monazide sands

Monazite is a reddish-brown phosphate mineral containing rare earth metals and is 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: • monazite-Ce (Ce, La, Pr, Nd, Th, Y)PO4 • monazite-La (La, Ce, Nd, Pr)PO4 • monazite-Nd (Nd, La, Ce, Pr)PO4 • monazite-Pr (Pr, Nd, Ce, La)PO4 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 and thorium. Due to the alpha decay of thorium and uranium, monazite contains a 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.

Mining history Monazite sand from Brazil was first noticed in sand carried in ship's ballast by Carl Auer von Welsbach in the 1880s. Von Welsbach was looking for a way to obtain thorium for his newly invented incandescent mantles.

Monazite sand was quickly adopted as the source of thorium for the mantles and was the foundation of what became the rare earth industry. Monazite sand was also briefly mined in North Carolina, but shortly thereafter the deposits in southern India were found. Brazilian and Indian monazite dominated the industry until the Second World War.

Besides the deposits in Brazil and India there are also large deposits in Australia. Monazite was the only significant source of commercial lanthanides until bastnaesite began to be processed in about 1965.

With declining interest in thorium as a potential nuclear fuel in the 1960s and increased concern over the disposal of the radioactive daughter products of thorium, bastnaesite came to displace monazite in the production of lanthanides due to its much lower thorium content.

However any future increase in interest in thorium for atomic energy will bring monazite back into commercial use. Mineralization and Extraction Because of their high density monazite minerals will concentrate in alluvial sands when released by the weathering of pegmatites.

These so-called placer deposits are often beach or fossil beach sands and contain other heavy minerals of commercial interest such as zircon and ilmenite. Monazite can be isolated as a nearly pure concentrate by the use of gravity, magnetic and electrostatic separation. Monazite sand deposits 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 concentrations 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. Steenkampskraal monazite provided a supply of the complete set of lanthanides.

Very low concentrations of the heaviest lanthanides in monazite justified the term "rare" earth for these elements, with prices to match. Thorium content of monazite is variable and sometimes can be up to 20 - 30 %. Monazite from certain carbonatites or from Bolivian tin veins is essentially thorium-free.

However, commercial monazite sands typically contain between 6 and 12% thorium oxide.