Primary zoning in pyrochlore group minerals from carbonatites

Hogarth, D. D.; Williams, C. T.; Jones, Peter Tom

doi:10.1180/002646100549544

Cited by 57 publications

(58 citation statements)

References 24 publications

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“…They discussed possible mechanisms to compensate for the charge defi ciency caused by the presence of Si (up to 0.70 apfu) on the B site. Contrary to this view, Hogarth et al (2000) suggested that the presence of Si (ranging from 3.03 to 3.84 wt% SiO 2 ) in the metamict, zoned uranpyrochlore from McCloskey's Field (Quebec) was due to impurities of sub-micrometer silicate phases rather than to a Si ↔ Nb substitution.…”

Section: Occurrence Of "Silicified" Pyrochloresmentioning

confidence: 78%

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

Bonazzi

Bindi

Zoppi

et al. 2006

American Mineralogist

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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.

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Section: Occurrence Of "Silicified" Pyrochloresmentioning

confidence: 78%

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

Bonazzi

Bindi

Zoppi

et al. 2006

American Mineralogist

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“…The pyrochlore structure (Fd3 ̅ m) with the ideal formula Y is an anion-deficient derivative of the fluorite structure (AX 2 ) type with ordered cation sites [8][9][10][11]. Pyrochlore consists of corner-sharing BX 6 octahedra and A 2 Y chains with eight-fold-coordinated A cations [11][12][13][14] (see Figure 1).…”

Section: Introductionmentioning

confidence: 99%

Thermal annealing of natural, radiation-damaged pyrochlore

Zietlow

Beirau

Mihailova

et al. 2016

Zeitschrift Für Kristallographie - Crystalline Materials

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Radiation damage in minerals is caused by the α-decay of incorporated radionuclides, such as U and Th and their decay products. The effect of thermal annealing (400-1000 K) on radiation-damaged pyrochlores has been investigated by Raman scattering, X-ray powder diffraction (XRD), and combined differential scanning calorimetry/thermogravimetry (DSC/TG). The analysis of three natural radiation-damaged pyrochlore samples from Miass/Russia [6.4 wt% Th, 23.1 · 10 18 α-decay events per gram (dpg)], Panda Hill/Tanzania (1.6 wt% Th, 1.6 · 10 18 dpg), and Blue River/Canada (10.5 wt% U, 115.4 · 10 18 dpg), are compared with a crystalline reference pyrochlore from Schelingen (Germany). The type of structural recovery depends on the initial degree of radiation damage (Panda Hill 28 %, Blue River 85 % and Miass 100 % according to XRD), as the recrystallization temperature increases with increasing degree of amorphization. Raman spectra indicate reordering on the local scale during annealing-induced recrystallization. As Raman modes around 800 cm −1 are sensitive to radiation damage . The most radiation damaged pyrochlore (Miass) shows an abrupt recovery of both, its short-(Raman) and long-range order (X-ray) between 800 and 850 K, while the weakly damaged pyrochlore (Panda Hill) begins to recover at considerably lower temperatures (near 500 K), extending over a temperature range of ca. 300 K, up to 800 K (Raman). The pyrochlore from Blue River shows in its initial state an amorphous X-ray diffraction pattern superimposed by weak Bragg-maxima that indicates the existence of ordered regions in a damaged matrix. In contrast to the other studied pyrochlores, Raman spectra of the Blue River sample show the appearance of local modes above 560 K between 700 and 800 cm −1 resulting from its high content of U and Ta impurities. DSC measurements confirmed the observed structural recovery upon annealing. While the annealing-induced ordering of Panda Hill begins at a lower temperature (ca. 500 K) the recovery of the highly-damaged pyrochlore from Miass occurs at 800 K. The Blue-River pyrochlore shows a multi-step recovery which is similarly seen by XRD. Thermogravimetry showed a continuous mass loss on heating for all radiation-damaged pyrochlores (Panda Hill ca. 1 %, Blue River ca. 1.5 %, Miass ca. 2.9 %).

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“…However, major Ta enrichment is rare in carbonatites (Bagdasarov, 2009). Oscillatory zoning in the primary pyrochlore may reflect supersaturation in Nb, Ti and Th in the carbonatite melt (Hogarth et al, 2000). On the other hand, changes in the proportions of HFSE in the A and B sites, coupled with changes in the F content may be important (see Bambi et al, 2012).…”

Section: Discussionmentioning

confidence: 99%

Niobium and rare earth minerals from the Virulundo carbonatite, Namibe, Angola

et al. 2012

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The Virulundo carbonatite in Angola is one of the largest in the world and contains pyrochlore as an accessory mineral in all of the carbonatite units (calciocarbonatites, ferrocarbonatites, carbonatite breccias and trachytoids). The primary magmatic pyrochlore is fluorine dominant and typically contains about equal molar quantities of Ca and Na at the A site. High-temperature hydrothermal processes have resulted in the pseudomorphic replacement of the primary pyrochlore by a second generation of pyrochlore with less F and Na. Low-temperature hydrothermal replacement of the first and second generation pyrochlore, associated with quartz-carbonate-fluorite vein formation in the carbonatite, has produced a third generation of pyrochlore, with a high Sr content. The Sr appears to have been released by low-temperature hydrothermal replacement of the primary magmatic carbonates. Finally, supergene alteration processes have produced late-stage carbonates, goethite, hollandite and rare earth element (REE) minerals (mainly synchysite-(Ce), britholite-(Ce), britholite-(La), cerite-(Ce)). Cerium separated from the other REEs in oxidizing conditions and Ce 4+ was incorporated into a late generation of supergene pyrochlore, which is strongly enriched in Ba and strongly depleted in Ca and Na.

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Primary zoning in pyrochlore group minerals from carbonatites

Cited by 57 publications

References 24 publications

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

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

Thermal annealing of natural, radiation-damaged pyrochlore

Niobium and rare earth minerals from the Virulundo carbonatite, Namibe, Angola

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