Mineral inclusions in an Arkansas diamond

Six isolated and totally enclosed inclusions were recovered from an Arkansas diamond by burning it in air at 850°C. They are identified as (a) three euhedral crystals of chromian diopside, (b) a euhedral bicrystal of chromian diopside + orthopyroxene with minor unidentified included matter, (c) anhedral olivine (Fo₉₂) + a small amount of attached unidentified vitreous-like silicate rich in Si and Al with minor Fe, Ti, Zn and K, (d) finely polycrystalline periclase + minor magnetite. X-ray diffraction, chemical, and morphological data are given. The periclase may have existed in the diamond as magnesite. If so, the observed inclusions bear resemblance to equilibrium phases recently reported for silicate + carbonate reactions under mantle-like conditions. Interpretation of P–T equilibrium conditions for the diamond-inclusion system based on the silicate-carbon ate reaction and two-pyroxene geothermometer suggests 5 × 10⁴ kbars and 1300°C, but the olivine + vitreous-like phase inclusion may indicate a pressure well below 5 × 10⁴ kbars.