SOLID CO2 AND CARBONATE MICRO-INCLUSIONS IN A GEM DIAMOND

Natural diamonds occasionally contain inclusions of carbonates, water, apatite, and silicates (e.g., Chrenko et al., Nature, 1973; Navon et al., Nature, 1988) which provide some glimpses into their crystallization history. Schrauder and Navon (Nature, 1993) discovered micro-inclusions of solid CO₂ in a small type IaA diamond crystal. They estimated these inclusions were under a pressure of 5 ± 0.5 GPa.

We examined a faceted 5.04 ct pear-shaped gem diamond (country of origin unknown) which appeared transparent with a slight whitish coloration. No fractures or visible inclusions were seen when it was viewed with an optical microscope (10x to 60x magnification). This diamond displays an undulating translucency throughout its entire volume. When exposing to short-wave ultraviolet radiation, a similar undulating appearance could also be seen in its weak blue fluorescence and phosphorescence.

The infrared absorption spectrum shows no nitrogen- or boron-related absorption bands – suggesting a type IIa diamond, which is confirmed by the absence of peaks in the ultraviolet – visible absorption spectrum. The infrared absorption spectrum exhibits distinct bands due to the fundamental modes of solid CO₂ at ~651 and ~2376 cm^-1. Weak bands corresponding to combination modes at 3625.3, 3753.8, 5019.5, and 5141.3 cm^-1 are also observed. Fitting the bands indicates that the solid CO₂ is under a pressure of 6.3 ± 0.4 GPa at room temperature (Hanson and Jones, J. Chem. Phys. 1981), which is higher than the value reported by Schrauder and Navon (1993). Assuming isochoric behavior, extrapolation to a mantle temperature of 1200°C results in a pressure of 9.0 – 10.0 GPa using the published equation of state (Belonoshko and Saxena, Geochim. Cosmochim. Acta, 1991), which may indicate the condition of diamond crystallization. Weak absorptions bands from micro carbonate inclusions at 1474.1 and 886.2 cm^-1 are also observed. These peaks are shifted from the room pressure positions due to high internal pressures. Raman spectroscopy shows no solid CO₂ or carbonate peaks, but reveals many photoluminescence emissions lines that cannot be assigned except for the emission at 503.5 nm from the 3H center.

Micro-inclusions of solid CO₂ and carbonate in this large gem diamond might suggest a stable localized pocket of carbonate-rich material deep within the mantle.