THE CRYSTAL STRUCTURES OF NIOBOPHYLLITE, KUPLETSKITE-(Cs) AND Sn-RICH ASTROPHYLLITE: REVISIONS TO THE CRYSTAL CHEMISTRY OF THE ASTROPHYLLITE-GROUP MINERALS
Bibliographic record
Abstract
The crystal structures of niobophyllite, K 2 Na Fe 2+ 7 (NbTi) (Si 4 O 12 ) 2 O 2 (OH) 4 O, kupletskite-(Cs), Cs 2 Na Mn 7 Ti 2 (Si 4 O 12 ) 2 O 2 (OH) 4 F, and Sn-rich astrophyllite, K 2 Na Fe 2+ 7 Ti 2 (Si 4 O 12 ) 2 O 2 (OH) 4 F, all members of the astrophyllite group, have been refined in space group P 1 to R 1 values 5.96, 3.33 and 3.53% for 3576, 3490 and 3791 observed [ F o > 4σ F ] unique reflections measured with Mo K α radiation on a Bruker AXS diffractometer with a CCD SMART APEX detector. The crystal structures of all three minerals are topologically identical to that of triclinic astrophyllite. The crystals used in the collection of the X-ray intensity data were analyzed by electron microprobe, Fe 3+ : (Fe 2+ + Fe 3+ ) ratios were measured by Mossbauer spectroscopy, and Li was determined by laser-ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS). The empirical formulae were calculated on an anion basis, (O + OH + F) = 31 apfu : niobophyllite, (K 1.79 Cs 0.03 ) ∑1.82 Na 1.01 (Fe 2+ 4.02 Mn 1.98 Fe 3+ 0.55 ) ∑6.55 (Nb 1.46 Ti 0.55 Ta 0.01 ) ∑2.02 [(Si 7.77 Al 0.20 ) ∑7.97 O 24 ] O 2.73 [(OH) 4.13 F 0.13 ] ∑4.26 ; kupletskite-(Cs), (Cs 1.42 K 0.36 Ca 0.08 Pb 0.06 Na 0.05 Sr 0.02 ) ∑1.99 (Na 0.73 Ca 0.27 ) ∑1 (Mn 3.74 Fe 2+ 1.82 Li 0.68 Zn 0.50 Fe 3+ 0.13 Mg 0.11 ) ∑6.99 (Ti 1.54 Nb 0.45 Zr 0.02 ) ∑2.01 [(Si 7.90 Al 0.05 ) ∑7.95 O 24 ] O 2.03 [(OH) 4.26 F 0.71 ] ∑4.97 ; Sn-rich astrophyllite, (K 1.70 Ca 0.04 Cs 0.03 Pb 0.01 ) ∑1.78 (Na 0.96 Ca 0.04 ) ∑1 (Fe 2+ 6.18 Mn 0.45 Fe 3+ 0.26 Ca 0.09 Zn 0.02 ) ∑7 (Ti 0.74 Sn 0.62 Nb 0.44 Zr 0.16 Ta 0.02 Mg 0.02 ) ∑2.00 [(Si 7.75 Al 0.16 ) ∑7.91 O 24 ] O 2 [(OH) 4.356 F 0.65 ] ∑5.00 ; Z = 1. In Sn-rich astrophyllite, Sn 4+ replaces Ti 4+ at the D site, and there is a corresponding expansion of the D octahedron: = 1.979 A, where φ represents O, F, OH. The discovery of Sn-rich astrophyllite indicates that there is potential for a new species in the astrophyllite group with Sn 4+ as the dominant cation at the D site. We have identified three distinct A sites, A (1), A (2) and A (3) with coordinations [13], [5] and [13], respectively, that are separated by ≤1.2 A such that locally adjacent sites cannot both be occupied. We suggest the possibility of Ca occupying the B site via the substitution B Ca 2+ + A □ → B Na + + A K + and producing a potential new mineral of the astrophyllite group. The general formula of the astrophyllite-group minerals should be written as A 2 B C 7 D 2 (T 4 O 12 ) 2 O 2 (OH) 4 X 0–1 as this (i) preserves the correct formula of the silicate radical in the structure, and (ii) identifies the O 2 anion component of the structure as not bonded to Si.
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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 | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| 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".