Monazite stability

New studies examining the textures, chemistry, zoning, and petrogenesis of phosphates in well-constrained metamorphic terranes to supplement the current, sparse data sets. Complete, accurate chemical analyses of natural phosphates from these studies would be especially revealing about the geochemical controls on monazite stability. 

Lanzirotti A, Hanson GN (1996) Geochronology and geochemistry of multiple generations of monazite from the Wepawaug Schist, Cormecticut, USA implications for monazite stability in metamorphic rocks. Contrib Mineral Petrol 125 332-340... 

To evaluate the factors affecting the structural stability of some crystalline materials that are potential hosts for radioactive wastes, the crystal structures of a series of 3+p5 xv5+o compounds, where A is lanthanum or a member of the rare-earth series, were determined. The end-member phosphates (APO4) have the monoclinic Monazite structure (P2 /n) for A La, Ce-Gd, and the tetragonal Zircon structure (l4]/amd) for A Tb - Lu. The corresponding vanadates have the Monazite structure only for LaVO, and the Zircon structure for A = Ce - Lu. When the end members are iso-structural, e.g., LaPO /LaVO, Monazite, YbPC /YbVOA,... 

The need for high chemical and physical stability in the substances that will hold nuclear waste ions in an immobilized state has stimulated extensive materials research. In support of the candidacy of the cation-linked tetrahedral compounds having the monazite, scheelite or zircon crystal structure, studies have sought to determine the extent to which various elements occurring in the waste can occupy stable substitutional positions on the host lattice sites.(l)... 

As a metal, europium is very reactive so that one usually finds it under its trivalent, triply oxidized form (Eu3+ ion) in oxides or salts. A divalent form (Eu2+) also displays some stability. Two minerals that contain many of the lanthanide elements, which are separated by liquid-liquid extraction, are commercially important monazite (found in Australia, Brazil, India, Malaysia, and South Africa) and bastnasite (found in China and the United States). 

In general, Y and the heavier lanthanides, Gd to Lu, are less abundant than the lighter lanthanides. La to Eu. However, there are two further complicating factors one is that the elements with even atomic number are more abundant than those of odd atomic number, reflecting the greater stability of such nuclei. Secondly, some ores (e.g. bastnasite, monazite) are richer in the lighter metals while others (e.g. xenotime) have more of the heavier metals. The abundance of yttrium in the Earth s crust is 31 ppm while the total abundance of the lanthanides is some 180 ppm cerium is the most abundant (66 ppm), while thulium and lutetium are the rarest (0.5 and 0.8 ppm, respectively). 

A full theoretical prediction for a specific bulk composition (Figure 20 Pyle and Spear, 2003) has intriguing implications for the control exerted by major minerals on accessory mineral stability (e.g.. Ferry, 2000), and for the repartitioning of trace elements among major and accessory minerals at different P and T. For example, the increase in monazite abundance at the expense of apatite with decreasing P accords with... 

Accessory minerals commonly contain high concentrations of radioactive elements, and are a common target of radiogenic isotope measurements. Specific elements include uranium (zircon, apatite, titanite, monazite, xenotime, allanite) and thorium (monazite and allanite). Each accessory mineral is stabilized in a rock via a single element or suite of related elements, specifically phosphorous (apatite), REE (allanite, monazite, xenotime), zirconium (zircon), and titanium (titanite). Trace elements also occur in the major minerals (particularly phosphorous, zirconium, and titanium), so accessory minerals participate directly in major mineral reactions (Pyle and Spear, 1999, 2000, 2003 Ferry, 2000 Pyle et al, 2001 ... 

The established chemical stability of monazite in the earth s crust over geological time scales (Floran et al. 1981b, Pasteels 1970). 

Floran RJ, Abraham MM, Boatner LA, Rappaz M (1981a) Geologic stability of monazite and its bearing on the immobilization of actinide wastes. In Scientific Basis for Nuclear Waste Management. Moore JG (ed) Plenum Publishing, New York, p 507-514... 

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... 

Figure 25. Ternary (CeP04-YP04-ThSi04) plots showing the stability fields for monazite, xenotime and thorite as a function of temperature (Seydoux-Guillaume et al. 2002).

Apatite, monazite, and xenotime are found in rocks of nearly every metamorphic grade and, apparently, have wide P-T stability ranges. Despite their widespread occurrence, surprisingly few studies on the prograde metamorphic evolution of phosphates have been published. Kapustin (1987) reported on apatite compositions as a function of metamorphic grade (e.g., Fig. lb). 

Gowda KA (1982) Heavy ion bombardment of zircon, monazite and other crystal structures. PhD dissertation, Marquette University, Milwaukee, Wisconsin Grambow B, Lutze W (1979) Chemical stability of phosphate glass under hydrothermal conditions. In Scientific Basis for Nuclear Waste Management, Vol. 2. Northrap, CJM, Jr (ed) Plenum Press, New York, p 109-116... 

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