FROM STRUCTURE TOPOLOGY TO CHEMICAL COMPOSITION. V. TITANIUM SILICATES: THE CRYSTAL CHEMISTRY OF NACARENIOBSITE-(Ce)
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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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 | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 0.001 |
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".