“Silicified” pyrochlore from nepheline syenite (mariupolite) of the Mariupol Massif, SE Ukraine: A new insight into the role of silicon in the pyrochlore structure

Pyrochlore-supergroup minerals containing relatively high Si concentration are quite common in various geochemical parageneses, e.g., carbonatites, alkaline syenites, pegmatites. However, the role of Si and the mechanism of its incorporation into the structure of these minerals, although widely which is the late-magmatic to early hydrothermal accessory component of the nepheline syenite in the alkaline Mariupol massif in Ukraine. It represents partly metamict, patchy-zoned, A-cation depleted, 2 (up to 13.01 wt%) that underwent both primary and secondary alterations, leading to kenopyrochlore or hydropyrochlore species. The primary alteration was induced by high-temperature, Ca2+- and Si4+-rich, and F– moderate A site vacancies mainly by Ca2+, but also Mn2+, Sr2+, and K+. The secondary alteration, induced by the exposure affected the whole pyrochlore crystals (both Si-enriched and Si-free domains) and caused, among oth- ers, the leaching of some A- and Y-site components. TEM investigations indicate that the selected-area electron diffraction patterns taken from Si-poor areas show strong and sharp diffraction spots related to well-crystalline pyrochlore, whereas the Si-rich areas show weaker spots with a diffuse diffraction halo that are typical of metamict material. Due to the fact that no intergrowth with other Si-bearing phases 4+ can occupy severely -decay damaged and chemically altered portions of this structure. The absence of Si in the sixfold-coordinated B site has been corroborated both by compositional relationships, and by the lack of any [6]Si4+ signal around –200 ppm in the MAS-NMR spectrum. A broad signal in the spectrum appearing at around –84 ppm, points to an amorphous species with tetrahedrally coordinated Si, close to Q(2)2(–)2], in the form A and B sites of the primary structure.