Pyrochlore

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Pyrochlore (Na,Ca)₂Nb₂O₆(OH,F) is a solid solution between the niobium end member (pyrochlore), and the tatntalum 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. So called from its becoming grass-green on being subjected to heat under the blowpipe.

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.