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 (Gd1.9Ca0.1Ti2O6.9), metallic conductors (Bi2Ru2O7-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.<ref name "tesla">J. Kouptsidis, F. Peters, D. Proch, W. Singer. "Niob für TESLA" (PDF). Retrieved 2008-09-02.</ref>

Pyrochlore ore typically contains greater than 0.05% of naturally occurring radioactive uranium and thorium.[4]

References

  1. ↑ 1.0 1.1 "pyrochlore at RRuff database" (PDF). rruff.info. Retrieved 2015-02-03.
  2. ↑ 2.0 2.1 "Pyrochlore Group: Pyrochlore Group mineral information and data.". mindat.org. Retrieved 2015-02-03.
  3. ↑ Barthelmy, Dave. "Pyrochlore Mineral Data". webmineral.com. Retrieved 2015-02-03.
  4. ↑ Dias da Cunha, K.; M. Santos, F. Zouain1, L. Carneiro1, G. Pitassi, C. Lima, C. V. Barros Leite and K. C. P. Dália (May 8, 2009). "Dissolution Factors of Ta, Th, and U Oxides Present in Pyrochlore". Water, Air, & Soil Pollution (Springer Netherlands) 205 (1-4): 251–257. doi:10.1007/s11270-009-0071-3. ISSN 0049-6979.

See also