THE CRYSTAL STRUCTURES OF NIOBOPHYLLITE, KUPLETSKITE-(Cs) AND Sn-RICH ASTROPHYLLITE: REVISIONS TO THE CRYSTAL CHEMISTRY OF THE ASTROPHYLLITE-GROUP MINERALS
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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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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 | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.002 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.002 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 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 it