Igor N. Kupriyanov

Mantle-slab interaction and redox mechanism of diamond formation.

Significance The primary question that we address in this study is what happens when a carbonate-bearing crust is subducted to depths where the Earth’s mantle is metal saturated. Subduction plays an important role in the evolution of the Earth’s interiors, but the mechanism of the interaction between the oxidized slab and reduced mantle remains unclear. Here we report the results of high-pressure redox-gradient experiments on the interaction between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle–slab boundary, and present mechanisms of diamond formation ahead of and behind the redox front. We demonstrate that the redox mechanism revealed in this study can explain the contrasting heterogeneity of natural diamonds on the composition of inclusions, carbon isotopic composition, and nitrogen impurity content. Subduction tectonics imposes an important role in the evolution of the interior of the Earth and its global carbon cycle; however, the mechanism of the mantle–slab interaction remains unclear. Here, we demonstrate the results of high-pressure redox-gradient experiments on the interactions between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle–slab boundary; thereby, we present mechanisms of diamond formation both ahead of and behind the redox front. It is determined that, at oxidized conditions, a low-temperature Ca-rich carbonate melt is generated. This melt acts as both the carbon source and crystallization medium for diamond, whereas at reduced conditions, diamond crystallizes only from the Fe-C melt. The redox mechanism revealed in this study is used to explain the contrasting heterogeneity of natural diamonds, as seen in the composition of inclusions, carbon isotopic composition, and nitrogen impurity content.

HPHT synthesis of diamond with high nitrogen content from an Fe3N–C system

Abstract The capability of iron nitride, Fe3N for converting graphite to diamond was explored at P=7 GPa and T=1550–1850 °C in experiments with a duration of 20 h. It was established that depending on the synthesis temperature the iron nitride melt provides conditions for crystallisation of diamond and/or graphite, with the minimal temperature for spontaneous diamond nucleation being approximately 1700 °C. Based on the results obtained it was argued that the iron nitride acts as the solvent-catalyst for diamond formation. The crystallised diamonds were found to contain nitrogen in concentration up to approximately 3300 ppm, which depending on the synthesis temperature was present in either the A form or both A and C forms. Absorption peaks caused by hydrogen-related defects were observed in IR spectra of all diamonds examined. For the 3107 cm−1 line a tendency to increase in intensity with increasing the nitrogen content was found. The well-known blue band-A, N3, H3 and 2.156 eV systems as well as a band with zero-phonon energy at 1.787 eV were observed in cathodoluminescence.

Germanium: a new catalyst for diamond synthesis and a new optically active impurity in diamond.

Diamond attracts considerable attention as a versatile and technologically useful material. For many demanding applications, such as recently emerged quantum optics and sensing, it is important to develop new routes for fabrication of diamond containing defects with specific optical, electronic and magnetic properties. Here we report on successful synthesis of diamond from a germanium-carbon system at conditions of 7 GPa and 1,500–1,800 °C. Both spontaneously nucleated diamond crystals and diamond growth layers on seeds were produced in experiments with reaction time up to 60 h. We found that diamonds synthesized in the Ge-C system contain a new optical centre with a ZPL system at 2.059 eV, which is assigned to germanium impurities. Photoluminescence from this centre is dominated by zero-phonon optical transitions even at room temperature. Our results have widened the family of non-metallic elemental catalysts for diamond synthesis and demonstrated the creation of germanium-related optical centres in diamond.

Effect of nitrogen impurities on the Raman line width in diamonds

The dependence of the Raman line width in diamonds on the nitrogen impurity is experimentally studied. A linear relation between the nitrogen content and the width is found. It is demonstrated that the slope of the linear fit depends on the type of nitrogen defect. A relation between the Raman experiment and results from lattice parameter experiments is found and discussed.

