پايروكلور

Pyrochlore
Pyrochlore from Russia
العامة
التصنيف	Oxide mineral
الصيغة (repeating unit)	(Na,Ca) 2Nb 2O 6(OH,F)
تصنيف سترونز	4.DH.15
تصنيف دانا	08.02.01.01 Pyrochlore group
النظام البلوري	Isometric
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m (No. 227)
Unit cell	a = 10.41(6) Å, Z = 8
التعرف
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
الجاذبية النوعية	4.45 to 4.90
الصفات البصرية	Isotropic, weak anomalous anisotropism
Refractive index	n = 1.9–2.2
سمات أخرى	Radioactive, often metamict
References

Pyrochlore (Na,Ca)
2Nb
2O
6(OH,F) is a mineral group of the niobium end member of the pyrochlore supergroup. Pyrochlore is also a term for the crystal structure Fd3m. The name is from the Greek πῦρ, fire, and χλωρός, green because it typically turns green on ignition in classic blowpipe analysis.

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 Mineral

The general formula, A
2B
2O
7 (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 control, catalysts, solid oxide fuel cell, ionic/electrical conductors etc.

The mineral is associated with the metasomatic end stages of magmatic intrusions. Pyrochlore crystals are usually well-formed (euhedral), occurring usually as octahedra of a yellowish or brownish color and resinous luster. It is commonly metamict due to radiation damage from included radioactive elements.

Pyrochlore occurs in pegmatites associated with nepheline syenites and other alkalic rocks. It is also found in granite pegmatites and greisens. It is characteristically found in carbonatites. Associated minerals include zircon, aegirine, apatite, perovskite and columbite.

 History

It was first described in 1826 for an occurrence in Stavern (Fredriksvärn), Larvik, Vestfold, Norway.

 Niobium mining

The three largest producers of niobium ore are mining pyrochlore deposits. The largest deposit in Brazil is the CBMM mine located south of Araxá, Minas Gerais, followed by the deposit of the Catalão mine east of Catalão, Goiás. The third largest deposit of niobium ore is Niobec mine west of Saint-Honoré near Chicoutimi, Quebec.

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

Lueshe in North Kivu, Democratic Republic of Congo, has substantial deposits of pyrochlore.

 Crystal structure

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 (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 to geometrical frustration and novel magnetic effects.

The pyrochlore structure shows varied physical properties spanning electronic insulators (e.g. La₂Zr₂O₇), ionic conductors (Gd_1.9Ca_0.1Ti₂O_6.9), metallic conductors (Bi₂Ru₂O_7−y), mixed ionic and electronic conductors, spin ice systems (Dy₂Ti₂O₇), spin glass 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.

The crystal structure has been investigated for use in solid electrolytes for lithium iron batteries. It is alleged to provide high conductivity while inhibiting dendrite growth.

 See also

• List of minerals

 References

• Queiroz, A. A. A. E.; Andrade, M. B. (2022). "Prospection of pyrochlore and microlite mineral groups through Raman spectroscopy coupled with artificial neural networks". Journal of Raman Spectroscopy. 53 (11): 1924–1930. Bibcode:2022JRSp...53.1924E. doi:10.1002/jrs.6433. S2CID 251463725.