Thorium silicate

Ce,La,Nd,Pr)P04 containing some thorium silicate (ThOj 1-18%). The principle lanthanide ore. 

Pabst A, Hutton CO (1951) Huttonite a new monoclinic thorium silicate. Am Mineral 36 60-69 Pedraza DF (1986) Mechanisms of the electron irradiation-induced amorphous transition in intermetallic compounds. J Mater Res 1 425-441... 

Pabst, A., Hutton, C. O., Osborne C., Huttonite, a new monoclinic thorium silicate its occurrence, analysis and properties. Am. Mineral, 36, (1951), 60-69. Cited on page 377. 

Most commercial thorium is extracted from monazite, which is not amenable to breakdown by dilute acids. Thorite, or thorium silicate, is an alternative thorium ore which occurs in fairly large quantities in some parts of the world, for example in the tailings from the tin beneficiation processes in Nigeria. The ore contains principally zircon (zirconium silicate), but this is of much smaller value than the 5 to 10 per cent thorite content. 

R. Ditz and co-workers, eds., Gmelin Handbook of Inorganic Chemistg, Thorium, Suppl Hoi A1a, Natural Occurrence, Minerals (Excluding Silicates), Springer-Vedag, Berlin, 1990. 

Thorium is widely but rather sparsely distributed and its only commercial sources are monazite sands (see p. 1229) and the mineral conglomerates of Ontario. The former are found in India, South Africa, Brazil, Australia and Malaysia, and in exceptional cases may contain up to 20% Th02 but more usually contain less than 10%. In the Canadian ores the thorium is present as uranothorite, a mixed Th,U silicate, which is accompanied by pitchblende. Even though present as only 0.4% Th02, the recovery of Th, as a co-product of the recovery of uranium, is viable. 

The most important minerals of the lanthanide elements are monazite (phosphates of La, Ce, Pr, Nd and Sm, as well as thorium oxide) plus cerite and gadolinite (silicates of these elements). Separation is difficult because of the chemical similarity of the lanthanides. Fractional crystallization, complex formation, and selective adsorption and elution using an ion exchange resin (chromatography) are the most successful methods. 

Glocker and Frohnmayer determined the characteristic constant c for nine elements (Reference 2, Table 4) ranging in atomic numbers from 42 (molybdenum) to 90 (thorium). They proved that identical results could be obtained with the sample in the primary (polychromatic) or in the diffracted (monochromatic) beam. The method was applied with good results to the determination of barium in glass of antimony in a silicate of hafnium in the mineral alvite and of molybdenum, antimony, barium, and lanthanum in a solution of their salts—for example, 5.45% barium was found on 90-minute exposure by the x-ray method for a glass that yielded 5.8% on being analyzed chemically. 

An alternative to the bridge technique was recently reported for thorium analysis in silicate rocks for which both Th and Th are measured on a single lon-counting detector (Rubin 2001). With careful chemistry and mass spectrometry, °Th/ Th ratios of igneous rocks can be measured with this technique with a precision that is similar to the bridge method. The disadvantage of this technique is that °Th ion-count rates are extremely low (around 10 cps) with normal silicate thorium ratios and are therefore subject to perturbations from background variation and low-level isobaric interferences in normal samples. 

Thorium. Multiple-collector measurement protocols by TIMS for thorium isotopic analysis typically involve the simultaneous measurement of Th and °Th (for silicate rocks), or Th and °Th, then Th and Th (for low- Th samples), using an axial ion counter and off-axis Faraday collector (Table 1). Various methods are used to correct for the relative gain between the low-level and Faraday detectors and 2a-uncertainties of l-5%o are typically obtained (Palacz et al. 1992 Cohen et al. 1992 McDermott et al. 1993 Rubin 2001). Charge-collection TIMS protocols enable Th, °Th and Th to be monitored simultaneously on a multiple-Faraday array and can achieve measurement uncertainties at the sub-permil level (Esat et al. 1995 Stirling et al. 1995). 

