Microstructural changes accompanying the opal‐A to opal‐CT transition: new evidence from the siliceous sinters of Geysir, Haukadalur, Iceland
Bibliographic record
Abstract
Abstract Opaline silica (opal‐A) has formed in marine, lacustrine and geothermal environments throughout geological time. During diagenesis opal‐A normally changes to opal‐CT, then opal‐C, and finally to quartz. Such changes commonly destroy the original fabrics and any fossils that opal‐A contained. The physical changes that accompany the opal‐A to opal‐CT transition, however, are known poorly. X‐ray diffraction analyses, electron microprobe analyses and high‐resolution, high‐magnification scanning electron microscope imagery of siliceous sinters from the Geysir geothermal area in Iceland show that opal‐A is formed of heterometric arrays of randomly packed microspheres (up to 5 μm diameter) with neighbouring spheres commonly being joined by small connection pads. In contrast, enlarged spheres, lepispheres, inverse opal (two types) and spindle frameworks with hexagonal motifs characterize opal‐CT. The textures in opal‐CT, which vary on a microscale, reflect the complex interplay between dissolution (e.g. inverse opal) and precipitation (e.g. enlarged spheres, spindle frameworks) that probably was mediated by groundwater in a near‐surface environment. The processes deciphered from these young rocks should, however, be applicable to sedimentary opal‐A and opal‐CT of all ages, irrespective of their origin.
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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 | 0.000 |
| 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 | 0.000 |
| 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".