Monazite is a primarily reddish-brown

phosphate mineral Phosphate minerals are minerals that contain the tetrahedrally coordinated phosphate () anion, sometimes with arsenate () and vanadate () substitutions, along with chloride (Cl−), fluoride (F−), and hydroxide (OH−) anions, that also fit in ...

that contains

rare-earth elements The rare-earth elements (REE), also called the rare-earth metals or rare earths, and sometimes the lanthanides or lanthanoids (although scandium and yttrium, which do not belong to this series, are usually included as rare earths), are a set of ...

. 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. It occurs usually in small isolated

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

s. It has a hardness of 5.0 to 5.5 on the

Mohs scale of mineral hardness The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by the Ger ...

and is relatively

dense Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be use ...

, about 4.6 to 5.7 g/cm³. There are five different most common species of monazite, depending on the relative amounts of the rare earth elements in the mineral: * monazite-(Ce), (the most common member), * monazite-(La), , * monazite-(Nd), , * monazite-(Sm), , * monazite-(Pr), . 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 Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

() is present in trace amounts, as well as small amounts of

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

and

thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...

. Due to the

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an a ...

of thorium and uranium, monazite contains a significant amount of

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

, which can be extracted by heating. The following analyses are of monazite from: (I.)

Burke County, North Carolina Burke County is a county in the U.S. state of North Carolina. It is part of the state's western mountain region, containing sections of the Blue Ridge Mountains including the Linville Gorge and South Mountains. As of the 2020 census, its po ...

, US; (II.)

Arendal Arendal () is a List of municipalities of Norway, municipality in Agder counties of Norway, county in southeastern Norway. Arendal belongs to the Districts of Norway, region of Southern Norway, Sørlandet. The administrative centre of the munici ...

, Norway; (III.) Emmaville, New South Wales, Australia. , , 30.88 , , 26.26 , , 30.21 , - , colspan=2, Yttrium oxide () , , — , , 3.82 , , — , - , colspan=2, Thorium oxide () , , 6.49 , , 9.57 , , 1.23 , - , colspan=2, Silica () , , 1.40 , , 1.86 , , 3.21 , - , colspan=2, Alumina () , , — , , —, , 3.11 , - , colspan=2, Iron oxide () , , —, , 1.13, , — , - , colspan=2, Lime (CaO) , , — , , 0.69 , , — , - , colspan=2, Water () , , 0.20 , , 0.52, , — , -line-height:50% , , , , , , , , , , - , , , , , 99.63 , , 100.60 , , 99.49 , - , colspan=2, Specific gravity , , 5.10 , , 5.15 , , 5.001 Monazite is an important

ore Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...

for thorium,Wolfgang Stoll "Thorium and Thorium Compounds" Ullmann's Encyclopedia of Industrial Chemistry 2012 Wiley-VCH, Weinheim. . lanthanum, and cerium. It is often found in

placer deposit In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation from a specific source rock during sedimentary processes. The name is from the Spanish language, Spanish word ''placer'', meaning "alluviu ...

India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...

Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...

, and

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

have large deposits of monazite sands. The deposits in

are particularly rich in monazite. Monazite is

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

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 geochronology is a geochronology, dating technique to study geological history using the mineral monazite. It is a powerful tool in studying the complex history of metamorphic rocks particularly, as well as Igneous rock, igneous, Sedimen ...

. 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 (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 . 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 (). Monazite also shares many structural similarities with;

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

, xenotime,

scheelite Scheelite is a calcium tungstate mineral with the chemical formula Ca W O4. It is an important ore of tungsten (wolfram). Scheelite is originally named after Swedish chemist Carl Wilhelm Scheele (1742–1786). Well-formed crystals are sought ...

anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...

barite Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate (Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...

, and rhabdophane.

Mining history

Monazite sand from

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

was first noticed in sand carried in ship's ballast by

Carl Auer von Welsbach Carl Auer von Welsbach (1 September 1858 – 4 August 1929), who received the Austrian noble title of Freiherr Auer von Welsbach in 1901, was an Austrian scientist and inventor, who separated didymium into the elements neodymium and praseody ...

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 North Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, South Carolina to the south, Georgia (U.S. stat ...

, but, shortly thereafter, extensive deposits in southern

were found. Brazilian and Indian monazite dominated the industry before World War II, after which major mining activity transferred to

. There are also large monazite deposits in

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. Monazite was the only significant source of commercial

lanthanides The lanthanide () or lanthanoid () series of chemical elements comprises at least the 14 Metal, metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. In the periodic table, they fill the 4f orbitals. Lutetium ...

, but because of concern over the disposal of the radioactive daughter products of thorium,

bastnäsite The mineral bastnäsite (or bastnaesite) is one of a family of three fluorocarbonate minerals, which includes bastnäsite-(cerium, Ce) with a formula of (Ce, La)CO3F, bastnäsite-(lanthanum, La) with a formula of (La, Ce)CO3F, and bastnäsite-(yt ...

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 A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...

. These so-called

s are often beach or fossil beach sands and contain other heavy minerals of commercial interest such as

and

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printi ...

. 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 Cerium is a chemical element; it has Chemical symbol, symbol Ce and atomic number 58. It is a hardness, soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it ...

