The crystal structure of alstonite, BaCa(CO<sub>3</sub>)<sub>2</sub>: an extraordinary example of ‘hidden’ complex twinning in large single crystals
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Bibliographic record
Abstract
Abstract Alstonite, BaCa(CO 3 ) 2 , is a mineral described almost two centuries ago. It is widespread in Nature and forms magnificent cm-sized crystals. Notwithstanding, its crystal structure was still unknown. Here, we report the crystal-structure determination of the mineral and discuss it in relationship to other polymorphs of BaCa(CO 3 ) 2 . Alstonite is trigonal, space group P 31 m , with unit-cell parameters a = 17.4360(6), c = 6.1295(2) Å, V = 1613.80(9) Å 3 and Z = 12. The crystal structure was solved and refined to R 1 = 0.0727 on the basis of 4515 reflections with F o > 4σ( F o ) and 195 refined parameters. Alstonite is formed by the alternation, along c , of Ba-dominant and Ca-dominant layers, separated by CO 3 groups parallel to {0001}. The main take-home message is to show that not all structure determinations of minerals/compounds can be solved routinely. Some crystals, even large ones displaying excellent diffraction quality, can be twinned in complex ways, thus making their study a crystallographic challenge.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| 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)
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Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
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