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Pyrochlore
General
Category Oxide mineral
Chemical formula (Na,Ca)2Nb2O6(OH,F)
Identification
Color Brown, yellow-brown, reddish brown, orange, black
Crystal habit disseminated granular - subhedral
Crystal system Isometric
Twinning 111 rare
Cleavage 111 indestinct
Fracture Subconchoidal to uneven, splintery
Mohs scale hardness 5.0–5.5
Luster vitreous to resinous
Streak White
Diaphaneity Subtranslucent to opaque
Specific gravity 4.2–6.4
Optical properties Isotropic, weak anomalous anisotropism
Refractive index n = 1.9–2.2
Other characteristics Variably radioactive

Pyrochlore (Na,Ca)2Nb2O6(OH,F) is a solid solution between the niobium end member (pyrochlore), and the tantalum end member (microlite). 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.

The name is from the Greek πΰρ, fire, and χλωρός, green because it typically turns green on ignition in classic blowpipe analysis.

Pyrochlore is also a more generic term for the pyrochlore crystal structure (Fd-3m). 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 intersticy between adjacent B-site cations. These systems are particularly susceptible to geometrical frustration and novel magnetic effects.

The pyrochlore structure shows varied physical properties ranging from electronic insulators, La2Zr2O7, ionic conductors, Gd1.9Ca0.1Ti2O6.9, metallic conductivity, Bi2Ru2O7-y, mixed ionic and electronic conductivity, spin ice systems Dy2Ti2O7, spin glass systems Y2Mo2O7 and superconducting materials Cd2Re2O7.

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 Saint-Honore mine west of Saint-Honore near Chicoutimi, Quebec.[1]

See also

References

  1. ^ J. Kouptsidis, F. Peters, D. Proch, W. Singer. "Niob für TESLA". http://tesla.desy.de/new_pages/TESLA_Reports/2001/pdf_files/tesla2001-27.pdf. Retrieved 2008-09-02.  
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