Monazite

Monazite is a reddish-brown phosphate mineral that contains rare-earth elements, and because its composition varies, it is actually considered a group of minerals. The most common member of this group is monazite-(Ce), where cerium is the dominant rare-earth element.

There are five main species of monazite, distinguished by which rare-earth element is most abundant: monazite-(Ce) (the most common), monazite-(La), monazite-(Nd), monazite-(Sm), and monazite-(Gd). In monazite-(Ce), for instance, cerium is the most plentiful rare-earth element, followed by lanthanum, neodymium, and thorium. The elements listed in parentheses are ordered by their relative proportions. Trace amounts of silica, uranium, and thorium are also present. Because thorium and uranium undergo alpha decay, monazite contains significant helium, which can be released by heating.

Monazite is an important source of thorium, lanthanum, and cerium. It is commonly found in placer deposits—concentrated in alluvial sands after weathering of pegmatites. Large deposits of monazite sands exist in India, Madagascar, and South Africa, with India’s being especially rich.

The mineral is radioactive due to its thorium and, less often, uranium content. This radioactivity allows monazite to be dated through monazite geochronology, as uranium and thorium decay into lead. Monazite crystals often contain multiple distinct zones formed during different geologic events, and dating these zones reveals the history of the host rocks.

The name "monazite" comes from the Greek word *monázein* (to be solitary), via German *Monazit*, referring to its isolated crystals.

All monazite minerals share the same atomic structure, similar to other M(III)PO₄ compounds. The M(III) centers are surrounded by eight oxygen atoms in a distorted arrangement, with bond lengths around 2.6 Å. The phosphate group is tetrahedral. This structure is also seen in lead chromate and resembles that of zircon, xenotime, scheelite, anhydrite, barite, and rhabdophane.

Monazite sand from Brazil was first noticed in ship ballast by Carl Auer von Welsbach in the 1880s, who was searching for thorium for incandescent mantles. It quickly became the main thorium source and launched the rare-earth industry. Mining briefly occurred in North Carolina, but soon larger deposits were found in southern India. Brazilian and Indian monazite dominated before World War II; after the war, South Africa became the main producer. In 1904, T. H. Laby and Douglas Mawson identified the first radium-bearing ore in Australia from monazite samples collected in the Pilbara, with Edgeworth David presenting their findings to the Royal Society of New South Wales. Australia also has large monazite deposits.

Monazite was once the only significant commercial source of lanthanides, but concerns over radioactive thorium byproducts led to bastnäsite replacing it in the 1960s, as bastnäsite contains much less thorium. Renewed interest in thorium for nuclear energy could bring monazite back into use.

Because of their high density, monazite minerals concentrate in placer deposits—often beach or fossil beach sands—along with other heavy minerals like zircon and ilmenite. Monazite can be purified using gravity, magnetic, and electrostatic separation. Most commercial monazite sands are of the monazite-(Ce) type, with lanthanide content typically around 45–48% cerium, 24% lanthanum, 17% neodymium, 5% praseodymium, and smaller amounts of samarium, gadolinium, and yttrium. Europium is 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, then the world’s largest lanthanide producer, supplying a full set of lanthanides. The very low concentrations of the heaviest lanthanides in monazite justified the term "rare" earth and their high prices. Thorium content varies, sometimes reaching 20–30%, though monazite from certain carbonatites or Bolivian tin ore veins is essentially thorium-free. Commercial sands typically contain 6–12% thorium oxide.

The original method for extracting thorium and lanthanides from monazite—called "cracking"—involved heating it with concentrated sulfuric acid at 120–150 °C for several hours, with variations in acid-to-ore ratio, heating time, and water addition.