Pyrochlore

Pyrochlore
General
Category	Oxide mineral
Formula	(Na,Ca)2Nb2O6(OH,F)
Crystal system	Isometric
Identification
Color	Brown, yellow-brown, reddish brown, orange, black
Crystal habit	disseminated granular - subhedral
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 also in granite pegmatites and greisens. It characteristically is 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, Gd_1.9Ca_0.1Ti₂O_6.9, metallic conductivity, Bi₂Ru₂O_7-y, Mixed Ionic and Electronic Conductivity, spin ice systems Dy₂Ti₂O₇, spin glass systems Y₂Mo₂O₇ and superconducting materials Cd₂Re₂O₇.

See also

• List of minerals

References

• Webmineral - Pyrochlore
• Mineral galleries
• Mindat
• Handbook of Mineralogy
• "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