In geology, the mineral monazite is a reddish-brown phosphate-containing rare earth metals and an important source of thorium, lanthanum, and cerium. It occurs usually in small isolated crystals. There are actually at least four different kinds of monazite, depending on relative elemental composition of the mineral:
The elements in parentheses are listed in the order in which they are in relative proportion within the mineral, so that lanthanum is the most common rare earth in monazite-La, and so forth. Silica, SiO2, will be present in trace amounts, as will small amounts of uranium. Due to the alpha decay of thorium and uranium, monazite contains significant amount of helium, which can be extracted by heating.
Monazite is an important ore for thorium, lanthanum, and cerium. It is often found in placer deposits. The deposits in India are particularly rich in monazite. It has a hardness of 5.0 - 5.5 and is relatively dense, about 4.6 to 5.7 g/cm3.
Because of the presence of thorium within monazite, it can be radioactive. If samples are kept, they should be placed away from minerals that can be damaged by radiation. Because of its radioactive nature, the monazite within rocks is a useful tool for dating geological events, such as heating or deformation of the rock.
The name monazite comes from the Greek μοναζειν (to be solitary), in allusion to its isolated crystals. India, Madagascar, and South Africa have large deposits of monazite sands.
Monazite sand from Brazil was first noticed in sand carried in ship's ballast by Auer von Welsbach, who was looking for a source of thorium for his incandescent mantles, in the 1880s. Monazite sand was quickly adopted as the source of thorium for the mantles, and became the foundation of what became the rare earth industry. Monazite sand soon was also coming (briefly) from North Carolina, but shortly thereafter the deposits in southern India were found, and thereafter Brazilian and Indian monazite dominated the industry, until WW2. Monazite was the only significant source of commercial lanthanides, until bastnaesite began to be processed in about 1965. With a decline in interest in thorium for atomic energy that set in about the same time, and increased concern about the disposal of radioactive daughter products of thorium, bastnaesite came to displace monazite as a source of lanthanides, due to its much lower thorium content. However, any future increase in interest in thorium as an energy source will bring monazite back into commercial use. Besides the deposits in Brazil and India, also occur large deposits in Australia. Monazite sand deposits that are large enough to be of commercial value have arisen by the erosion of large bodies of granite and gneiss, and are inevitably of the monazite-(Ce) composition. Typically, the lanthanides in such monazites contain about 45-48% cerium, about 24% lanthanum, about 17% neodymium, about 5% praseodymium, and minor quantities of samarium, gadolinium, and yttrium. Europium contents tend to be low, about 0.05%. South African "rock" monazite, from Steenkampskraal, was processed in the 1950s and early 1960s by the Lindsay Chemical Division of American Potash and Chemical Corporation, at the time the largest producer of lanthanides in the world, so as to provide the complete set of lanthanides. The very low content of the heaviest lanthanides in monazite justified the term "rare" earth for these (with prices to match). Thorium content of monazite is variable and sometimes may amount to up to 20 - 30 %. Monazite from certain carbonatites, or from the Bolivian tin veins, is essentially thorium-free. However, commercial monazite sands typically contain between 6 and 12% thorium oxide.
The original process for "cracking" monazite so as to extract the thorium and lanthanide content was to heat it with concentrated sulfuric acid to temperatures between 120 and 150 °C for several hours. Variations in the ratio of acid to ore, the extent of heating, and the extent to which water was added afterwards led to several different processes to separate thorium from the lanthanides. One of the processes caused the thorium to precipitate out as a phosphate or pyrophosphate in crude form, leaving a solution of lanthanide sulfates from which the lanthanides could be easily precipitated as a double sodium sulfate. The acid methods led to the generation of considerable acid waste, and loss of the phosphate content of the ore.
More recently, opening monazite has been done with hot sodium hydroxide solution (73 %) at about 140 °C. This process allows the valuable phosphate content of the ore to be recovered as crystalline trisodium phosphate. The lanthanide/thorium hydroxide mixture can be treated with hydrochloric acid to provide a solution of lanthanide chlorides, and an insoluble sludge of the less-basic thorium hydroxide.
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