Single-crystal diffraction and transmission electron microscopy studies of "silicified" pyrochlore from Narssarssuk, Julianehaab district, Greenland

Abstract

Pyrochlore-group minerals that exhibit high Si contents are fairly common in geochemically evolved parageneses. However, the role of Si in the structure of these minerals is unclear. Different explanations have been invoked to clarify the way in which Si is incorporated in natural pyrochlores. These include the presence of impurities, the presence of Si in an amorphous or dispersed state, and its presence as an essential part of the structure.This paper reports an integrated XREF, SEM, EMPA, and TEM study on pyrochlore samples with high SiO 2 content (up to 11.51 wt%) from Narssârssuk, Julianehaab district (Greenland). TEM observations reveal that Si-poor areas have strong and sharp diffraction peaks, whereas the Si-rich areas showed weaker spots with the diffuse diffraction halo typical of a metamict material. No evidence of crystalline phases other than pyrochlore was observed. Two single crystals having the unit-cell parameter a = 10.4200(7) and 10.3738(7) Å, respectively, were analyzed by X-ray diffraction and the structure was refi ned to R obs = 2.62 and 4.35%. On the basis of both the refi ned site scattering and the octahedral bond distance and the results of the TEM investigation, only a fraction (~30-50%) of the Si detected by EMPA is incorporated in the structure. A comparison with structural data of Si-free pyrochlores reported in the literature supports this assumption and allows a linear multiple regression to model the effect of the substitution of (Nb,Ta) by Ti and Si. The remaining 50-70% of the total silicon detected is incorporated in the radiation-damaged portions of pyrochlore.

Role of vein-phases in nanoscale sequestration of U, Nb, Ti, and Pb during the alteration of pyrochlore

“…They have, however, been reported in the other localities (Lumpkin and Ewing, 1995;Nasraoui and Bilal, 2000;Chakhmouradian and Mitchell, 2002;Bonazzi et al, 2006). The relatively low content of SiO 2 in the pyrochlore analyzed here, <1.3 wt.…”

“…%, suggests minor incorporation of Si into the crystal structure during alteration. Based on experimental studies, Si accumulation was most likely facilitated by the amorphous nature of the pyrochlore, rather than being incorporated into the original crystalline pyrochlore structure (Bonazzi et al, 2006).…”

Liandratite from Karkonosze pegmatites, Sudetes, Southwestern Poland

“…zircon and pyrochlore supergroup minerals) has been extensively investigated (e.g. Geisler et al 2003;Rayner et al 2005;Bonazzi et al 2006;Dumańska-Słowik et al 2014). In various minerals of the above-named assemblage there are different elements; however, they share a common feature-they all concentrate in the intensively radiation-damaged, amorphous domains outside the crystal structure of the host mineral.…”

Ti-Nb Mineralization of Late Carbonatites and Role of Fluids in Its Formation: Petyayan-Vara Rare-Earth Carbonatites (Vuoriyarvi Massif, Russia)

“…Examples include brookite and rutile from the Nb (Ti, REE) deposit Morro dos Seis Lagos ( [25]; see their Figure 12). The unusual pattern of the excessive Nb entry that is not compensated by Fe requires further research [39] Later generation brookite, Brk-2 overgrowths both Brk-1, and the rutile developed on it. Its Nb content also varies, providing the mineral with a mosaic appearance in BSE-images.…”

“…Examples include brookite and rutile from the Nb (Ti, REE) deposit Morro dos Seis Lagos ( [25]; see their Figure 12). The unusual pattern of the excessive Nb entry that is not compensated by Fe requires further research [39]. Thus, attempts at balancing Fe 2+ and Fe 3+ according to the scheme Fe 2+ /(Fe 2+ + Fe 3+ ) ≈ 1 − (Nb/Fe) suggested by Werner and Cook [23] have failed and need further refinement.…”