Compressibility of synthetic Mg-Al tourmalines to 60 GPa
Bibliographic record
Abstract
Abstract High-pressure single-crystal X-ray diffraction patterns on five synthetic Mg-Al tourmalines with near end-member compositions [dravite NaMg3Al6Si6O18(BO3)3(OH)3OH, K-dravite KMg3Al6Si6O18(BO3)3(OH)3OH, magnesio-foitite □(Mg2Al)Al6Si6O18(BO3)3(OH)3OH, oxy-uvite CaMg3Al6Si6O18(BO3)3(OH)3O, and olenite NaAl3Al6Si6O18(BO3)3O3OH, where □ represents an X-site vacancy] were collected to 60 GPa at 300 K using a diamond-anvil cell and synchrotron radiation. No phase transitions were observed for any of the investigated compositions. The refined unit-cell parameters were used to constrain third-order Birch-Murnaghan pressure-volume equation of states with the following isothermal bulk moduli (K0 in GPa) and corresponding pressure derivatives (K0′ = ∂K0/∂P)T: dravite K0 = 97(6), K0′ = 5.0(5); K-dravite K0 = 109(4), K0′ = 4.3(2); oxy-uvite K0 = 110(2), K0′ = 4.1(1); magnesio-foitite K0 = 116(2), K0′ = 3.5(1); olenite K0 = 116(6), K0′ = 4.7(4). Each tour-maline exhibits highly anisotropic behavior under compression, with the c axis 2.8–3.6 times more compressible than the a axis at ambient conditions. This anisotropy decreases strongly with increasing pressure and the c axis is onlŷ14% more compressible than the a axis near 60 GPa. The octahedral Y- and Z-sites' composition exerts a primary control on tourmaline's compressibility, whereby Al content is correlated with a decrease in the c-axis compressibility and a corresponding increase in K0 and K0′. Contrary to expectations, the identity of the X-site-occupying ion (Na, K, or Ca) does not have a demonstrable effect on tourmaline's compression curve. The presence of a fully vacant X site in magnesio-foitite results in a decrease of K0′ relative to the alkali and Ca tourmalines. The decrease in K0′ for magnesio-foitite is accounted for by an increase in compressibility along the a axis at high pressure, reflecting increased compression of tourmaline's ring structure in the presence of a vacant X site. This study highlights the utility of synthetic crystals in untangling the effect of composition on tourmaline's compression behavior.
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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 | 0.000 | 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) | 0.002 | 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".