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{{Short description|Mineral containing rare-earth elements}}
{{distinguish|text=the feldspathic igneous intrusive rock [[Monzonite]]}}
{{Infobox mineral
| name = Monazite
| category = [[Phosphate minerals]]
| boxwidth =
| boxbgcolor =
| image = Monazite-(Ce)-164025.jpg
| imagesize = 260px
| caption = Monazite-(Ce)
| formula = ([[Cerium|Ce]],[[Lanthanum|La]],[[Thorium|Th]])[[Phosphate|PO4]]
| molweight =
| strunz = 8.AD.50
| system = [[Monoclinic]]
| class = Prismatic (2/m) (same [[H–M symbol]])
| symmetry = ''P2''1/n
| color = Orange, Purple, Reddish brown, brown, pale yellow, pink, blue, green, gray,
| habit = Commonly as prismatic or wedge-shaped crystals
| twinning = Contact twins common
| cleavage = Distinct on [100] poor on [010]
| fracture = Conchoidal to uneven
| prop1 = Magnetism | prop1text = Paramagnetic, moderately strongly
| mohs = 5.0–5.5
| luster = Resinous, vitreous to adamantine
| refractive = ''n''α = 1.770–1.793 ''n''β = 1.778–1.800 ''n''γ = 1.823–1.860
| opticalprop = Biaxial (+)
| 2V = 10–26°
| birefringence =
| pleochroism = Weak
| streak = White
| gravity = 4.6–5.7 (4.98–5.43 for monazite-Ce)
| density =
| melt =1900–2100
| fusibility =
| diagnostic =
| solubility =
| diaphaneity = Translucent to opaque
| other = [[Image:Radioactive.svg|25px]] [[Radioactive]] if uranium and/or thorium-rich, dull brown [[cathodoluminescence]], [[paramagnetic]]
| references = <ref>[https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Monazite Mineralienatlas].</ref><ref>{{cite web |last1=Anthony |first1=John W. |last2=Bideaux |first2=Richard A. |last3=Bladh |first3=Kenneth W. |last4=Nichols |first4=Monte C. |title=Monazite |url=http://www.handbookofmineralogy.org/pdfs/monazite.pdf |website=Handbook of Mineralogy |publisher=Mineral Data Publishing |access-date=23 July 2022 |date=2005}}</ref>
}}

'''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.<ref>[http://www.mindat.org/min-2750.html Monazite group on Mindat.org]</ref> The most common species of the group is [[monazite-(Ce)]], that is, the cerium-dominant member of the group.<ref>[https://www.mindat.org/min-2751.html Monazite-(Ce) on Mindat.org]</ref> It occurs usually in small isolated [[crystal]]s. It has a hardness of 5.0 to 5.5 on the [[Mohs scale of mineral hardness]] and is relatively [[Density|dense]], 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:<ref>[http://www.mindat.org/min-2750.html Monazite group on Mindat.org]</ref>
* [[monazite-(Ce)]], ([[Cerium|Ce]],[[Lanthanum|La]],[[Neodymium|Nd]],[[Thorium|Th]])[[Phosphate|PO4]] (the most common member),
* [[monazite-(La)]], (La,Ce,Nd)PO4,
* [[monazite-(Nd)]], (Nd,La,Ce)PO4,
* [[monazite-(Sm)]], ([[Samarium|Sm]],[[Gadolinium|Gd]],Ce,Th)PO4,
* [[monazite-(Pr)]], ([[Praseodymium|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]] (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.<ref>[https://books.google.com/books?id=S-QDAAAAMBAJ&dq=Popular+Mechanics+1931+%22all-metal%22&pg=PA460 "Helium From Sand"], March 1931, ''Popular Mechanics'' p. 460.</ref>

The following analyses are of monazite from: (I.) [[Burke County, North Carolina]], US; (II.) [[Arendal]], Norway; (III.) [[Emmaville, New South Wales]], Australia.<ref>{{cite EB1911 |wstitle=Monazite |volume=18 |page=692 |first=Leonard James |last=Spencer |authorlink=Leonard James Spencer}}</ref>
{|style="margin:auto; font-size:92%; line-height:120%; text-align:right;"
|-
| || ||I. ||II. ||III.
|-
|colspan=2|Phosphorus pentoxide (P2O5)|| 29.28|| 27.55|| 25.09
|-
|colspan=2|Cerium oxide (Ce2O3) ||31.38 || 29.20 ||36.64
|-
|Lanthanum oxide (La2O3) Didymium oxide (Di2O3)||<math>\Big\}</math> ||30.88 || 26.26 ||30.21
|-
|colspan=2|Yttrium oxide (Y2O3) || — || 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%
| || ||{{rule|3em|align=left}}||{{rule|3em|align=left}}||{{rule|3em|align=left}}
|-
| || ||99.63 ||100.60 ||99.49
|-
|colspan=2|Specific gravity || 5.10 || 5.15 || 5.001
|}

Monazite is an important [[ore]] for thorium,<ref name=Ullmann>Wolfgang Stoll "Thorium and Thorium Compounds" Ullmann's Encyclopedia of Industrial Chemistry 2012 Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a27_001}}.</ref> lanthanum, and cerium.<ref>McGill, Ian (2005) "Rare Earth Elements" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a22_607}}.</ref> It is often found in [[placer deposit]]s. [[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.<ref name="WilliamsJercinovic2007">{{cite journal|last1=Williams|first1=Michael L.|last2=Jercinovic|first2=Michael J.|last3=Hetherington|first3=Callum J.|title=Microprobe Monazite Geochronology: Understanding Geologic Processes by Integrating Composition and Chronology|journal=Annual Review of Earth and Planetary Sciences|volume=35|issue=1|year=2007|pages=137–175|issn=0084-6597|doi=10.1146/annurev.earth.35.031306.140228|bibcode=2007AREPS..35..137W}}</ref> These domains can be dated to gain insight into the geologic history of its host rocks.

