A. A. Chepurov

Diamond crystallization in the Fe-Co-S-C and Fe-Ni-S-C systems and the role of sulfide-metal melts in the genesis of diamond

Diamond crystals 0.1–0.8 carats were synthesized in experiments conducted in a BARS split-sphere multianvil high-pressure apparatus in the systems Fe-Co-S-C and Fe-Ni-S-C at a pressure of 5.5 GPa and temperature of 1300°C. The microtextures of the samples and the phases accompanying diamond (carbides, graphite, monoslufide solid solution, pentlandite, and taenite) are examined in much detail, the properties of metal-sulfide-carbon alloys are discussed, and issues related to the genesis of sulfide inclusions in diamonds and graphite crystallization in the diamond stability field are considered. The experiments demonstrate that diamonds can be synthesized and grow in pre-eutectic metal-sulfide melts with up to 14 wt % sulfur at relatively low P-T parameters, which correspond to the probable temperatures and pressures of natural diamond-forming processes at depths of approximately 150 km in the Earth’s upper mantle.

Problem of water in the upper mantle: Antigorite breakdown

The subducting oceanic crust has a heterogeneous composition but mainly is composed of a mixture of anhydrous dolerite and gabbro with mafic green schist (albite + epidote + chlorite + actinolite) and amphib� olite [1]. An increase in pressure and temperature results in rock dehydration in the subducting oceanic crust [2]. As this takes place, dehydration proceeds successively depending on phase transitions in hydrous phases (chlorite, lawsonite, amphibole, phengite, zoisite–clinozoisite, and others). The sub� ducting oceanic crust contains >5 wt % Н2О at the ini�

Interaction of diamond with ultrafine Fe powders prepared by different procedures

We have studied the interaction of synthetic diamond crystals with ultrafine Fe powders during catalytic diamond gasification in a hydrogen atmosphere at 900°C. The Fe powders were prepared by three procedures: reduction of Fe2O3 nanopowder; evaporation using an ELV-6 electron accelerator, followed by condensation; and reduction of ferric chloride. The surface-processed diamond crystals were examined by electron microscopy. The results indicate that ultrafine powders produced by the first two procedures cause predominantly lateral etching of diamond. The Fe particles prepared by the third procedure penetrate into the bulk of diamond crystals and produce etch pits and “tunnels,” thereby markedly increasing the specific surface area of the crystals.

On the formation of element carbon during decomposition of CaCO3 at High P-T parameters under reducing conditions

This work presents data on the experimental study of CaCO3 stability (3.0–5.5 GPa; 1300–1400°C) under reducing conditions modeling the presence of metallic iron. It is established that CaCO3 is stable at the above P-T parameters under reducing conditions (in the presence of metallic Ti). CaCO3 decomposed only when it chemically interacted with iron, forming Ca-ferrites and releasing solid carbon in the form of graphite in the closed system (in sealed Pt-ampoules).

Experimental estimation of the rate of gravitation fractionating of xenocrysts in kimberlite magma at high P-T parameters

This report considers experimental studies of the gravitation fractionating of xenocrysts (diamond, garnet, and olivine) in kimberlite magma at 4.0 GPa and 1400°C. The values obtained (0.6–0.7 m/h for 0.3-mm diamond crystals, 0.36 m/h for garnet grains, and 0.6–0.29 m/h for olivine grains) point to a high rate of xenocryst sinking in the ultralow-viscous kimberlite magma (as high as 1 m/h and more, depending on the densities and grain sizes of minerals). A high rate of xenocryst sinking in kimberlite magma results in the impossibility of preservation of heterogeneity in these melts for a sufficiently long time.

Experimental and thermodynamic study of the crystallization of diamond and silicates in a metal-silicate-carbon system

Silicate inclusions are widespread in natural diamonds, which also may contain rare inclusions of native iron. This suggests that some natural diamonds crystallized in metal-silicate-carbon systems. We experimentally studied the crystallization of diamond and silicate phases from the starting composition Fe0.36Ni0.64 + silicate glass + graphite and calculated the Fe mole fractions of the silicate phases crystallizing under these conditions. The silicates synthesized together with diamond had low Fe mole fractions [Fe/(Fe + Mg + Ca)] in spite of strong Fe predominance in the system. The Fe mole fractions of the silicates decreased in the sequence garnet-pyroxene-olivine, which is consistent with the results of our thermodynamic calculations. The Fe mole fraction of silicates under various redox conditions under which metal-carbon melts are stable drastically decreases with decreasing fo2. The low Fe mole fractions of silicate inclusions in diamond from the Earth’s mantle can be explained by the highly reducing crystallization conditions, under which Fe was concentrated as a metallic phase of the magmatic melts and could be only insignificantly incorporated in the structures of silicates.

