Pyrochlore

Pyrochlore
Pyrochlore from Russia
General
Category	Oxide mineral
Formula (repeating unit)	(Na,Ca)2Nb2O6(OH,F)
Strunz classification	4.DH.15
Dana classification	08.02.01.01 Pyrochlore group
Crystal system	Isometric
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m
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	Variably radioactive, often metamict
References

Pyrochlore (Na,Ca)₂Nb₂O₆(OH,F) is a mineral group of the niobium end member of the pyrochlore supergroup.

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.

It was first described in 1826 for an occurrence in Stavern (Fredriksvärn), Larvik, Vestfold, Norway. The name is from the Greek πῦρ, 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 interstice between adjacent B-site cations. These systems are particularly susceptible to geometrical frustration and novel magnetic effects.