THE CRYSTAL STRUCTURE OF VLADIMIRITE, WITH A REVISED CHEMICAL FORMULA, Ca4(AsO4)2(AsO3OH){middle dot}4H2O
Bibliographic record
Abstract
Two samples of vladimirite, one from Bou Azzer, Morocco, and the other from a new occurrence in Copiapo, Chile (designated as R100075 and R080001, respectively), were examined with an electron microprobe, single-crystal X-ray diffraction, and Raman spectroscopy. Our results show that vladimirite is monoclinic with space group P 2 1 / c and unit-cell parameters a 5.8279(2), b 10.1802(4), c 22.8944(10) A, β 96.943(2)°, and V 1348.35(9) A 3 for R100075 and a 5.8220(1), b 10.1750(2), c 22.8816(6) A, β 96.902(1)°, and V 1345.66(5) A 3 for R080001. The structure determinations, with R 1 = 0.022 and 0.023 for R100075 and R080001, respectively, yielded an ideal chemical formula Ca 4 (AsO 4 ) 2 (AsO 3 OH)·4H 2 O (Z = 4) for this mineral, in contrast to Ca 5 (AsO 4 ) 2 (AsO 3 OH) 2 ·5H 2 O (Z = 3) documented in the literature. The chemical compositions for R100075 and R080001 are Ca 4.03 (AsO 4 ) 2 (As 0.99 O 3 OH)·4H 2 O and Ca 3.97 (AsO 4 ) 2 (As 1.01 O 3 OH)·4H 2 O, with trace Zn and Mn, respectively. The structure is characterized by undulating layers formed by the four nonequivalent, rather irregular Ca polyhedra [Ca1O 6 (H 2 O), Ca2O 6 (H 2 O), Ca3O 4 (H 2 O) 3 , and Ca4O 5 (H 2 O) 3 ] linked through the sharing of edges and vertices. These undulating layers are parallel to (010) and are interconnected by AsO 4 and AsO 3 OH tetrahedra, as well as hydrogen bonds, along the b axis. Vladimirite is remarkable inasmuch as one of the hydrogen-bonded O–H···O distances (O12H–H···O3) is only 2.465(2) A, which is the shortest donor–acceptor distance of all known Ca-bearing arsenate minerals, similar to the short donor–acceptor distances observed in several synthetic compounds containing AsO 3 OH groups.
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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 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| 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".