The name monazite comes from the {{lang-grc|μονάζειν|monázein}} (to be solitary), via German ''Monazit'', in allusion to its isolated crystals.<ref>''[[Oxford English Dictionary]]'', 3rd edition, 2002.</ref>

==Structure==
[[File:PbCrO4.jpg|thumb|left|220 px|Structure of monazite. Color scheme: red = O, pale blue = P, dark gray = Ce(III) and other lanthanides and actinides.]]
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).<ref>Quareni, S.; de Pieri, R. "A three-dimensional refinement of the structure of crocoite, PbCrO4" ''Acta Crystallographica'' 1965, volume 19, pp. 287–289.</ref>

==Mining history==
[[Image:monazitemineshelbync.jpg|thumb|left|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 thorium for his newly invented incandescent [[gas mantle|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]], but, shortly thereafter, extensive deposits in southern [[India]] were found. Brazilian and Indian monazite dominated the industry before World War II, after which major mining activity transferred to [[South Africa]]. There are also large monazite deposits in [[Australia]].

Monazite was the only significant source of commercial [[lanthanides]], but concern over the disposal of the [[Decay product|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-based nuclear power|thorium for nuclear energy]] may bring monazite back into commercial use.{{citationneeded|date=June 2019}}

==Mineralization and extraction==
[[Image:MonaziteUSGOV.jpg|thumb|left|Monazite powder]]
[[Image:Monazit - Madagaskar.jpg|thumb|left|Monazite - Locality: Madagascar - Exposed in the Mineralogical Museum, Bonn, Germany]]
Because of their high density, monazite minerals concentrate in alluvial sands when released by the weathering of [[pegmatites]]. These so-called [[placer deposit]]s 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-([[cerium|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 mine|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]]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]] to temperatures between {{convert|120 and 150|°C|sigfig=2}} 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.
[[File:Monazite acid cracking process.svg|600px|center]]

=== Alkaline cracking ===
A more recent process uses hot [[sodium hydroxide]] solution (73%) at about {{Convert|140|°C|sigfig=2}}. 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.
[[Image:Monazit opening alkaline.gif|600px|center]]

===Extraction of rare-earth metals from monazite ore===
{{refimprovesection|date=July 2022}}
[[File:Hydrometallury of rare earth metals from monazite ore process flow diagram.jpg|thumb|400px|Process flow diagram for extraction of rare-earth metals from monazite ore using hydrometallurgy]]

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.<ref>Gupta, C. K. and T. K. Mukherjee. ''Hydrometallurgy in Extraction Processes''. Boca Raton, Florida: CRC, 1990. Print.</ref><ref>Gupta, C. K., and N. Krishnamurthy. ''Extraction Metallurgy of Rare Earths''. Boca Raton, Florida: CRC, 2005. Print.</ref>

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]] 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.

==References==
{{Reflist}}

==Further reading==
* J. C. Bailar et al., ''Comprehensive Inorganic Chemistry'', Pergamon Press, 1973.
* R. J. Callow, ''The Industrial Chemistry of the Lanthanons, Yttrium, Thorium and Uranium'', Pergamon Press 1967. {{LCCN|6714541}}.
* Gupta, C. K. and N. Krishnamurthy, ''Extactive Metallurgy of Rare Earths'', CRC Press, 2005, {{ISBN|0-415-33340-7}}.
* Gupta, C. K., and T. K. Mukherjee. ''Hydrometallurgy in Extraction Processes'', Boca Raton, Florida: CRC Press, 1990. Print.
* Price List, Lindsay Chemical Division, American Potash and Chemical Corporation, 1960.
* R. C. Vickery, ''Chemistry of the Lanthanons'', Butterworths and Academic Press, 1953.

==External links==
{{commons category|Monazite}}
* [https://web.archive.org/web/19980215073411/http://mineral.galleries.com/minerals/phosphat/monazite/monazite.htm Monazite]
* [http://www.roaldhoffmann.com/sites/all/files/an_unusual_state_of_matter.pdf An Unusual State Of Matter] Poem about monazite by [[Roald Hoffman]]
* [https://web.archive.org/web/20120215121509/http://dc.lib.unc.edu/cdm4/item_viewer.php?CISOROOT=%2Fnc_post&CISOPTR=3240&CISOBOX=1&REC=1 "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]
* {{YouTube|id=vdHHUGwFoJE|title=radiation (in) paradise – the secret of the sand}}; the third in a series of videos about a Monazite beach in Brazil.
* [https://archive.org/details/monazitethoriumm00kithiala Monazite, thorium, and mesothorium] (1915)

{{Phosphate minerals}}

{{Authority control}}

[[Category:Lanthanide minerals]]
[[Category:Thorium minerals]]
[[Category:Phosphate minerals]]
[[Category:Monoclinic minerals]]
[[Category:Minerals in space group 14]]

Monazite - Revision history

EJEBoris894787: expand citations for clarity and updated info

Ajay Kumar: Created a new article