Pyrochlore

Pyrochlore
Pyrochlore from Uganda
General
Category	Oxide mineral
Formula (repeating unit)	(Na,Ca)2Nb2O6(OH,F)
Strunz classification	04.DH.15
Dana classification	08.02.01.01 Pyrochlore group
Crystal symmetry	Isometric 4/m 3 2/m
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
Crystal system	Isometric - Hexoctahedral
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	[1][2][3]

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.^[1]

Name and discovery

It was first described in 1826 for an occurrence in Stavern (Fredriksvärn), Larvik, Vestfold, Norway. The name is from the Greek πΰρ, fire, and χλωρός, green because it typically turns green on ignition in classic blowpipe analysis.^[2]

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), haldane chain systems (Tl₂Ru₂O₇) 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 Niobec mine west of Saint-Honore near Chicoutimi, Quebec.^[4]

Pyrochlore ore typically contains greater than 0.05% of naturally occurring radioactive uranium and thorium.^[5]

References

• "Oxide Pyrochlores - A Review", M.A. Subramanian, G. Aravamudan and G. V. Subba Rao, Progress in Solid State Chemistry, Volume 15 (1983) pp. 55-143
• Atencio, D., Andrade, M. B., Christy, A. G., Gieré, R., & Kartashov, P. M. (2010). The pyrochlore supergroup of minerals: nomenclature. The Canadian Mineralogist, 48(3), 673-698.doi: 10.3749/canmin.48.3.673

See also

• List of minerals