Monazite is a primarily reddish-brown

phosphate mineral Phosphate minerals contain the tetrahedrally coordinated phosphate (PO43−) anion along sometimes with arsenate (AsO43−) and vanadate (VO43−) substitutions, and chloride (Cl−), fluoride (F−), and hydroxide (OH−) anions that also fit i ...

that contains

rare-earth elements The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...

. Due to variability in composition, monazite is considered a group of minerals. The most common species of the group is

monazite-(Ce) Monazite-(Ce) is the most common representative of the monazite group. It is the cerium-dominant analogue of monazite-(La) Monazite-(La) is a relatively rare representative of the monazite group, with lanthanum being the dominant rare earth elemen ...

, 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, macro ...

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 various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by th ...

and is relatively

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

, 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: *

, ( Ce, La, Nd, Th) PO₄ (the most common member), *

monazite-(La) Monazite-(La) is a relatively rare representative of the monazite group, with lanthanum being the dominant rare earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the ...

, (La,Ce,Nd)PO₄, *

monazite-(Nd) Monazite-(Nd) is a relatively rare representative of the monazite group, with neodymium being the dominant rare earth element in its structure. This variety of monazite is typically colored bright rose-red. It is the neodymium analogue of monazite- ...

, (Nd,La,Ce)PO₄, *

monazite-(Sm) Monazite-(Sm) is an exceedingly rare representative of the monazite group, with samarium being the dominant rare earth element in its structure. It is the samarium analogue of monazite-(Ce), monazite-(La), and monazite-(Nd) Monazite-(Nd) is a rela ...

, ( Sm, Gd,Ce,Th)PO₄, * monazite-(Pr), ( Pr,Ce,Nd,Th)PO₄. 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 , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

(SiO₂) is present in trace amounts, as well as small amounts of

uranium Uranium is a chemical element with the 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. Uranium is weak ...

and

thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high me ...

. 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) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...

of thorium and uranium, monazite contains a significant amount of

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

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

Burke County, North Carolina Burke County is a county located in the U.S. state of North Carolina. As of the 2020 census, the population was 87,570. Its county seat is Morganton. Burke County is part of the Hickory–Lenoir–Morganton, NC Metropolitan Statistical Area. ...

, USA; (II.) Arendal, Norway; (III.)

Emmaville, New South Wales Emmaville is a town on the Northern Tablelands, New South Wales, Northern Tablelands in the New England (Australia), New England region of New South Wales, Australia. It is in the Glen Innes Severn Council district. Emmaville is at an elevati ...

, Australia. , , 30.88 , , 26.26 , , 30.21 , - , colspan=2, Yttrium oxide (Yt₂O₃) , , — , , 3.82 , , — , - , colspan=2, Thorium oxide (ThO₂) , , 6.49 , , 9.57 , , 1.23 , - , colspan=2, Silica (SiO₂) , , 1.40 , , 1.86 , , 3.21 , - , colspan=2, Alumina (Al₂O₃) , , — , , —, , 3.11 , - , colspan=2, Iron oxide (Fe₂O₃) , , —, , 1.13, , — , - , colspan=2, Lime (CaO) , , — , , 0.69 , , — , - , colspan=2, Water (H₂O) , , 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 containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 Apr ...

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 word ''placer'', meaning "alluvial sand". Placer mi ...

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...

, and

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...

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 consi ...

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 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, sedimentary and hydrothermal rocks. ...

. 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 grc, μονάζειν, 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 Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, l ...

(PbCrO₄).

Mining history

Monazite sand from

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

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 praseo ...

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 state in the Southeastern region of the United States. The state is the 28th largest and 9th-most populous of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, Georgia and So ...

, 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 Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

. Monazite was the only significant source of commercial

lanthanides The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and ytt ...

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

bastnäsite The mineral bastnäsite (or bastnaesite) is one of a family of three carbonate-fluoride minerals, which includes bastnäsite-( Ce) with a formula of (Ce, La)CO3F, bastnäsite-( La) with a formula of (La, Ce)CO3F, and bastnäsite-( Y) with a formul ...

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 co ...

. These so-called

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

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 the r ...

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, printing ...

. 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 with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 ...

, about 24%

lanthanum Lanthanum is a chemical element with the 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 between lant ...

, about 17%

neodymium Neodymium is a chemical element with the 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 metals. It is a hard, slightly malleable, silvery metal that quickly tarnishe ...

, about 5%

praseodymium Praseodymium is a chemical element with the symbol Pr and the atomic number 59. It is the third member of the lanthanide series and is considered to be one of the rare-earth metals. It is a soft, silvery, malleable and ductile metal, valued for i ...

, and minor quantities of

samarium Samarium is a chemical element with symbol Sm and atomic number 62. It is a moderately hard silvery metal that slowly oxidizes in air. Being a typical member of the lanthanide series, samarium usually has the oxidation state +3. Compounds of samar ...

gadolinium Gadolinium is a chemical element with the symbol Gd and atomic number 64. Gadolinium is a silvery-white metal when oxidation is removed. It is only slightly malleable and is a ductile rare-earth element. Gadolinium reacts with atmospheric oxygen ...

, and

yttrium Yttrium is a chemical element with the 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 always found in co ...

. 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

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

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 ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

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 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 orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

pyrophosphate In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a P–O–P linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate (Na2H2P2O7) and tetrasodium pyrophosphate (Na4P2O7), among othe ...

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. Monazite acid cracking process

Alkaline cracking

A more recent process uses hot sodium hydroxide 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 the 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 a ...

. The lanthanide/thorium hydroxide mixture can be treated with

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

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.Gupta, C. K., and N. Krishnamurthy. ''Extraction Metallurgy of Rare Earths''. Boca Raton, Florida: CRC, 2005. Print. 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 In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay direc ...

, particularly

radium Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rathe ...

will be present in trace amounts and form a radiotoxic hazard. While

radium-228 Radium (88Ra) has no stable or nearly stable isotopes, and thus a standard atomic weight cannot be given. The longest lived, and most common, isotope of radium is 226Ra with a half-life of . 226Ra occurs in the decay chain of 238U (often referred ...

(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.

References

External links

Monazite

An Unusual State Of Matter
Poem about monazite by

Roald Hoffman Roald Hoffmann (born Roald Safran; July 18, 1937) is a Polish-American theoretical chemist who won the 1981 Nobel Prize in Chemistry. He has also published plays and poetry. He is the Frank H. T. Rhodes Professor of Humane Letters, Emeritus, a ...

"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 Phosphate minerals Monoclinic minerals Minerals in space group 14