Monazite uranium content

Uranium and thorium are widely distributed in the earth s crust, with average concentrations of 2 and 8 ppm, respectively (Grainger 1958 Boyle 1982 Frondel et al. 1967 Wills 1997). Over 100 different uranium-based minerals are known, but deposits of high-grade ore (hke uraninite and pitchblende, with as much as 70% uranium by weight) are rare. Most of the world s supply of recovered uranium has been extracted from ores with a uranium content of only about 0.2%. Conversely, the mineralogy of thorium is less varied, and deposits of its principal mineral, monazite, are fairly common. The thorium content of pure monazite is typically between 5% and 10%. 

If a mineral is sufficiently old and rich in Th and U, Pb concentrations can be high enough to be detectable using an electron microprobe. Analysis of all three elements then provides an estimate of age. This method has been widely used in uranium-ore studies, as pitchblende produces high Pb contents in relatively short times. Recently this technique has been applied with success to monazite (Suzuki and Adachi 1991, 1994 Montel et al. 1994, 1996 Williams and Jercinovic 2002), and, with more difficulty, to zircon, and xenotime (Suzuki and Adachi 1991, Geisler and Schleicher 2000)... 

With respect to the actinides, monazite tends to concentrate thorium, whereas xenotime tends to concentrate uranium, but can take up also appreciable amounts of thorium. According to Deer et al. (2013), common varieties of monazite have 4-12 mol% of Th02, whereas uranium occurs in minor amounts. Van Emden et al. (1997) mention Th02 contents in monazite ranging 1.2-21.9 wt%, whereas UO2 contents are from detection limit up to 0.75 wt%. Xenotime analyses show UO2 contents ranging from detection limit to 5.82 wt%, while Th02 varies from detection limit to 8.44 wt%. Watt (1995) lists monazite compositions showing wt% of Th to vary from 5.17-21.41 wt%, and UO2 from 0.22-3.17 wt%. 

At Baiyunebo in Inner Mongoha, 135 km from Baotou, RE minerals are found together with minerals of iron, titanium and niobium. The deposit is worked for iron, which keeps the costs low for production of the RE metals. Both monazite and bastnaesite are mined and, because of that, the mine is very important as a source of the cerium group metals. The bastnaesite has a higher content of samarium and europium than the CaHfornian ore in Mountain Pass. As the monazite is low in thorium and free from uranium its radioactivity is low, which is a benefit for ore handhng. 

Coastal and river placer deposits in Guandong and Hainan provinces have been worked for ilmenite and zircon, giving monazite and xenotime as by-products. Owing to the high content of thorium and uranium in the material - giving safety problems - these ores have gradually been replaced by others for the extraction of REMs. 

In the earth s crust thorium is almost four times more common than uranium, and the radioactive decay of thorium generates a considerable part of the geothermal energy. The primary source of thorium is monazite (Figure M25), a yellow or reddish-brown rare-earth phosphate. Seashore deposits at Travancore in India contain very big quantities of monazite. The mineral formula is (Ce,La,Nd,Th)PO and the thorium content in monazite ores is 5-10%. In 2001 the production of monazite concentrate in the world was 5710 tonnes [52.13], of which India accounted for 5000 toimes or 88%. Other producers were Brazil and Malaysia. 

Other additional uranium sources, associated with unconventional deposits or exploited as a by-product of other minerals (e.g. copper and gold), are those found in old mine dumps (gold mines in South Africa), phosphate rocks (Morocco, the U.S.A. and the U.S.S.R.), with a content ranging from 0.001 to 0.07%, in copper deposits, such as the porphyry coppers , in marine black shales with a content ranging from 0.001 to 0.008% (the U.S.A. and Sweden), in coal and lignite deposits with a content normally of 0.001%, exceptionally reaching 1% (the U.S.A.), in monazite deposits with 0.3% (India, Brazil, Australia and Malaysia), in igneous rocks, such as the alkaline intrusives distributed in various parts of the world, and, as has already been mentioned, in sea water. 

---