Synthesis and crystal-chemistry of alkali amphiboles in the system Na2O-MgO-FeO-Fe2O3-SiO2-H2O as a function of fO2
Bibliographic record
Abstract
This paper reports the results of hydrothermal synthesis in the system Na2O-MgO-FeO-Fe2O3-SiO2- H2O. Four samples of stoichiometric magnesioriebeckite composition, ideally square 3Na(2)Mg(3)Fe(2)(3+)O(22)(OH)(2), were run at 700-800 degrees C, 0.4 GPa, and redox conditions varying from NNO (Nickel-Nickel Oxide) to NNO + 2.3 log fo(2) Powder XRD and SEM-EDX show a high (> 85%) amphibole yield for all samples; however, in no case was the end-member composition attained. EMP analyses show that the amphiboles obtained deviate strongly from nominal stoichiometry toward magnesio-arfvedsonite [NaNa2Mg4Fe3+Si8O22(OH)(2)]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reflect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mossbauer spectra show that in all samples, Fe3+ is completely ordered at M2, whereas Fe2+ occurs at the M1, M3, and M4 sites. The Fe3+/Fe2+ ratio is a function of f(o2): for increasing oxidation conditions, there is significant increase in Fe-M2(2+) and decrease in Fe2+, notably in Fe-M4(2+). Mossbauer spectra also show significant variation in Fe-M1(2+) and Fe-M3(2+) quadrupole splitting as a function of the Fe3+ content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies < 3680 cm(-1), assigned to octahedral (M1.3)(Mg, Fe2+)-OH-(A)square configurations, and a broad band consisting of four overlapping components related to (M1,3)(Mg, Fe2+) configurations associated with occupied A-sites. Quantitative evaluation of the relative band intensities suggests a linear increase of A-site occupancy with decreasing fo(2) of synthesis. The composition of the amphiboles synthesized, can be best described by a combination of the (C)(Mg,Fe2+)(1) (B)(Mg,Fe2+) Fe-C(-1)3+ Na-B(-1) and the Na-A(1) (C)(Mg,Fe2+)(1) (A)square(1) Fe-C(-1)3+ exchange vectors. The experimental trend is in accord with the trend documented for natural amphiboles, and suggests that the amphibole composition can in fact be used to monitor changes in f(o2) during crystallization.
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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 | 0.002 | 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 | 0.001 | 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 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".