New Finding of Andradite (Demantoid Variety) from the Mine of Sa Spinarbedda, Domus De Maria (Cagliari): Mineralogical Characterization and Spectroscopy Analysis
Bibliographic record
Abstract
This work reports a new occurrence of demantoid, the green variety of andradite [Ca3Fe2(SiO4)3], belonging to the garnet group, recently discovered in an abandoned mine of Sa Spinarbedda, Domus de Maria (Cagliari), Italy. \nThe well-known deposits of this appreciated gem are located in Russia, northern Madagascar, southeast Iran, Canada northern Pakistan and central Namibia (1-6). Recently, gem-quality demantoids have occasionally reported in the Muslim Bagh area of Balochistan, a large province in south-west Pakistan (7). In Italy isolated andradite crystals (demantoid variety) have been found in Val Malenco (Sondrio) and in Montjovet (Aosta) (8-9). \nBy gemmological point of view, the brilliance is considered one of the most important qualities of this gem and its name is referred to the similarities with the diamond gem.The general formula is X3Y2(SiO4)3 in which X site can be occupied by divalent cations (Ca2+, Mg2+, Fe2+, Mn2+) while the Y site usually hosts six-fold coordinated trivalent cations, as Al3+, Fe3+, Cr3+, Ti4+. In this work gemological, geochemical and mineralogical properties of two Sa Spinarbedda potential gem quality andratites are investigated to verify their belonging to the demantoid class. The samples examined by standard gemological methods were transparent, with evidence of graphite inclusions as macroscopic features. \nThey have refraction index (RI) values > 1.81, UV fluorescence in the region of 366-254 nm, and hydrostatic Specific Gravity (SG) in the range of 3.7-4.1. Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) microanalyses and X-ray powder diffraction (XRPD) techniques were carried out to determine the chemical composition as well as the structural features of garnets. Electron probe microanalyzer (EPMA) results showed that the analysed samples are chemically a nearly pure andradite (TiO2 0.035 %wt, Na2O 0.018 %wt, Al2O3 0.433 %wt), but with very low Cr contents (~ 8 ppm). This is consistent with the chemical features of the demantoid gem variety.Strong similarities with the chemical composition of \nNamibian and Madagascar demantoids are highlighted, especially concerning the skarn matrix (9). The match of the results obtained through the different techniques suggested that a new occurrence of demantoids in Italy, and Sardinia can be added to the list of demantoid deposits providing high quality gems.
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How this classification was reachedexpand
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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.000 | 0.001 |
| 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 teacher head, 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".