Oxidation/alteration of pentlandite and pyrrhotite surfaces at pH 9.3: Part 1. Assignment of XPS spectra and chemical trends
Bibliographic record
Abstract
Using X-ray photoelectron spectroscopy, the oxidation/alteration of massive coexisting pentlandite [(Fe,Ni) 9 S] and pyrrhotite (Fe 1-x S) from Voiseys Bay has been studied in aqueous solution at pH 9.3. The Fe 2p, Ni 2p, S 2p, and O1s spectra were obtained for each mineral from polished surfaces, and from surfaces reacted with the solutions for up to 180 minutes. Both minerals oxidize rapidly to give mostly a relatively thin layer (tens of angstroms) of Fe(III) oxyhydroxide (probably FeOOH). In addition, violarite (FeNi 2 S 4 ), Ni(OH) 2 , and NiSO 4 are found in the pentlandite oxidized overlayer, and Fe(III)-S and polysulfide species are found in the overlayers of both minerals. A protocol was developed to fit quantitatively all spectra in a consistent fashion. Fe 2p 3/2 spectra were fitted with two Fe(II)-S peaks depending upon Fe coordination (four and six) in pentlandite, pyrrhotite, and violarite. An Fe(III)-S multiplet, an Fe(III) oxyhydroxide multiplet, as well as Ni Auger peaks, were also required to fit the Fe 2p spectra. The Ni 2p 3/2 spectra for pentlandite were fit with two Ni(II)-S peaks from the original pentlandite or violarite (four and six coordination), as well as Ni(OH) 2 and NiSO 4 peaks. The S 2p spectra for both minerals were fit with four and five coordinate S peaks from the original minerals and violarite, as well as polysulfides and sulfate. Finally, the O 1s spectra of both minerals were fit with peaks from oxide, hydroxide, and adsorbed water. The pyrrhotite surface oxidizes more rapidly than the pentlandite surface. For example, after five minutes of reaction the extent of oxidation of pyrrhotite to give an FeOOH overlayer is similar to that of pentlandite after thirty minutes of reaction. In both minerals, iron diffuses from the bulk to the surface and is oxidized. Pentlandite shows small amounts of Ni(OH) 2 and NiSO 4 in the Fe(lII) oxyhydroxide- dominated overlayer even after 180 minutes of reaction. These observations of the oxidation and alteration of the pentlandite surface are entirely consistent with thermodynamic calculations of the relative stabilities of pentlandite, violarite, Ni(OH) 2 , NiSO 4 , and FeOOH. The first stage of reaction is characterized by formation of violarite and FeOOH on pentlandite. With continued oxidation, this assemblage yields NiSO 4 plus FeOOH plus violarite, which is eventually converted to Ni(OH) 2 plus violarite plus FeOOH. Finally, after the violarite is consumed, the NiSO 4 plus FeOOH combination occurs as the thermodynamically stable assemblage.
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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 | 0.000 | 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".