INSIGHTS INTO ASTROPHYLLITE-GROUP MINERALS. II. CRYSTAL CHEMISTRY
Bibliographic record
Abstract
The crystal chemistry of 20 specimens of astrophyllite-group minerals from seven localities has been studied using a combination of single-crystal X-ray analysis of the structure and electron-microprobe analyses. Members of the astrophyllite group are triclinic ( P 1) and monoclinic ( C2/c or A 2) heterophyllosilicates with the general formula A 2 BC 7 D 2 T 8O26(OH)4 X 0–1, where A = [10]–[13]K, Rb, Cs, Na, H3O+, H2O, or □; B = [10]Na or Ca; C = [6]Mn, Fe2+, Fe3+, Na, Mg, or Zn; D = [6]Ti, Nb, or Zr; T = Si and Al, and X = F, OH, O, or □. The structure consists of two composite sheets, a heterogeneous ( H ) sheet and a sheet of octahedra ( O ), stacked along [001] in a 2:1 ratio. These 2:1 composite layers are bonded via a shared apical D ϕ6 anion through an interlayer containing an ordered array of K and Na, resulting in a layered HOH structure reminiscent of the TOT structure in 2:1 phyllosilicates. There are four crystallographically distinct M O6 octahedra in the O sheet; they host Mn, Fe2+, Fe3+, Na, Mg and Zn. The M (1) octahedron, the largest and least distorted within the O sheet, is host predominantly to Mn and Fe2+, with significant Na (up to 0.274 apfu ). The M (2) octahedron is highly distorted and contains predominantly Mn and Fe2+, whereas the M (3) and M (4) octahedra are the smallest and are occupied by Mn, Fe2+, Fe3+, Mg and Zn. The H sheet consists of open-branched zweier [100] single [Si4O12]8− chains cross-linked by corner-sharing D ϕ6 octahedra in triclinic species and kupletskite- Ma 2 b 2 c , and D O5 polyhedra in magnesium astrophyllite. Distortion of the D ϕ6 octahedron is minimized by (Nb,Zr) ⇔ Ti substitution, resulting in displacement of the cation toward the center of the octahedron. The composition of the [Si4O12]8− chains does not vary; there is essentially no Al, and Mossbauer spectroscopy did not indicate the presence of [4]Fe3+. Lateral fit of the O and H sheets is accomplished primarily by: (1) flattening of O -sheet octahedra, (2) tilting of the H -sheet tetrahedra out of the (001) plane, and (3) elongation or thickening of the tetrahedra along [001].
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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 | 0.001 |
| 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".