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Pyrochlore
Chemical Formula	(Na,Ca)2Nb2O6(OH,F)
Species	Oxides & Hydroxides
Crystal System	Hexagonal
Mohs Scale	5-5.5
Specific Gravity	4.45-4.90
Color	Black to brown, chocolate-brown, reddish brown, amber-orange, red-orange
Streak	White
Luster	Vitreous to resinous
Refractive Index	n = 1.9–2.2
Diaphaneity	Subtranslucent to opaque
Cleavage	111 indistinct, may be a parting.
Fracture	Subconchoidal to uneven, splintery
Crystal Habit：Typically octahedra, disseminated granular, massive

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

Occurrence

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.

Name and discovery

It was first described in 1826 for an occurrence in Stavern (Fredriksv?rn), Larvik, Vestfold, Norway. The name is from the Greek π?ρ, fire, and χλωρ??, 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 (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 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), 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 Catal?o mine east of Catal?o, Goiás. The third largest deposit of niobium ore is Saint-Honore mine west of Saint-Honore near Chicoutimi, Quebec.

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