Defect contributions to the heat capacities and stabilities of some chain, ring, and sheet silicates, with implications for mantle minerals
Bibliographic record
Abstract
At temperatures less than ~1500 K, previously published C P data demonstrate that the heat capacities of orthoenstatite, proto-enstatite, diopside, and pseudowollastonite include primarily Debye type vibrational and anharmonic contributions, whereas the alkali chain, sheet, and ring silicates, Na 2 SiO 3 , Li 2 SiO 3 , K 2 SiO 3 , and Na 2 Si 2 O 5 include a third contribution. The third contribution to C P arises from defect formation due to the mobility Na, K, Li, and O 2− . The contribution becomes apparent at temperatures above 700–800 K for Na and K silicates, and above 900–1000 K for Li metasilicate. With strong thermal agitation, alkali-non-bridging oxygen (NBO) bonds are ruptured with the cations exiting their structural sites to occupy interstitial sites, thereby producing intrinsic Frenkel defects, which contribute to the C P of the alkali silicates. The magnitudes of the C P defect contributions correlate inversely with cation-oxygen bond strengths, as measured by bond dissociation energies. K-O and Na-O bond strengths are weak (239 and 257 kJ/mol) and defect contributions are large for these alkali chain, ring, and sheet silicates. The greater bond strength of Li-O (341 kJ/mol) correlates with a weaker defect contribution to the C P of Li 2 SiO 3 . Mg-O and Ca-O bonds are stronger still (394 and 464 kJ/mol) and no C P defect contributions are observed for the pyroxenes and pseudowollastonite up to ~1500 K. Above ~800 K a polymerization reaction occurs in Na 2 SiO 3 , which produces some Q 3 species and free oxygen (O 2− or oxide ion). The polymerization reaction annihilates an oxygen structural site so that the O 2− produced must reside on non-structural sites thus producing intrinsic anionic defects. The same reactions likely occur in Na 2 Si 2 O 5 and K 2 SiO 3 . Raman spectra of Na 2 SiO 3 indicate >10% of Na + and ~1.7% of O 2− on interstitial sites at 1348 K. Ca- and Mg-bearing mantle minerals subjected to temperature greater than ~1500 K experience the destabilizing effects of disordering (Frenkel defect formation). The minerals may respond either by changing their composition or by changing phase. An abundance of Ca and Na defects in pyroxenes, for example, likely promotes production of new components (e.g., CaAl 2 SiO 6 , NaAlSi 2 O 6 ) in pyroxenes. By their production, Ca and Na defect concentrations are reduced thereby stabilizing the phases. Mg-O bond dissociation and production of intrinsic Mg 2+ and O 2− point defects within olivine likely destabilize it and promote the phase transition to wadsleyite at the base of the upper mantle.
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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 | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.001 |
| 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".