Experimental modeling of the conditions of crystallization of subcalcium chromium pyropes

1062 Subcalcium chromium pyropes with a CaO concen� tration of ~3 wt % or lower and widely variable Cr2O3 concentration of ~5 wt % and higher are observed as inclusions in diamonds of peridotitic assemblage, xeno� liths of harzburgite (dunite) and lherzolite, and as indi� vidual grains in the heavy fraction of kimberlite [1–3]. Of special interest are chromium pyropes with a CaO concentration of <2 wt %. Independently of the method of analysis recalculation, these minerals con� tain a significant admixture of the MgCr component (knorringite). The main problem in the continuing, long discussion about the conditions of the formation of these pyropes is the composition of the protolith. The authors of one of the hypotheses [4] suggested that subcalcium chromium garnets, similarly to dia� monds, observed only within cratons are accessory minerals of restitic rocks from the komatiitic process of deep melting. This hypothesis was supported by the data on the Archean Sm–Nd model age of subcalcium pyropes included in diamonds of some South African kimberlites. However, experiments performed at high pressure [5] demonstrated that subclacium chromium pyropes of the diamond assemblage [1] could not be in equilibrium with the komatiitic melt being character� ized by too high a Cr 2

High-pressure, high-temperature processing of low-nitrogen boron-doped diamond

We have studied high-pressure, high-temperature processing (7.0 GPa, 2000–2100°C) of low-nitrogen boron-doped synthetic diamonds grown in the Fe-Ni-C system (5.5–6.0 GPa, 1350–1450°C) with boron and titanium additions. The results indicate that, during the growth of low-nitrogen boron-doped diamonds, there is a competition between different acceptors (boron and nickel). The system of point defects and their distribution over the crystal are not influenced by the processing; the uniformity of coloration in natural diamonds is governed by the prevalence of octahedron growth sectors.

Study of the Surface of Natural Diamonds by the Method of Atomic Force Microscopy

Crystal morphology is widely applied for analysis of homomorphic and typomorphic signs of natural dia� monds, which is an important link in reconstruction of their genesis. Optical and scanning electron micros� copy is mainly used for the study of diamond mor� phology. Currently atomic force microscopy (AFM) is widely applied for investigation of the surfaces of vari� ous materials. This method is based on registration of interatomic forces of interaction between the surface of studied object and the probe of the microscope [1]. This allows investigation of relief of the surface and its local properties while obtaining quantitative charac�

Diamond interaction with ultradispersed particles of iron in a hydrogene environment: Surface micromorphology

1284 The morphology of natural diamonds is controlled not only by the conditions of crystal growth, but by epigenetic processes as well, which, first, influence diamond preservation and, second, provide evidence for the conditions of their occurrence in the natural environment. Because of this, the crystal morphology is an important element of reconstruction of these conditions. In this connection of special interest are so called metallic films on diamonds from different sources dis covered by A.B. Makeev and coauthors using precise methods of investigations [1, 2]. Most likely metallic films on diamonds are quite widely abundant. The thickness of the films varies from 0.1 to 1.0 μm. According to the composition, two types of films (including iron bearing) are distinguished. The authors of the above mentioned papers, with out respect to the long experience of the study of the composition and parageneses of mineral and fluid inclusions in natural diamonds discussed in numerous publications (e. g., [3–5]), consider metallic films as membranes, through which growth of diamonds pro ceeded by analogy with the known method of diamond synthesis in the metal–carbon systems at high pres sure. However, we suggest that additional investiga tions of diamond micromorphology beneath films are necessary, since they may have an epigenetic origin, as it is assumed in [6], and consequently may leave traces of interaction with the diamond surface. The very presence of native metals requires reduc ing conditions, i.e., quite low values of oxygen fugac ity. At high pressure, this is possible in the presence of hydrogen–methane fluid, which may reduce metals from the silicate and oxide phases. Hydrogen and methane do not interact with diamond [7, 8]. Interac tion with hydrogen is possible in the presence of cata lysts, among which are transitional metals, including iron group metals.