Thorite structure

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 us to detect Eu " and uranyl emission centers (Eig. 4.70). 

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). 

Because of its ability to accept U and Th, monazite is one of the most radioactive minerals after uraninite, thorianite or thorite. It is the most common radioactive mineral (Overstreet 1967), and in many rocks the main host of U and Th. The possibility for monazite to accept Pb in the same site as U and Th is obviously important for geochronology. Pb produced by U and Th has a place in the structure (Quarton et al. 1984). Therefore there is a not a natural tendency for Pb to be released from monazite, as might be anticipated by considering the structure of zircon. Another consequence of the ability of monazite to incorporate various ions is that, in the three U and Th decay chains, most elements can be incorporated in the A site. If all actinides and a small amount of Ra (because Ba is favourably partitioned) can be accepted in monazite, all elements with half-lives greater than about one year are incorporated in the mineral structure. This suggests that at any moment in the radioactive decay chains of U and Th, there is little tendency for any intermediate decay products to escape from the mineral structure. 

Taylor, M., and Ewing, R.C. (1978) The erystal structure of the ThSi04 polymorphs huttonite and thorite, Acta Cryst., 834,1074-1079. 

Coffinite is isostructural with zircon, thorite and hafnon and may show significant solid solution with each of these minerals. Its structure is shown in Fig. 3. No specific structural study has been done on coffinite. Crystals are always extremely small. The structure of the (OH)4 group may be implied by analogy with hydrogarnets. " Its role in the stability of coffinite is totally unknown. 

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