| Pyrochlore | |
|---|---|
 Pyrochlore from Russia | |
| General | |
| Category | Oxide mineral |
| Formula | (Na,Ca)2Nb2O6(OH,F) |
| IMA symbol | Pcl[1] |
| Strunz classification | 4.DH.15 |
| Dana classification | 08.02.01.01 Pyrochlore group |
| Crystal system | Isometric |
| Crystal class | Hexoctahedral (m3m) H-M symbol: (4/m3 2/m) |
| Space group | Fd3m (No. 227) |
| Unit cell | a = 10.41(6) Å,Z = 8 |
| Identification | |
| Color | Black to brown, chocolate-brown, reddish brown, amber-orange, red-orange |
| Crystal habit | Typically octahedra, disseminated granular, massive |
| Twinning | 111 rare |
| Cleavage | 111 indistinct, may be a parting. |
| Fracture | Subconchoidal to uneven, splintery |
| Tenacity | Brittle |
| Mohs scale hardness | 5.0–5.5 |
| Luster | Vitreous to resinous |
| Streak | White |
| Diaphaneity | Subtranslucent to opaque |
| Specific gravity | 4.45 to 4.90 |
| Optical properties | Isotropic, weak anomalous anisotropism |
| Refractive index | n = 1.9–2.2 |
| Other characteristics |  Radioactive, oftenmetamict |
| References | [2][3][4][5] |
Pyrochlore (Na,Ca)2Nb2O6(OH,F) is a mineral group of theniobium end member of the pyrochlore supergroup. Pyrochlore is also a term for the crystal structureFd3m. The name is from theGreekπῦρ,fire, andχλωρός,green because it typically turns green on ignition in classic blowpipe analysis.[4]
The general formula,A2B2O7 (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, automobileexhaust gas control, catalysts, solid oxide fuel cell, ionic/electrical conductors etc.
The mineral is associated with themetasomatic end stages of magmatic intrusions. Pyrochlore crystals are usually well-formed (euhedral), occurring usually asoctahedra of a yellowish or brownish color andresinous luster. It is commonlymetamict due to radiation damage from included radioactive elements.
Pyrochlore occurs inpegmatites associated withnepheline syenites and other alkalic rocks. It is also found ingranite pegmatites andgreisens. It is characteristically found incarbonatites. Associated minerals includezircon,aegirine,apatite,perovskite andcolumbite.[3]
It was first described in 1826 for an occurrence in Stavern (Fredriksvärn),Larvik, Vestfold, Norway.[4]
The three largest producers of niobium ore are mining pyrochlore deposits. The largest deposit inBrazil is the CBMM mine located south ofAraxá, Minas Gerais, followed by the deposit of the Catalão mine east ofCatalão, Goiás. The third largest deposit of niobium ore is Niobec mine west ofSaint-Honoré nearChicoutimi, Quebec.[6]
Pyrochlore ore typically contains greater than 0.05% of naturally occurring radioactiveuranium andthorium.[7]
Lueshe inNorth Kivu, Democratic Republic of Congo, has substantial deposits of pyrochlore.[8]
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 simplefluorite structure (AO2 = A4O8), where the A and B cations are ordered along the⟨110⟩ direction. The additional anion vacancy resides in the tetrahedral interstice between adjacent B-site cations. These systems are particularly susceptible togeometrical frustration and novel magnetic effects.
The pyrochlore structure shows varied physical properties spanning electronicinsulators (e.g. La2Zr2O7),ionic conductors (Gd1.9Ca0.1Ti2O6.9),metallic conductors (Bi2Ru2O7−y), mixed ionic and electronic conductors,spin ice systems (Dy2Ti2O7),spin glass systems (Y2Mo2O7), haldane chain systems (Tl2Ru2O7) andsuperconducting materials (Cd2Re2O7).[9] More disordered structures, such as the bismuth pyrochlores,[10] have also been investigated due to interesting high-frequency dielectric properties.[11]
The crystal structure has been investigated for use in solid electrolytes forlithium iron batteries. It is alleged to provide high conductivity while inhibiting dendrite growth.[12]
