Monazite

Monazite
General
Category	Phosphate minerals
Chemical formula	(Ce,La)PO4
Strunz classification	08.AD.50
Identification
Color	Reddish brown, brown, pale yellow, pink, green, gray
Crystal habit	Commonly as prismatic or wedge-shaped crystals
Crystal system	Monoclinic
Twinning	Contact twins common
Cleavage	Distinct on [100] poor on [010]
Fracture	Conchoidal to uneven
Mohs scale hardness	5.0 to 5.5
Luster	Resinous, vitreous to adamantine
Streak	White
Diaphaneity	Translucent to opaque
Specific gravity	4.6–5.7 (4.98–5.43 for Monazite-Ce)
Optical properties	Biaxial (+)
Refractive index	nα = 1.770–1.793 nβ = 1.778–1.800 nγ = 1.823–1.860
Pleochroism	Weak
2V angle	10–26°
Other characteristics	Radioactive if thorium-rich, dull brown cathodoluminescence, paramagnetic
References	[1]

Monazite is a reddish-brown phosphate mineral containing rare earth metals. 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.^[2]

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 to 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

Postcard view of a monazite mine in Shelby, North Carolina, showing cart tracks and a bridge

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, deposits in southern India were found. Brazilian and Indian monazite dominated the industry before the Second World War, after which major mining activity transferred to South Africa and Bolivia. There are also large deposits in Australia.

Monazite was the only significant source of commercial lanthanides until bastnäsite 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, bastnäsite 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

Monazite powder

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.

Acid opening

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.

Alkaline opening

A more recent process uses 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.

References

1. ^ Monazite. Handbook of Mineralogy. (PDF) . Retrieved on 2011-10-14.
2. ^ "Helium From Sand", March 1931, Popular Mechanics article bottom of page 460

Further reading

• R.J. Callow, The Industrial Chemistry of the Lanthanons, Yttrium, Thorium and Uranium, Pergamon Press 1967. LC Cat. 67-14541
• C.K. Gupta, N. Krishnamurthy, Extactive Metallurgy of Rare Earths, CRC Press, 2005, ISBN 0-415-33340-7
• Price List, Lindsay Chemical Division, American Potash and Chemical Corporation, 1960
• R.C. Vickery, Chemistry of the Lanthanons, Butterworths and Academic Press, 1953
• J.C. Bailar et al., Comprehensive Inorganic Chemistry, Pergamon Press, 1973

External links

• Monazite
• An Unusual State Of Matter Poem about monazite by Roald Hoffman
• "British Monazite Mine, Shelby, N.C." in Durwood Barbour Collection of North Carolina Postcards (P077), North Carolina Collection Photographic Archives, Wilson Library, UNC-Chapel Hill