of strong exchange interaction of outer 5d orbitals of neighboring Ce ions. Hereby its luminescence is not observed in REE minerals. From the other side, other trivalent REE can achieve some concentrations without quenching of luminescence, because outer electron shells, such as 5s and 5p , shield their inner 4f-4f transitions. 

The monazite structure consists of distorted PO4 tetrahedra with each metal atom roughly equidistant from nine oxygen atoms. Minor amounts of other rare-earth elements may occur. Steady-state luminescence under X-ray excitation of monazite 

Laser-induced time-resolved luminescence enables to detect Eu and Nd  

Luminescence under steady-state laser excitation with 780-785 nm revealed several narrow lines in IR range of the spectrum (Fig. 4.179). Natural monazite luminescence under cw laser excitations have been studied together with synthetic samples activated by different REE (Lenz et al. 2015). It was proved that the luminescence lines are mostly connected to trivalent REE such as Nd, Er, Pr, Sm. The broadening of the Stark sublevels correlates well with Th content and the degree of amorphization of the various monazite (Fig. 4.180). 

Thorite and orangite (orange thorite) have a tetragonal structure and are isostructural with zircon. Steady-state spectra under X-ray and laser (337 nm) excitations are connected with REE , namely Sm , Tb , Dy and Eu . Reabsorption lines of Nd have been also detected (Gorobets and Rogojine 2001). Laser-induced time-resolved luminescence enables to detect Eu nyl emission centers (Fig. 4.178a). 

The mineral matrix is formed chiefly by La or Ce . The last one is widely regarded as the luminescence impurity center, but in rare-earth bearing minerals it is subjected to concentration quenching because of strong exchange 

The monazite structure consists of distorted PO4 tetrahedra with each metal atom roughly equidistant from nine oxygen atoms. Minor amounts of other rare-earth elements may occur. Steady-state liuninescence under X-ray excitation of monazite revealed emission of Gd, Tb, Dy and Sm (Gorobets and Rogojine 2001). Laser-induced time-resolved liuninescence enables us to detect Sm +, Eu and Nd emission centers (Fig. 4.70). 

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Uraninite is associated with the following minerals zircon, monazite, carbonaceous material (thucolite ), mica, feldspar, and with rare-earth-bearing minerals. It occurs in small distinct crystals or may be massive. Some grains are discoloured, especially in the centre, metamict, and fractured. 

Cations of the lanthanide elements also produce colours in some minerals through intra-electronic transitions within 4/orbitals (Adams, 1965 Bernstein, 1982). Absorption bands are usually sharp and weak, leading to pastel shades. Examples of such coloured minerals are monazite, bastnaesite, rhabdophane, xenotime, gadolinite, and certain apatites, calcites, scheelites and fluorites. As noted earlier, some rare earth-bearing minerals, notably fluorite and monazite, also display the alexandrite effect (Berstein, 1982 Schmetzer et al., 1980). 

The principal rare earth-bearing minerals, bastnasite, monazite, and xenotime, are separated from othermin-... 

The ionic radius of U is very similar to that of tetravalent Th, and to those of many rare-earth ions (Table 2). This fact determines the occurrence of both Th and U in many rare earth bearing minerals. In the surficial environment, however, U is readily oxidized to U, which forms UOi , uranyl ion. ... 

Rare-earth minerals occur in a variety of geologic environments. Concentrations exist in igenous, sedimentary and metamorphic rocks. The rare-earths are constituents in over 160 of minerals [3], but only a few are recovered for commercial production. Bastnasite, Monazite, Loparite, Xenotime and Rare-earth bearing Clay are the major sources of the world s rare-earth supply. Bastnasite, Monazite and Lopariie are considered to be the principle cerium ores (Table 1.1). 

Hughes JM, Cameron M, Crowley KD (1990) Crystal stiuctures of natural ternary apatites Solid solution in Cas(P04)3X (X = F, OH, Cl) system. Am Mineral 75 295-304 Hughes JM, Cameron M, Mariano AN (1991) Rare-earth-element ordering and stractmal variations in natural rare-earth-bearing apatites. Am Mineral 76 1165-1173 Ito J (1968) Silicate apatites and oxyapatites. Am Mineral 53 890-907... 

Ytterby, a village in Sweden near Vauxholm) Yttria, which is an earth containing yttrium, was discovered by Gadolin in 1794. Ytterby is the site of a quarry which yielded many unusual minerals containing rare earths and other elements. This small town, near Stockholm, bears the honor of giving names to erbium, terbium, and ytterbium as well as yttrium. 

Of the principal minerals of Ca listed in Table 1, the most important ores are the various deposits of CaCOj, especially limestones, which occur as immense sedimentary beds over extensive parts of the earth s surface. Extraction of Ca from CaCOj is a simple and relatively inexpensive process. Although the other Ca-bearing minerals are rarely considered as potential Ca sources, they are widely distributed and extensively mined fluorite and apatite for their fluoride and phosphate content, gypsum and anhydrite for their use in construction. 

Literature on flotation of gold, PGMs, rare earths and various oxides is rather limited, compared to literature on treatment of sulphide-bearing ores. As mentioned earlier, the main problem arises from the presence of gangue minerals in the ore, which have flotation properties similar to those of valuable minerals. These minerals have a greater floatability than that of pyrochlore or columbite. In the beneficiation of oxide minerals, finding a selectivity solution is a major task. 

Phosphates. The two major phosphate bearing ores are monazite and xenotime, the former being a source of light lanthanides and the latter a source of the heavy rare earths, see Table IV. Deposits in the form of heavy mineral sands are the major source of monazite. They are usually exploited as a byproduct of rutile, ilmenite, and zircon mining operations. 

Rapp RP and Watson EB (1986) Monazite solubihty and dissolution kinetics Implications for the thorium and light rare earth chemistry of felsic magmas. Contrib Mineral Petrol 94 304-316 Rapp RP, Ryerson FJ, Miller CF (1987) Experimental evidence bearing on the stability of monazite during crystal anatexis. Geophys. Res Letters 14 307-310... 

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