Melting and subsolidus phase relations in the system Na2CO3-MgCO3±H2O at 6 GPa and the stability of Na2Mg(CO3)2 in the upper mantle

Abstract Phase relations in the Na2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900-1400 °C temperatures. Sub-solidus assemblages are represented by Na2CO3+Na2Mg(CO3)2 and Na2Mg(CO3)2+MgCO3, with the transition boundary near 50 mol% MgCO3 in the system. The Na2CO3-Na2Mg(CO3)2 eutectic is established at 1200 °C and 29 mol% MgCO3. Melting of Na2CO3 occurs between 1350 and 1400 °C. We propose that Na2Mg(CO3)2 disappears between 1200 and 1250 °C via congruent melting. Magnesite remains as a liquidus phase above 1300 °C. Measurable amounts of Mg in Na2CO3 suggest an existence of MgCO3 solid-solutions in Na2CO3 at given experimental conditions. The maximum MgCO3solubility in Na-carbonate of about 9 mol% was established at 1100 and 1200 °C. The Na2CO3 and Na2Mg(CO3)2 compounds have been studied using in situ X‑ray coupled with a DIA-type multi-anvil apparatus. The studies showed that eitelite is a stable polymorph of Na2Mg(CO3)2 at least up to 6.6 GPa and 1000 °C. In contrast, natrite, γ-Na2CO3, is not stable at high pressure and is replaced by β-Na2CO3. The latter was found to be stable at pressures up to 11.7 GPa at 27 °C and up to 15.2 GPa at 1200 °C and temperatures at least up to 800 °C at 2.5 GPa and up to 1000 °C at 6.4 GPa. The X‑ray and Raman study of recovered samples showed that, under ambient conditions, β-Na2CO3 transforms back to γ-Na2CO3. Eitelite [Na2Mg(CO3)2] would be an important mineral controlling insipient melting in subducting slab and upwelling mantle. At 6 GPa, melting of the Na2Mg(CO3)2+MgCO3 assemblage can be initiated, either by heating to 1300 °C under “dry” conditions or at 900-1100 °C under hydrous conditions. Thus, the Na2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the Na- and Mg-rich carbonatite melts similar to those found as inclusions in olivines from kimberlites and the deepest known mantle rock samples-sheared peridotite xenoliths (190-230 km depth).

High-pressure synthesis and characterization of diamond from a sulfur–carbon system

Abstract Diamond crystallization in the sulfur–graphite system has been studied at P=7 GPa and T=1750–1850°C in experiments with a duration up to 7 h. It has been found that diamond nucleation and crystallization occur both at the interface between the graphite and sulfur melt and directly within the carbon–saturated sulfur melt. Diamond crystals with maximum size up to 500 μm were synthesized. The crystals had cube–octahedral morphology with minor faces of trapezohedron. Goniometric measurements revealed that crystallographic indexes of the trapezohedron faces are {411} and {944}. Spectroscopic characterization of sulfur–synthesized diamonds by means of infrared absorption microscopy and cathodoluminescence has been made for the first time. It was found that crystals contain nitrogen impurity in the form of A aggregates with concentration up to approximately 700 at. ppm. An absorption band with a maximum at 1050 cm−1, whose origin is not clear, was observed in the IR spectra. The cathodoluminescence spectra of these diamonds were found to comprise of the well-known H3 and 575-nm systems as well as a broad emission band.

Phase relations in the system FeCO3-CaCO3 at 6 GPa and 900–1700 °C and its relation to the system CaCO3-FeCO3-MgCO3

