Pyrochlore () is a mineral group of the

niobium Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has sim ...

end member of the pyrochlore supergroup. The general formula, (where A and B are metals), represent a family of phases isostructural to the mineral pyrochlore. Pyrochlores are an important class of materials in diverse technological applications such as luminescence, ionic conductivity, nuclear waste immobilization, high temperature thermal barrier coatings, automobile

exhaust gas Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...

control, catalysts, solid oxide fuel cell, ionic/electrical conductors etc.

Occurrence

The mineral is associated with the

metasomatic Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical com ...

end stages of magmatic intrusions. Pyrochlore crystals are usually well formed (euhedral), occurring usually as

octahedra In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

of a yellowish or brownish color and

resin In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...

ous luster. It is commonly

metamict Metamictisation (sometimes called metamictization or metamiction) is a natural process resulting in the gradual and ultimately complete destruction of a mineral's crystal structure, leaving the mineral amorphous. The affected material is therefore ...

due to radiation damage from included radioactive elements. Pyrochlore occurs in

pegmatite 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 ...

s associated with

nepheline syenite Nepheline syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are al ...

s and other alkalic rocks. It is also found in

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

pegmatites and

greisen Greisen is a highly altered granitic rock or pegmatite, usually composed predominantly of quartz and micas (mostly muscovite). Greisen is formed by self-generated alteration of a granite and is a class of moderate- to high-temperature magmatic-hy ...

s. It is characteristically found in

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. Associated minerals include

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 ...

aegirine Aegirine is a member of the clino pyroxene group of inosilicate minerals. Aegirine is the sodium endmember of the aegirine- augite series. Aegirine has the chemical formula Na Fe Si2 O6 in which the iron is present as Fe3+. In the aegirine-augit ...

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...

perovskite Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as (XIIA2+VIB4+X2−3), known as ...

and

columbite Columbite, also called niobite, niobite-tantalite and columbate [], is a black mineral group that is an ore of niobium. It has a submetallic Lustre (mineralogy), luster and a high density and is a niobate of iron and manganese. This mineral group w ...

Name and discovery

It was first described in 1826 for an occurrence in Stavern (Fredriksvärn),

Larvik Larvik () is a List of cities in Norway, town and Municipalities of Norway, municipality in Vestfold in Vestfold og Telemark Counties of Norway, county, Norway. The administrative centre of the municipality is the city of Larvik. The municipality ...

Vestfold Vestfold is a traditional region, a former county and a current electoral district in Eastern Norway. In 2020 the county became part of the much larger county of Vestfold og Telemark. Located on the western shore of the Oslofjord, it bordered th ...

Norway Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of Jan Mayen and t ...

. The name is from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

, ''fire'', and , ''green'' because it typically turns green on ignition in classic blowpipe analysis.

Crystal structure

Pyrochlore is also a more generic term for the pyrochlore crystal structure ( ''F''dm). The more general crystal structure describes materials of the type A₂B₂O₆ and A₂B₂O₇ where the A and B species are generally rare-earth or transition metal species; e.g. Y₂Ti₂O₇.The pyrochlore structure is a super structure derivative of the simple

fluorite structure In solid state chemistry, the fluorite structure refers to a common motif for compounds with the formula MX2. The X ions occupy the eight tetrahedral interstitial sites whereas M ions occupy the regular sites of a face-centered cubic (FCC) structure ...

(AO₂ = A₄O₈, where the A and B cations are ordered along the direction. The additional anion vacancy resides in the tetrahedral interstice between adjacent B-site cations. These systems are particularly susceptible to

geometrical frustration In condensed matter physics, the term geometrical frustration (or in short: frustration) refers to a phenomenon where atoms tend to stick to non-trivial positions or where, on a regular crystal lattice, conflicting inter-atomic forces (each one fa ...

and novel magnetic effects. The pyrochlore structure shows varied physical properties spanning electronic

insulators Insulator may refer to: * Insulator (electricity), a substance that resists electricity ** Pin insulator, a device that isolates a wire from a physical support such as a pin on a utility pole ** Strain insulator, a device that is designed to work ...

