Thermodynamics and crystal chemistry of rhomboclase, (H<sub>5</sub>O<sub>2</sub>)Fe(SO<sub>4</sub>)<sub>2</sub>·2H<sub>2</sub>O, and the phase (H<sub>3</sub>O)Fe(SO<sub>4</sub>)<sub>2</sub>and implications for acid mine drainage
Bibliographic record
Abstract
The system Fe 2 O 3 -SO 3 -H 2 O contains the most important minerals of acid mine drainage (AMD), iron oxides, and iron sulfates. For geochemical modeling of the AMD systems, reliable thermodynamic data for these phases are needed. In this work, we have determined thermodynamic data for the most acidic sulfates rhomboclase [(H 5 O 2 )Fe(SO 4 ) 2 ⋅2H 2 O or (H 3 O)Fe(SO 4 ) 2 ⋅3H 2 O] and the phase (H 3 O)Fe(SO 4 ) 2 . The actual compositions of the studied phases are (H 3 O) 1.34 Fe(SO 4 ) 2.17 (H 2 O) 3.06 (molecular mass of 344.919 g/mol) and (H 3 O) 1.34 Fe(SO 4 ) 2.17 (289.792 g/mol). Structural details for both phases were refined from synchrotron powder X-ray diffraction data. Enthalpies of formation were determined by acid-solution calorimetry. Low-temperature heat capacity was measured for rhomboclase by relaxation calorimetry but a critical analysis of entropies for several oxysalts showed that these data are too high. Entropies for both phases were estimated from a Kopp-rule algorithm. The enthalpies of formation and entropies were combined with previously published temperature-relative humidity brackets to generate an internally consistent thermodynamic data set for rhomboclase: Δ f H ° = –3202.03 kJ/mol, S° = 378.7 J/(mol⋅K); and for (H 3 O) 1.34 Fe(SO 4 ) 2.17 : Δ f H ° = –2276.25 kJ/mol, S° = 253.2 J/(mol⋅K). Solubility experiments at room temperature and at T = 4 °C agree well with previously reported data in the system Fe 2 O 3 -SO 3 -H 2 O. An inspection of the extended Pitzer model for Fe 3+ -SO 4 solutions shows that this model reproduces the general topology of the phase diagram, but the position of the calculated solubility curves deviates substantially from the experimental data. Solid-state 2 H MAS NMR spectra on deuterated rhomboclase show two isotropic chemical shifts δ iso ( 2 H) = of 8 ± 1 and 228 ± 1 ppm, assigned to D5O2+ $\text{D}_5\text{O}^+_2$ and Fe-OD 2 groups, respectively. Canonical ensemble (NVT) molecular dynamics simulations for (H 3 O)Fe(SO 4 ) 2 at T = 300 K showed that the H 3 O + groups maintain their trigonal pyramidal geometry and perform different types of motion.
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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 | 0.001 |
| 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".