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 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 (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.
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 ranging from electronic insulators, La2Zr2O7, ionic conductors, Gd1.9Ca0.1Ti2O6.9, metallic conductivity, Bi2Ru2O7-y, mixed ionic and electronic conductivity, spin ice systems Dy2Ti2O7, spin glass systems Y2Mo2O7 and superconducting materials Cd2Re2O7.
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