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Pyrochlores are a fascinating group of minerals, whose structure is a 3-D network of (Nb,Ti,Ta,Zr)O6 octahedra (pink on the diagram at right) connected via oxygen atoms at their corners (blue spheres). "Voids" in this framework are occupied by large cations in an eight-fold coordination by O2- and monovalent anions (F- and [OH]-, purple spheres). These coordination polyhedra are shown as distorted green cubes on the diagram. The pyrochlore structure may be interpreted as an anion-deficient, cation-ordered derivative (A2B2X7) from the fluorite structure type (A4X8). The pyrochlore structure is very flexible, which is reflected in the complex chemistry of natural pyrochlores, in their ability to accommodate a large proportion of vacancies in the A and (F,OH) sites, to take up water, and to exchange cations with fluids. The composition of pyrochlore-group minerals from alkaline rocks and carbonatites may be generalized as follows: (Na,Ca,U,Th,REE,Sr,Ba,Pb,K,Mn)2-x(Nb,Ti,Ta,Zr,Fe,Si)2O6(F,OH)1-ynH2O. Cool, isn't it?.. Shown below are:
[Left] Zoned pyrochlore crystal in phoscorite from the Sokli carbonatite complex (Finland). The zoning, so obvious in back-scattered electrons, is due to enrichment of the crystal's core in U and Ta relative to the rim. Terry Williams and I described pyrochlore-group minerals from carbonatitic rocks in great detail in "Phoscorites and Carbonatites from Mantle to Mine" (Chapter 9).
Another important milieu for pyrochlore-group minerals is feldspathoid syenites and associated metasomatic rocks (like albitites and fenites). The images below show oscillatory- zoned and altered (ion-leached and hydrated) pyrochlores in albitite from Mt. Selsurt, Lovozero, Russia (as seen under the microscope in plane-polarized light [middle] and in back-scattered electrons [at right]). Roger Mitchell and I described these fascinating pyrochlores in 2002 in the European Journal of Mineralogy (volume 14, pp. 821-836).
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Crystal structure of pyrochlore
Pyrochlore, Sokli Pyrochlore, Lovozero Pyrochlore, Lovozero