Cerium Speciation in Durango Fluorapatite from X-Ray Photoelectron and X-Ray Absorption Spectroscopy: Implications for the Ce-in-Apatite Oxybarometer
Bibliographic record
Abstract
Abstract As a redox-sensitive rare-earth element (REE) in apatites, knowledge about the speciation of cerium (Ce) has broad applications, from understanding geochemical and geological processes in the Earth’s crust and mantle to developing sustainable REE extraction techniques. However, Ce speciation in apatites has rarely been determined experimentally but is often inferred from the presence or absence of Ce anomalies in normalized REE patterns. In this contribution, Ce speciation in gem-quality fluorapatite from Durango (Mexico) has been determined by combined X-ray photoelectron spectroscopy (XPS) and synchrotron X-ray absorption near-edge structure (XANES) spectroscopy. The Ce4+/ΣCe (where ΣCe denotes the total Ce = Ce3+ + Ce4+) values in Durango fluorapatite from Ce 3d XPS and bulk Ce L3-edge XANES techniques are similar (0.32–0.34 versus 0.27–0.30). However, microbeam Ce L3-edge XANES (µXANES) spectra measured on oriented sections parallel and perpendicular to the crystallographic c-axis yielded significantly different Ce4+/ΣCe values (0.18–0.21 versus 0.34–0.36), suggesting a marked crystal-orientation effect of the µXANES technique. On the other hand, the Ce4+/ΣCe values of 0.27–0.31 from µXANES measured on an inclined section are similar to those from the XPS and bulk XANES techniques. The mixed Ce3+ and Ce4+ states in Durango fluorapatite are consistent with its formation under oxidizing magmatic-hydrothermal conditions but contradict the general lack of Ce anomalies (CeN/CeN* = 0.86–1.01, where CeN* = Sqrt(LaN × PrN)) in chondrite-normalized REE patterns. This study shows that the lack of Ce anomalies in chondrite-normalized REE patterns of apatites does not guarantee an absence of Ce4+ and that the development of the Ce-in-apatite oxybarometer requires direct determination of Ce speciation in apatites.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 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 | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 source (direct Gemma or distilled Codex), 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".