Monazite

Monazite is a primarily reddish-brown phosphate mineral that contains rare-earth elements. Due to variability in composition, monazite is considered a group of minerals. The most common species of the group is monazite-(Ce), that is, the cerium-dominant member of the group.

Types There are five different most common species of monazite, depending on the relative amounts of the rare-earth elements in the mineral:

monazite-(Ce), (Ce,La,Nd,Th)PO4 (the most common member) monazite-(La), (La,Ce,Nd)PO4 monazite-(Nd), (Nd,La,Ce)PO4 monazite-(Sm), (Sm,Gd,Ce,Th)PO4 monazite-(Gd), PO4 Monazite-(Ce) is by far the most common of the group. It occurs usually in small isolated crystals. It has a hardness of 5.0 to 5.5 on the Mohs scale of mineral hardness and its specific gravity (aka relative density) is also 5 to 5.5. The elements in parentheses are listed in the order of their relative proportion within the mineral: lanthanum is the most common rare-earth element in monazite-(La), and so forth. Silica (SiO2) is present in trace amounts, as well as 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. The following analyses are of monazite from: (I.) Burke County, North Carolina, US; (II.) Arendal, Norway; (III.) Emmaville, New South Wales, Australia.

Monazite is an important ore for thorium, lanthanum, and cerium. It is often found in placer deposits. India, Madagascar, and South Africa have large deposits of monazite sands. The deposits in India are particularly rich in monazite. Monazite is radioactive due to the presence of thorium and, less commonly, uranium. The radiogenic decay of uranium and thorium to lead enables monazite to be dated through monazite geochronology. Monazite crystals often have multiple distinct zones that formed through successive geologic events that lead to monazite crystallization. These domains can be dated to gain insight into the geologic history of its host rocks. The name monazite comes from the Ancient Greek: μονάζειν, romanized: monázein (to be solitary), via German Monazit, in allusion to its isolated crystals.

Structure

All monazites adopt the same structure, meaning that the connectivity of the atoms is very similar to other compounds of the type M(III)PO4. The M(III) centers have a distorted coordination sphere being surrounded by eight oxides with M–O distances around 2.6 Å in length. The phosphate anion is tetrahedral, as usual. The same structural motif is observed for lead chromate (PbCrO4). Monazite also shares many structural similarities with; zircon, xenotime, scheelite, anhydrite, barite, and rhabdophane.

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 thorium for his newly invented incandescent mantles. Monazite sand was quickly adopted as the thorium source and became the foundation of the rare-earth industry. Monazite sand was also briefly mined in North Carolina, United States, but, shortly thereafter, extensive deposits in southern India were found. Brazilian and Indian monazite dominated the industry before World War II. After World War II, monazite mining shifted to South Africa. It was in monazite that the discovery of the first radium-bearing ore was identified in Australia in 1904 by T. H. Laby and Douglas Mawson, who analysed samples of monazite collected from the Pilbara in Western Australia. The samples were tested in the University of Sydney engineering laboratory. Edgeworth David made the formal presentation of their paper describing their findings to the Royal Society of New South Wales on 5 October 1904. There are large monazite deposits in Australia. Monazite was the only significant source of commercial lanthanides, but because of concern over the disposal of the radioactive daughter products of thorium, bastnäsite came to displace monazite in the production of lanthanides in the 1960s due to its much lower thorium content. Increased interest in thorium for nuclear energy may bring monazite back into commercial use.

Mineralization and extraction

Because of their high density, monazite minerals 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 prevalently 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 ore veins is essentially thorium-free. However, commercial monazite sands typically contain between 6 and 12% thorium oxide.

Acid cracking 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 (250 and 300 °F) 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.

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