Identification of diagenetic calcium arsenates using synchrotron-based micro X-ray diffraction
Bibliographic record
Abstract
In this work, we identify the type of calcium arsenates found in sediment samples from an aquifer located in Matehuala, San Luis Potosí, México.Sediments in contact with levels up to 158 mg/L of arsenic in neutral pH water were studied by X-ray diffraction, scanning electron microscopy coupled to energy dispersive X-ray analyses (SEM-EDS), and synchrotron based X-ray diffraction.Identification of these calcium arsenates by X-ray analysis has proved to be very difficult to achieve because the precipitates of interest are on the microscale and immerse in a matrix of calcite, gypsum, and quartz comprising nearly 100 % of the samples.Needle-like specimens composed of calcium, arsenic, and oxygen were, however, commonly observed in sediment samples during SEM-EDS analyses in backscattered mode.Synchrotron based X-ray analyses revealed some peaks that were compared with published data for guerinite, haindingerite, and pharmacolite suggesting that these were the calcium arsenates present in sediments, the calcium arsenates that control the solubility of arsenic in the contaminated aquifer in Matehuala, and the calcium arsenates that prevail in the long-term in the environment after cycles of dissolution and precipitation.The identification of these calcium arsenates is consistent with the environmental conditions prevailing at the study area and the SEM-EDS observations.However, its identification is not unequivocal as the comparison of experimental data collected in single crystal specimens against X-ray diffraction references collected in powders prevents a strictly proper identification of the specimens analyzed.In this way, scorodite was also identified by synchrotron based Xray analyses however its presence is inconsistent with the environmental conditions and the calcium arsenate associations found in this study.Scorodite identification was therefore considered tentative.A thorough examination, with additional and/or improved analytical techniques, should be undertaken to find an environmentally sound explanation for the diffraction peaks assigned to scorodite, which might be from a clay a mineral, probably with no arsenic.
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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 | 0.001 | 0.001 |
| 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 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".