SIMS techniques have occupied somewhat of a narrower niche in uranium-series analysis, but have significantly improved Th isotope analysis relative to TIMS for chemically separated samples. The major improvement relative to TIMS is an improvement by about an order of magnitude in efficiency or sample size requirements for silicates. For uranium and/or thorium rich minerals such as carbonates and zircons, both SIMS and laser-ablation MC-ICPMS have been used for the direct in situ analysis of U and Th isotopes (Reid et al. 1997 Stirling et al. 2000) on very small (pg to ng levels of total U and Th) samples, at 10-100 pm scale resolution. 

Nakai S, Fidcuda S, Nakada S (2001) Thorium isotopic measurements on silicate rock samples with a multicollector inductively coupled plasma mass spectrometer. Analyst 126 1707-1710... 

Van den Bogaard P, Schimick C (1995) " °Ar/ Ar laser probe age of Bishop Tuff quartz phenocrysts substantiate long-lived silicic magma chamber at Long Valley, United States. Geology 23 759-762 Van Orman J.A., Grove T.L., Shimizu N (1998) Uranium and thorium diffusion in diopside. Earth Planet Sci Lett 160 505-519... 

Thorium is widely but rather sparsely distributed its only commercial sources are monazite (together with the rare earths) and uranothorite (a mixed Th, U silicate). Uranium is surprisingly common and more abundant than mercury, silver or cadmium in the earth s crust. It is widely distributed and it is found scattered in the faults of old igneous rocks. Concentration by leaching followed by re-precipitation has produced a number of oxide minerals of which the most important are uranite (also called pitchblende) U308 and carnotite, K UC HVO -SF O. 

Although the mineral which H. M. T. Esmark discovered looked a great deal like gadolinite, his father believed it to be new, possibly a kind of tantalite. Berzelius analysis of it proved it to be a silicate of a new metal, which he named thorium (50, 60). Although Pastor Esmark wished to name the mineral berzelite, Berzelius preferred the shorter name thorite (45). 

The refractory condensate model has fallen out of favor, including with Lewis (1988). Nevertheless, it is a useful end-member case. Goettel (1988) calculated the composition of the silicate portion of an ultrarefractory Mercury (Table 2, column 2). This model composition contains no FeO or volatiles, and has large concentrations of the refractory elements—aluminum, calcium, and magnesium. We calculated the thorium and uranium contents of such refractory condensates by assuming chondritic Al/Th and Al/U ratios. A surface of this composition will contain many of the phases in calcium-aluminum-rich inclusions (CAls), such as forsterite, anorthite, spinel, perovskite, hibonite, and melilite. 

More controversial (although sometimes cited as proven fact) have been claims (e.g., Taylor and Jakes, 1974 Taylor, 1982) that the bulk Moon is enriched roughly twofold in the cosmochemically refractory lithophile elements (a class that includes the REEs, the heat sources thorium and uranium, and the major elements aluminum, calcium, and titanium), and that compared to Earth s primitive mantle, the Moon s silicate mg ratio is much lower, i.e., its EeO concentration is much higher. Neither of these claims has been confirmed by recent lunar science developments, which include the advent of global thorium and samarium maps (Lawrence et al., 2002a Prettyman et al., 2002), data from lunar meteorites, and some radically changed interpretations of the Apollo seismic database. 

Niobium and tantalum are roughly interpolated between thorium and lanthanum in the Iherzolites, but the Nb/Ta values are variable. Lenoir et al. (2001) have revealed an abrupt change in the Nb/Ta ratio across the Ronda recrystallization front (Van der Wal and Vissers, 1993, 1996), from subchondritic values (—10) in the spinel tectonite domain ( ahead of the front) to near-chondritic (—16) in the granular domain ( behind the front). They tentatively ascribed this observation to a change from lithospheric conditions, under which Nb-Ta would be dominantly controlled by very small amounts of titanium oxides precipitated from volatile-rich small-volume melts (Bodinier et al., 1996), to more astheno-spheric conditions, under which the titanium oxides would be dissolved into basaltic partial melt and Nb-Ta redistributed between silicate phases. The harzburgites are systematically enriched in niobium and tantalum relative to... 

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