FROM STRUCTURE TOPOLOGY TO CHEMICAL COMPOSITION. V. TITANIUM SILICATES: THE CRYSTAL CHEMISTRY OF NACARENIOBSITE-(Ce)
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Bibliographic record
Abstract
The crystal structure of nacareniobsite-(Ce), ideally Na3 Ca3 REE Nb (Si2O7)2 OF3, a 7.468(2), b 5.689(1), c 18.891(4) A, β 101.37(3)°, V 786.9(1) A3, space group P 21/ c , Z = 2, D calc = 3.539 g.cm−3, from Kvanefjeld in the Ilimaussaq alkaline complex, South Greenland, has been solved by direct methods and refined to R 1 = 6.5% on the basis of 1866 unique reflections | F o ≥ 4σ F | collected on a Bruker P 4 diffractometer with a CCD 4K Smart detector and Mo K α radiation from a crystal consisting of an epitactic intergrowth of nacareniobsite-(Ce) domains. An electron-microprobe analysis gave (wt.%): SiO2 28.30, Nb2O5 12.01, TiO2 2.24, Ta2O5 0.33, Ce2O3 10.55, La2O3 4.42, Nd2O3 4.34, Pr2O3 1.12, Sm2O3 0.64, Gd2O3 0.47, Y2O3 0.57, SrO 0.45, CaO 19.79 Na2O 10.07, F 6.15, total 98.86. The empirical formula is Na2.77 (Ca3.01 Sr0.04)Σ 3.05 (Ce0.55 La0.23 Nd0.22 Pr0.06 Sm0.03 Gd0.02 Y0.04)Σ 1.15 (Nb0.77Ti4+0.24Ta0.01)Σ 1.02 (Si2O7)2 (O1.24 F0.76) Σ2.00 F2, Z = 2, calculated on a basis of 18 (O + F) apfu . Nacareniobsite-(Ce) belongs to a group of titanium disilicate minerals containing the TS block, which consists of HOH sheets (H: heteropolyhedral sheet, O: trioctahedral sheet). In the O sheet, there are two octahedrally coordinated sites: the M O(1) site is primarily occupied by Nb with minor Ti4+ and trace Ta, giving ideally 1 Nb apfu , = 2.01 A, and the M O(3) site is occupied by Na, giving 2 Na apfu , = 2.41 A. The [8]-coordinated M O(2) site is occupied by Na with subordinate Ca, = 2.50 A. The MO polyhedra each share six common edges to form a close-packed O sheet. The H sheet consists of (Si2O7) groups and MH and A P polyhedra. In the H sheet, there are two tetrahedrally coordinated sites occupied by Si with a grand distance of 1.625 A. There are two [7]-coordinated sites, MH and A P , occupied by Ca and REE, giving in total (Ca3 REE1.0), = 2.46 and = 2.49 A, respectively. An H sheet is connected to an O sheet through vertices of (SiO4) tetrahedra and MH and A P polyhedra. Within one TS block in nacareniobsite-(Ce), two (Si2O7) groups, one from each H sheet, link to the MO(2) polyhedron of the O sheet, and two H sheets are approximately related by a pseudo-mirror plane, m z. In the structure of nacareniobsite-(Ce), TS blocks repeat along [001] and are connected through common vertices and edges of (SiO4) tetrahedra and MH and A P polyhedra. Nacareniobsite-(Ce) has the same type of linkage of H and O sheets in the TS block as the Ti disilicate minerals of Group I: gotzenite, hainite, seidozerite, grenmarite, mosandrite, rinkite, kochite and rosenbuschite. In the minerals of Group I, (Ti + Nb) equals 1 apfu and occurs in the O sheet of the TS block. Nacareniobsite-(Ce) is the only disilicate mineral with the TS block in which a pentavalent cation, Nb5+, is dominant at one site in the O sheet. The topology of the crystal structure of nacareniobsite-(Ce) is identical to that of mosandrite [ideally Na2 Ca4 REE Ti (Si2O7)2 O2 F3]. Nacareniobsite-(Ce) and mosandrite differ in the cations in the O sheet of the TS block: {[8]Na [6]Na2[6]Nb5+} in nacareniobsite- (Ce) and {[8]Na [6](NaCa) [6]Ti4+} in mosandrite. In nacareniobsite-(Ce), dominance of Nb5+ at the M O(1) site results in partial substitution of Na for Ca at the [6]-coordinated M O(3) site in the O sheet.
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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 | 0.000 | 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 | 0.000 | 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) | 0.002 | 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