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- Title
Natural "CO<sub>2</sub>-rich" Colored Diamonds.
- Authors
Hainschwang, Thomas; Notari, Franck; Fritsch, Emmanuel; Massi, Laurent; Breeding, Christopher M.; Rondeau, Benjamin
- Abstract
This study of apparently monocrystalline, so-called CO2-rich diamonds was performed on several hundred samples that were light to dark brown (appearing black) with various modifying colors, including "olive" (mixtures of green, brown, gray, and yellow), yellow, and almost red. The color was usually heterogeneously distributed in the form of patches or non-deformation-related color banding. Characteristic plate-like inclusions were present in nearly all samples. These appeared small, extremely thin, transparent to opaque, and rounded or hexagonal. In general, they were concentrated within certain colored sectors of the diamonds that often exhibited distinct birefringence. The FTIR spectra were characteristic for all samples, with two bands at various positions from 2406 to 2362 cm-1 and 658 to 645 cm-1. These bands have been previously interpreted as the v3 and v2 bands of CO2 due to inclusions of pressurized solid carbon dioxide (CO2) in its cubic form (Schrauder and Navon, 1993; Wang et al., 2005). Practically all samples fluoresced a distinct yellow to greenish yellow to long- and short-wave UV radiation, and they showed lasting yellow phosphorescence. The one-phonon IR absorptions varied dramatically from standard type Ia peaks to very complex bands, which in many cases were inclusion-related. In some diamonds, unknown absorptions dominating the one-, two-, and three-phonon regions were observed, and no satisfactory explanation for their presence could be given. In many samples, the bands observed in the FTIR spectra corresponded to inclusions of carbonates and silicates, notably calcite, mica, and hydrous minerals. Some of the diamonds showed a more-or-less distinct type Ib character. Our calculations of the theoretical v3 and v2 band positions at various pressures have caused us to strongly doubt the previous interpretation of the IR bands at 2406-2362 cm-l and 658-645 cm-l. In most cases, the observed absorption positions and shifts (up to 50 cm-1) did not correspond to the calculated values and appeared to be random. Furthermore, the v3 and v2 bands exhibited highly variable widths (FWHM) and intensity ratios. HPHT-treatment experiments on "CO2-rich" diamonds also brought unexpected results. A possible reason for this is that the CO2 molecules are integrated into the structure of the diamonds and that the CO2 is not present as inclusions. There are some indications that the hexagonal polymorph of diamond (lonsdaleite) could be present in these diamonds. Further analysis may confirm the identity of the hexagonal platelets as lonsdaleite inclusions, as was previously suggested by Kliya and Milyuvene (1984).
- Subjects
CRYSTALS; CARBON dioxide; DIAMONDS; ABSORPTION; CARBONATES; SILICATES
- Publication
Gems & Gemology, 2006, Vol 42, Issue 3, p97
- ISSN
0016-626X
- Publication type
Article