Abstract The subsolidus and melting phase relations in the CaCO3-siderite system have been studied in multianvil experiments using graphite capsules at pressure of 6 GPa and temperatures of 900-1700 °C. At low temperatures, the presence of ankerite splits the system into two partial binaries: siderite + ankerite at 900 °C and ankerite + aragonite up to 1000 °C. Extrapolated solvus curves intersect near 50 mol% just below 900 °C. At 1100 and 1200 °C, the components appear to form single-phase solid solutions with space group symmetry R3c, while CaCO3 maintains aragonite structure up to 1600 °C and 6 GPa. The FeCO3 solubility in aragonite does not exceed 1.0 and 3.5 mol% at 900-1000 and 1600 °C, respectively. An increase of FeCO3 content above the solubility limit at T > 1000 °C, leads to composition-induced phase transition in CaCO3 from aragonite, Pmcn, to calcite, R3c, structure, i.e., the presence of FeCO3 widens the calcite stability field down to the P-T conditions of sub-cratonic mantle. The siderite-CaCO3 diagram resembles a minimum type of solid solutions. The melting loop for the FeCO3-CaCO3 join extends from 1580 °C (FeCO3) to 1670 °C (CaCO3) through a liquidus minimum near 1280 ± 20 °C and 56 ± 3 mol% CaCO3. At X(Ca) = 0-30 mol%, 6 GPa and 1500-1700 °C, siderite melts and dissolves incongruently according to the reaction: siderite = liquid + fluid. The apparent temperature and X(Ca) range of siderite incongruent dissolution would be determined by the solubility of molecular CO2 in (Fe,Ca)CO3 melt. The compositions of carbonate crystals and melts from the experiments in the low-alkali carbonated eclogite (Hammouda 2003; Yaxley and Brey 2004) and peridotite (Dasgupta and Hirschmann 2007; Brey et al. 2008) systems are broadly consistent with the topology of the melting loop in the CaCO3- MgCO3-FeCO3 system at 6 GPa pressure: a Ca-rich dolomite-ankerite melt coexists with Mg-Fe-calcite in eclogites at CaO/MgO > 1 and Mg-dolomite melt coexists with magnesite in peridotites at CaO/MgO < 1. However, in fact, the compositions of near solidus peridotite-derived melts and carbonates are more magnesian than predicted from the (Ca,Mg,Fe)CO3 phase relations.

Diamond crystallization from an Mg–C system under high pressure, high temperature conditions

Diamond nucleation and growth in the magnesium–carbon system were studied at a pressure of 7 GPa and temperatures in the range of 1500–1900 °C. To explore the effects of kinetics in diamond crystallization processes the duration of experiments was varied from 5 min to 20 h. It was established that the induction period preceding diamond nucleation decreased with increasing temperature from about 17.5 h at 1500 °C to almost zero at 1900 °C, while the rate of diamond growth increased by almost three orders of magnitude, from 10 μm h−1 (1500 °C) to 8.5 mm h−1 (1900 °C). The cubic morphology was found to be the stable growth form of diamond over the entire range of temperatures used in the study. Based on the data obtained it was suggested that diamond growth in the Mg–C system took place in the kinetically controlled regime. Spectroscopic characterization revealed that the synthesized diamond crystals contained boron and silicon impurities. A specific continuous absorption, giving rise to the abundant brown coloration of the produced crystals, and a band at about 1480 cm−1 found in the Raman spectra were tentatively assigned to defects involving π-bonded carbon atoms.

Crystal Growth of Diamond

Abstract In this chapter we have presented a review of the state-of-the-art of diamond crystal growth using high pressure high temperature (HPHT) and chemical vapor deposition (CVD) approaches. We considered the main methods, techniques, and equipment used for single crystal diamond growth. Special attention is given to discussing the effects of growth conditions on the growth and properties of diamond. Current achievements in HPHT and CVD growth of single crystal diamond and its applications are considered.

Crystal growth and characterization of HPHT diamond from a phosphorus-carbon system

Crystallization of diamond in the phosphorus-graphite system has been studied at 7 GPa and 1750 °C in a series of experiments with duration from 5 to 20 h. Spontaneous diamonds crystallized through both film growth (FG) and temperature gradient growth (TGG) processes as well as diamond layers grown on seed crystals were obtained. Morphology of the crystallized diamonds was studied by scanning electron microscopy and goniometry. For TGG diamonds faces of tetrahexahedron and trapezohedron indexed as {310} and {911}, respectively, which are the new growth forms of synthetic diamond, were established. By FTIR measurement it was shown that bluish coloration characteristic of crystallized diamonds arises from a continuum of absorption, which is similar to the phosphorus photo-ionization continuum seen in P-doped CVD diamond films.