THE CRYSTAL STRUCTURE OF CERVANDONITE-(Ce), AN INTERESTING EXAMPLE OF As3+ -> Si DIADOCHY
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Bibliographic record
Abstract
Cervandonite-(Ce), a unique arsenosilicate from Alpine fissures at Pizzo Cervandone (Scherbadung) in the Central Alps between the Binn Valley (CH) and Ossola (I), has proven to be a problem in view of the extreme scarcity of single crystals suitable for the determination of its complex structure. The monoclinic supercell (Z = 6) originally described by the discoverers is confirmed; we have established the crystal structure of a trigonal subcell, with space group R 3 m and a 6.508(1), c 18.520(3) A, V 679.4(2) A 3 and Z = 3, to a final R(F) = 0.0380 for 441 observed independent reflections [ I > 2σ ( I )]. The mechanism of the very unusual As 3+ → Si (not As 5+ → Si) diadochy is accounted for by considering the presence in two different sites of sorosilicate anions Si 2 O 7 6− , which are statistically replaced by arsenite [AsO 3 3− ] anions and minor amounts of OH − ; the positions of the oxygen atoms common to these anions are the same. The asymmetric unit also contains one eight-coordinated atomic site occupied by Ce and other REE lying on the three-fold axis; a bipyramidal six-coordinated site (of symmetry .m ) occupied by Fe, Ti, or Al also is present. The originally proposed chemical formula (Ce,Nd,La)(Fe 3+ ,Fe 2+ ,Ti 4+ ,Al) 3 SiAs(Si,As)O 13 is accordingly revised to (Ce,Nd,La)(Fe 3+ ,Fe 2+ ,Ti 4+ ,Al) 3 O 2 (Si 2 O 7 ) 1− x + y (AsO 3 ) 1+ x − y (OH) 3 x −3 y , where x and y are 0.47 and 0.31, respectively, in the sample here examined.
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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 | 0.000 | 0.000 |
| 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 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
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Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
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