Fe SPECIATION IN WEATHERED PYROCHLORE-GROUP MINERALS FROM THE LUESHE AND ARAXA (BARREIRO) CARBONATITES BY 57Fe MOSSBAUER SPECTROSCOPY

Nasraoui, Mohamed; Waerenborgh, J.C.

doi:10.2113/gscanmin.39.4.1073

Cited by 18 publications

(11 citation statements)

References 21 publications

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“…At n = 0, A is an eightfold-coordinated cation (mostly Ca and Na, with K, Sr, Ba, Sn 2+ , Pb 2+ , REE 3+ , Y, Sb 3+ , Bi 3+ , U 4+ , and Th 4+ occurring less commonly or in lesser amounts) whereas B is an octahedrally coordinated cation, namely Nb, Ta, and Ti 4+ (or Sb 5+ in the stibiconite mineral group) with minor quantities of Fe 3+ , Sn 4+ , and W 6+ . The X position is mostly occupied by O 2-or, to a lesser extent, OH - (Ercit et al 1993(Ercit et al , 1994Nasraoui and Waerenborgh 2001), whereas Y may be O 2-, OH -, or F -. Pyatenko (1959) was the fi rst to recognize that the pyrochlore structure is a derivative of the fl uorite structure.…”

Section: Introductionmentioning

confidence: 99%

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: Introductionmentioning

confidence: 99%

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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“…Weathered pyrochlore supergroup minerals are commonly hydrated and deficient in A-site cations (Nasraoui & Waerenborgh 2001). Only the B-site lacks deficiencies (Atencio et al 2010).…”

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confidence: 99%

Pyrochlore and columbite-tantalite as indicator minerals for specialty metal deposits

Mackay

Simandl

2015

GEEA

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Pyrochlore supergroup and columbite-tantalite series minerals can be used as indicators for specialty metal deposits. Their mineral chemistry can be used to target specific specialty metal deposit-types and provide a vector towards mineralisation. A ternary Nb-Ti-Ta discrimination diagram for the pyrochlore supergroup shows that minerals from carbonatites and related deposits occupy the pyrochlore field and extend slightly into the betafite and microlite fields (>35% Nb, <65% Ti, and <45% Ta). Those from peralkaline complexes define a narrow zone paralleling the Nb-Ti join (>40% Nb, <60% Ti, and <10% Ta). Pyrochlore supergroup minerals from Li-Cs-Ta enriched (LCT) pegmatites occupy the microlite field (<90% Nb, ≤40% Ti, and > 15% Ta) whereas those from peraluminous granites plot in the microlite field along the Nb-Ta join, extending slightly into the pyrochlore field (<65% Nb, <10% Ti, and >35% Ta) on the Nb-Ti-Ta ternary diagram. The discrimination diagram for pyrochlore supergroup minerals is based on 349 new electron microprobe analyses from British Columbia and Québec, Canada, combined with 879 published analyses from 50 deposits worldwide. A columbite-tantalite series discrimination quadrilateral indicates that minerals from carbonatites and related deposits are Nb-rich, with Mn/(Mn+Fe) and Ta/(Ta+Nb) atomic ratios ≤ 0.25 and ≤ 0.20, respectively. Columbite from peralkaline complexes plot along the FeNb 2 O 6 –MnNb 2 O 6 join of the quadrilateral and have Mn/(Mn+Fe) ratios of ≤ 0.65 and Ta/(Ta+Nb) ratios of ≤ 0.15. Fe-Nb-rich varieties within this zone slightly overlap with those from carbonatites. Columbite-tantalite series minerals from Li-Cs-Ta (LCT) pegmatites and peraluminous granites are not well constrained but do slightly overlap with columbite-(Fe) from carbonatites. A minor overlap exists between the fields from carbonatites, peralkaline complexes, LCT pegmatites, and peraluminous granites near the columbite-(Fe) corner of the quadrilateral. The discrimination diagram for columbite-tantalite series minerals is based on 153 new electron microprobe analyses from British Columbia combined with over 2500 analyses complied from the literature. Combining the two discrimination diagrams presented here enhances their effectiveness to explore for specialty metal deposits.

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“…Calculation of other components such as O and OH was done using charge balance and stoichiometry. Nasraoui and Waerenborgh (2001) demonstrated that the oxidation state of Fe can change in secondary pyrochlores and that some Fe 2+ can be present in the B position along with Fe 3+ . However, the amounts of total Fe are very low in all of the secondary pyrochlores, therefore, the iron oxidation state does not have a significant influence on the charge valence.…”

Section: Methodsmentioning

confidence: 99%

Subsolidus processes as a key factor on the distribution of Nb species in plutonic carbonatites: The Tchivira case, Angola

Melgarejo

Costanzo

Bambi

et al. 2012

Lithos

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Fe SPECIATION IN WEATHERED PYROCHLORE-GROUP MINERALS FROM THE LUESHE AND ARAXA (BARREIRO) CARBONATITES BY 57Fe MOSSBAUER SPECTROSCOPY

Cited by 18 publications

References 21 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

Pyrochlore and columbite-tantalite as indicator minerals for specialty metal deposits

Subsolidus processes as a key factor on the distribution of Nb species in plutonic carbonatites: The Tchivira case, Angola

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