V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia

nechaev@igm.nsc.ru

Graphite is frequently found as inclusions in natural diamond crystals [1] and diamond-containing xenoliths [2]. Until now, the parameters of diamond crystallization in such associations and syngenetic graphite inclusions in diamond remain subject to discussions. The possibility of joint crystallization of diamond and graphite in the diamond stability field was experimentally proved [3]. In this connection the purpose of this study was to identify and characterize the graphite inclusions in crystals of diamond, forsterite and orthopyroxene, obtained in water-containing silicate, water-containing chloride and carbonate-silicate systems as containing or not containing of metastable graphite in the run products.

Plate-like graphite inclusions with sizes up to 30 μm were found in diamond crystals grown in systems: Mg₂SiO₄‑H₂O–C, SiO₂‑H₂O–C [4], KCl‑K₂CO₃‑H₂O–C, K₂CO₃‑H₂O–C [5], NaCl‑H₂O–C, kimberlite‑H₂C₂O₄·2H₂O‑С и CaCO₃‑MgCO₃‑SiO₂‑H₂C₂O₄·2H₂O‑С at 7.5 GPa and 1600°C for 15–40 hours. There are graphite plates in diamond layers grown near the interface with the seed and oriented mainly along {111} of the diamond host. Graphite inclusions were found in central part of spontaneous diamonds. Plate-like inclusions have hexagonal, rounded or irregular angular form. No strain or microcracks were observed around the graphite inclusions. The form of the graphite inclusions likely depends on the degree of graphite stability. When metastable graphite crystallizes during the entire period of the experiment, the entrapped graphite inclusions either have a form inherent to graphite (hexagonal plates) or have an irregular angular form, which is determined by the form of diamond growth layers. When metastable graphite is absent, the graphite inclusions in diamond are mainly of rounded shape. The occurrence of graphite inclusions of hexagonal shape near the interface with the seed, which were entrapped at the initial stages of diamond growth, indicates, however, that crystallization of metastable graphite occurred during the initial phase of diamond precipitation. As shown in [6], with increasing duration of the experiments crystallization of graphite changes into crystallization of diamond. The transition from graphite to diamond crystallization is accompanied by the dissolution of newly formed graphite. Diamond crystals growing under these conditions entrap partly dissolved rounded plates of graphite.

Quench carbon was detected only in systems in which the metastable graphite absent. It is formed pattern on the diamond faces decorating the edges and growth layers. The smallest scales of quench carbon with sizes up to 3 μm were found also in quenching aggregate.

Plate-like graphite inclusions with sizes up to 40 μm were also observed in liquidus crystals of forsterite and orthopyroxene obtained in system Mg₂SiO₄‑Fe₂SiO₄‑MgCO₃‑H₂O in the diamond stability field (6.3 GPa and 1400°C) for 40 hours. Diamond formation in the system with parameters of the experiment was not found. Thin graphite plates were present at grain boundaries forsterite and orthopyroxene. Inclusions have a different crystallographic orientation and are irregularly distributed inside the host minerals. Graphite inclusions have hexagonal or rounded form. Microcracks were observed around the graphite inclusions in orthopyroxene.

The diamond crystals, metastable graphite, quench carbon and silicate phase with graphite inclusions were analyzed using optical and scanning electron microscopes. Graphite inclusions were studied using Raman spectroscopy on a Ramanor U1000 (Jobin Yvon) multichannel micro-spectrometer with the Nd:YAG laser at a wavelength of 532 nm. It was found that metastable graphite corresponded to highly ordered graphite (HOPG). The Raman spectra exhibited only G band at 1581 cm^-1 (FWHM 22cm^-1). For the quench carbon broad D (1360 cm^-1) and G (1580 cm^-1) bands of similar intensities were found in the Raman spectra, indicating a low-ordered structure. Graphite inclusions of irregular angular or rounded shape in diamonds corresponded to highly ordered graphite with the G band at 1590‑1600 cm^-1. The shift of the G band position to higher frequencies can be accounted for by compressive strains in the inclusions, which is characteristic for most inclusions in diamond. Graphite in forsterite and orthopyroxene has a highly ordered structure. In addition to main bands of forsterite and orthopyroxene the Raman spectra have graphite G peak at 1583 cm^-1 (FWHM 14cm^-1). Using the available data on the pressure-induced shift of the G band of graphite [7], residual pressure in the inclusions was calculated to be approximately 2.3 GPa.

Thus, as a result of studies show those graphite inclusions in diamond can form not only in the thermodynamic stability field of graphite prior or after diamond crystallization, but also in the field of diamond stability as a result of joint crystallization. The presence of graphite inclusions in other mantle minerals (olivine and pyroxene) may indicate that similar mineral assemblages which do not contain diamonds were crystallized also in the diamond stability field, but at relatively low temperatures. The obtained experimental data on the formation of syngenetic graphite inclusions should be taken into consideration for interpreting genesis of graphite inclusions.

This work was supported by the Integration project of SB RAS (№ 31)

References:

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