, about 24%

lanthanum Lanthanum is a chemical element; it has symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements bet ...

, about 17%

neodymium Neodymium is a chemical element; it has Symbol (chemistry), symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth element, rare-earth metals. It is a hard (physics), hard, sli ...

, about 5%

praseodymium Praseodymium is a chemical element; it has symbol Pr and atomic number 59. It is the third member of the lanthanide series and is considered one of the rare-earth metals. It is a soft, silvery, malleable and ductile metal, valued for its magnetic ...

, and minor quantities of samarium,

gadolinium Gadolinium is a chemical element; it has Symbol (chemistry), symbol Gd and atomic number 64. It is a silvery-white metal when oxidation is removed. Gadolinium is a malleable and ductile rare-earth element. It reacts with atmospheric oxygen or moi ...

, and

yttrium Yttrium is a chemical element; it has Symbol (chemistry), symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost a ...

Europium Europium is a chemical element; it has symbol Eu and atomic number 63. It is a silvery-white metal of the lanthanide series that reacts readily with air to form a dark oxide coating. Europium is the most chemically reactive, least dense, and soft ...

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

carbonatite Carbonatite () is a type of intrusive rock, intrusive or extrusive rock, extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geoche ...

s 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 Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

to temperatures between for several hours. Variations in the ratio of acid to

, 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 Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

pyrophosphate In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate () and tetrasodium pyrophosphate (), among others. Often pyrophosphates a ...

in crude form, leaving a solution of lanthanide sulfates, from which the lanthanides could be easily precipitated as a double

sodium sulfate Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound with formula Na2SO4 as well as several related hydrates. All forms are white solids that are highly soluble in water. With an annual production of 6 mill ...

. The acid methods led to the generation of considerable acid waste, and loss of the phosphate content of the ore. Monazite acid cracking process

Alkaline cracking

A more recent process uses hot

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula . It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly corrosive base (chemistry), ...

solution (73%) at about . This process allows the valuable phosphate content of the ore to be recovered as crystalline

trisodium phosphate Trisodium phosphate (TSP) is an inorganic compound with the chemical formula . It is a white, granular or crystalline solid, highly soluble in water, producing an alkaline solution. TSP is used as a cleaning agent, builder, lubricant, food addi ...

. The lanthanide/thorium hydroxide mixture can be treated with

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

to provide a solution of lanthanide chlorides, and an insoluble sludge of the less-basic thorium hydroxide.

Extraction of rare-earth metals from monazite ore

Hydrometallury of rare earth metals from monazite ore process flow diagram

The extraction of rare-earth metals from monazite ore begins with digestion with sulfuric acid followed by aqueous extraction. The process requires many neutralizations and filtrations. The final products yielded for this process are thorium-phosphate concentrate, RE hydroxides, and uranium concentrate. Depending on the relative market prices of uranium, thorium, and rare earth elements as well as the availability of customers and the logistics of delivering to them, some or all of those products may be economical to sell or further process into a marketable form, while others constitute tailings for disposal. Products of the uranium and thorium decay series, particularly

radium Radium is a chemical element; it has chemical symbol, symbol Ra and atomic number 88. It is the sixth element in alkaline earth metal, group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, ...

will be present in trace amounts and form a radiotoxic hazard. While radium-228 (a product of thorium decay) will be present only in extremely minute amounts (less than one milligram per metric ton of thorium), and will decay away with a half life of roughly 5.75 years, radium-226 will be present at a ratio above 300 milligrams per metric ton of uranium and due to its long half life (~1600 years) will essentially remain with the residue. As radium forms the least soluble alkaline earth metal sulfate known, radium sulfate will be present among the solid filtration products after sulfuric acid has been added.

Containing nuclear waste

In two studies, one testing synthetic monazites radioactive waste storage capabilities by submerging it in a contaminated wastewater system for an extended period of time, and the other comparing the durability of the crystalline structures of multiple minerals, they investigate the ability of monazite to act as a host for nuclear byproducts from high-grade plutonium in decommissioned nuclear weapons and spent fuel from nuclear reactors. Results from both investigations show that monazite is one of the better options for storage in comparison to the previously used borosilicate glass. One study done at Oak Ridge National Laboratory in Tennessee the performance of synthetic monazite to borosilicate glass in radioactive waste management is compared. This experiment involved synthetic monazite and borosilicate glass being soaked in a contaminated simulated Savannah River defense wastes for 28 days, during the time period the leaching rates from both materials were measured. The results show that the synthetic monazite is a far more effective material for containing radioactive waste due to its low leaching rates and slow corrosion rate. In a second study natural monazite is found to have an enhanced ability to deal with radiation byproducts due the property of radiation "resistance" as it is able to remain crystalline after being subjected to high amounts of alpha-decay radiation and becoming amorphized. Due to this high durability, it is seen as a better alternative for hosting materials such as radioactive strontium than other tested minerals. Synthetic monazite is also shown to have similar durability to that of the natural crystalline samples after it becomes fully amorphized.

References

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
* ; the third in a series of videos about a Monazite beach in Brazil.
Monazite, thorium, and mesothorium
(1915) {{Authority control Lanthanide minerals Thorium minerals Radioactive gemstones Phosphate minerals Monoclinic minerals Minerals in space group 14