On sub-micrometre inclusions in diamond coat: crystallography and composition of ankerites and related rhombohedral carbonates
| Autor: | A. R. Lang, J. C. Walmsley |
|---|---|
| Rok vydání: | 1992 |
| Předmět: |
0301 basic medicine
education.field_of_study Chemistry Population Space group Diamond Crystal structure engineering.material 010502 geochemistry & geophysics 01 natural sciences Apatite Grain size 03 medical and health sciences Crystallography 030104 developmental biology Geochemistry and Petrology visual_art engineering visual_art.visual_art_medium education Chemical composition Ankerite 0105 earth and related environmental sciences |
| Zdroj: | Mineralogical Magazine. 56:533-543 |
| ISSN: | 1471-8022 0026-461X |
| DOI: | 10.1180/minmag.1992.056.385.09 |
| Popis: | Crystallographic studies of micro-inclusions in diamond coat, carried out by analytical electron microscopic techniques, show that structures possessing rhombohedral symmetry form a significant fraction of the population of well-crystaUised particles. They are, however, less frequent than apatite or biotite, which were identified earlier by the same investigative methods. Thin-foil specimens of diamond coat were prepared by mechanical fine-polishing, and subsequently ion-beam milling, sawn and polished plates of coated diamonds oriented parallel to (100) or (110). The majority of crystalline inclusions analysed were ≤0.2 p,m in diameter. Data obtained on 15 individual inclusions, including composition analysis by energy-dispersive X-ray spectroscopy, are reported and discussed. The a-axis of the hexagonal unit cell, and the c/a ratio, were determined for all specimens, and are believed to be accurate to ≈1% in most cases. In 12 out of 15 specimens, the cations with Z ≥ 11 identified comprised substantially only Mg, Fe and Ca. Values of a in these specimens ranged from 4.81 to 4.92 Å; and for three of them the space group R, corresponding to the dolomite structure, was positively identified. These crystals are classed as ankerites. Representative Mg:Fe:Ca ratios in the ankerites are 28 : 18 : 54, with cell dimensions a = 4.87 Å, c/a = 3.37. The ankerites contained small amounts of Ba, typically 1-3%, and smaller amounts of Sr, typically 0.2-2%. Three out of the 15 specimens contained >10% Ba. In two of these, Ba and Ca concentrations were roughly equal, and in one of the two, which had cell parameters a = 5.11 Å, c/a = 3.50, the space group symmetry Rc, corresponding to the calcite structure, was verified. |
| Databáze: | OpenAIRE |
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