(e.g. La₂Zr₂O₇), ionic conductors (Gd_1.9Ca_0.1Ti₂O_6.9), metallic conductors (Bi₂Ru₂O_7−''y''), mixed ionic and electronic conductors,

spin ice A spin ice is a magnetic substance that does not have a single minimal-energy state. It has magnetic moments (i.e. "spin") as elementary degrees of freedom which are subject to frustrated interactions. By their nature, these interactions preven ...

systems (Dy₂Ti₂O₇),

spin glass In condensed matter physics, a spin glass is a magnetic state characterized by randomness, besides cooperative behavior in freezing of spins at a temperature called 'freezing temperature' ''Tf''. In ferromagnetic solids, component atoms' magne ...

systems (Y₂Mo₂O₇), haldane chain systems (Tl₂Ru₂O₇) and superconducting materials (Cd₂Re₂O₇). More disordered structures, such as the bismuth pyrochlores, have also been investigated due to interesting high-frequency dielectric properties.

Niobium mining

The three largest producers of niobium ore are mining pyrochlore deposits. The largest deposit in

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 ...

is the CBMM mine located south of

Araxá Araxá () is a municipality in Western Minas Gerais state, Brazil. Its estimated population by IBGE (Brazilian Institute of Geography and Statistics) in 2020 is 107,337 inhabitants and the area of the municipality is , with making up the urban p ...

Minas Gerais Minas Gerais () is a state in Southeastern Brazil. It ranks as the second most populous, the third by gross domestic product (GDP), and the fourth largest by area in the country. The state's capital and largest city, Belo Horizonte (literally ...

, followed by the deposit of the Catalão mine east of

Catalão Catalão () is a city and municipality located in the south of the state of Goiás, in Brazil. It is a large producer of grains, cattle, and phosphates and has a John Deere and Mitsubishi factory. Demographics *Population density: 17.85 inhab ...

Goiás Goiás () is a Brazilian state located in the Center-West region. Goiás borders the Federal District and the states of (from north clockwise) Tocantins, Bahia, Minas Gerais, Mato Grosso do Sul and Mato Grosso. The state capital is Goiânia. ...

. The third largest deposit of niobium ore is Niobec mine west of Saint-Honoré near

Chicoutimi Chicoutimi () is the most populous borough (arrondissement) of the city of Saguenay in Quebec, Canada. It is situated at the confluence of the Saguenay and Chicoutimi rivers. During the 20th century, it became the main administrative and com ...

Quebec Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirtee ...

. Pyrochlore ore typically contains greater than 0.05% of naturally occurring radioactive

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 ...

Lueshe The Lueshe mine, or Lweshe mine, is a niobium mine located in Rutshuru Territory in North Kivu province of the Democratic Republic of the Congo. The site is one of the most economically significant pyrochlore deposits in the world. The rare niobi ...

North Kivu North Kivu (french: link=no, Nord-Kivu) is a province bordering Lake Kivu in the eastern Democratic Republic of the Congo. Its capital is Goma. North Kivu borders the provinces of Ituri to the north, Tshopo to the northwest, Maniema to the so ...

, Democratic Republic of Congo, has substantial deposits of pyrochlore.

References

*{{cite journal, last2= Andrade, first2= M. B. , date=2022 , title=Prospection of pyrochlore and microlite mineral groups through Raman spectroscopy coupled with artificial neural networks , journal=Journal of Raman Spectroscopy , doi=10.1002/jrs.6433, last1=Queiroz, first1= A. A. A. E. Sodium minerals Calcium minerals Niobium minerals Oxide minerals Cubic minerals Minerals in space group 227 Minerals described in 1826

Occurrence

Name and discovery

Crystal structure

Niobium mining

See also

References