Fe SPECIATION IN WEATHERED PYROCHLORE-GROUP MINERALS FROM THE LUESHE AND ARAXA (BARREIRO) CARBONATITES BY 57Fe MOSSBAUER SPECTROSCOPY
Bibliographic record
Abstract
The minerals of the pyrochlore group exhibit a large range of chemical composition. The general structural formula can be written as A2–mB2X6–wY1–n•pH2O. The occupancy of the eight-fold-coordinated A site can reflect the degree of alteration of the pyrochlore. In unaltered pyrochlore-group minerals, this site is completely filled by Na and Ca, whereas in the altered equiva-lents, the A site is only partly filled by cations as K, Ba, Sr and Ce. In the literature, the structural formula is systematically calculated assuming an ideal stoichiometry, i.e., a total of two six-fold-coordinated B-site cations per formula unit, dominantly Nb, with subordinate Ti and Ta in certain rock-types, e.g., carbonatites. Fe is systematically considered as Fe3+ and assigned to the B site. The present 57Fe Mössbauer study focuses on altered pyrochlore-group minerals from two Nb ore deposits in laterite: Lueshe in the Democratic Republic of Congo, and Araxá in Brazil. In contrast to the common assumption that all Fe is present as Fe3+, 47 ± 2 % of the total Fe is in the Fe2+ oxidation state in the kalipyrochlore sample from Lueshe. Spectra of both samples are consistent with the presence of all the Fe3+ at the B site, but with two different coordination spheres. Fe3+ coordinated by 6 O2 – is observed in both cases. In the Lueshe sample, it is also coordinated by 5 O2 – and 1 OH–, whereas in bariopyrochlore from Araxá, the unusually large value of the quadrupole splitting, 3 mm/s, affecting ~19 ± 2 % of the Fe3+, suggests that this Fe3+ is coordinated by only 5 O2 – owing to the presence of anion vacancies at the X site.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".