Lanthanide mineral zircon

The heavy mineral sand concentrates are scmbbed to remove any surface coatings, dried, and separated into magnetic and nonmagnetic fractions (see Separation, magnetic). Each of these fractions is further spHt into conducting and nonconducting fractions in an electrostatic separator to yield individual concentrates of ilmenite, leucoxene, monazite, mtile, xenotime, and zircon. Commercially pure zircon sand typically contains 64% zirconium oxide, 34% siUcon oxide, 1.2% hafnium oxide, and 0.8% other oxides including aluminum, iron, titanium, yttrium, lanthanides, uranium, thorium, phosphoms, scandium, and calcium. 

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. 

In clay samples Zr-Th-rich coffinite was found around remnants of zircon. It is likely that it is the result of solid solution with zircon, ZrSi04 and thorite, ThSi04, which are isostruc-tural with coffinite (Finch Murakami 1999 Jensen Ewing 2001). The presence of phosphorus and sulphur in coffinite suggests that both elements substituted for Si in the coffinite structure. A previous study at the Bangombe site in Gabon has clearly shown that coffinites are most important secondary minerals for the retention of fissiogenic lanthanides and actinides (Stille et al. 2003). 

Apparently, formation of actinide (IV) compounds with more complex cationic and anionic compositions with the monazite or zircon (xenotime) structure types is possible. These compounds can be considered to be solid solutions. The possibility of forming this kind of compound is realized in the minerals mentioned above. These minerals are characterized by the complex cation compositions monazite - (La, Ce, other lanthanides, Y, Ca, Th)(P, Si)04, xenotime - (Y, lanthanides. Sc, Zr, Th, U)(Si, P)04 zircon - (Zr, Hf, Th, U, lanthanides, Ca, Fe, Nb, Ta)(Si,P)04 [71]. The ionic radii and cationic proportions, anionic sizes and synthesis conditions affect the formation of each type considered. 

Quantitative modelling has been less successfully applied to rocks cff more felsic composition, such as granodiorites, dacites, granites and rhyolites. This is principally due to the ubiquitous presence in these evolved rocks of minor mineral phases, such as sphene, allanite, apatite and zircon, whose lanthanide contents may account for a substantial fraction of the total rock budget. Thus Gromet and Silver (1983) found that sphene and allanite, in a granodiorite from the Peninsular Ranges, California, contained 80-95% of the lanthanide content of the total rock. Distribution coefficients are not well known for these phases and the abundances of these trace minerals are difficult to determine accurately. 

Fig. 42. Lanthanide abundance patterns for accessory minerals. Note in particular the high abundances are light lanthanide enrichment of monazite and al-lanite, and the extreme heavy lanthanide enrichment of zircon. (Data are from table 24.)...

Placer deposits of potential future value as lanthanide ores invariably have the lanthanides present in common lanthanide or lanthanide-concentrating minerals (e.g., apatite, allanite, monazite, xenotime, zircon). Most pegmatites of potential value as ore have the lanthanides as common lanthanide-bearing minerals too (those previously mentioned plus fluorite). Alkalic rock complexes may produce commercially useful concentrations of common lanthanide-bearing minerals (e.g., apatite, perovskite) or rare ones (e.g., bastnaesite). 

Zircon, ZrSi04, is a common but sparse accessory mineral in alkaline igneous rocks and some pegmatites. It is fairly resistant to weathering and metamorphic processes and can be concentrated in placer or beach sand deposits. Zircons accept a spectrum of lanthanide distributions as is evident from the two shown in fig. 21.32. 

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