Elsevier

Diamond and Related Materials

Volume 10, Issue 12, December 2001, Pages 2131-2136
Diamond and Related Materials

Diamond and graphite crystallization from C–O–H fluids under high pressure and high temperature conditions

https://doi.org/10.1016/S0925-9635(01)00491-5Get rights and content

Abstract

Crystallization of diamond was studied in the CO2–C, CO2–H2O–C, H2O–C, and CH4–H2–C systems at 5.7 GPa and 1200–1420°C. Thermodynamic calculations show generation of CO2, CO2–H2O, H2O and CH4–H2 fluids in experiments with graphite and silver oxalate (Ag2C2O4), oxalic acid dihydrate (H2C2O4·2H2O), water (H2O), and anthracene (C14H10), respectively. Diamond nucleation and growth has been found in the CO2–C, CO2–H2O–C, and H2O–C systems at 1300–1420°C. At a temperature as low as 1200°C for 136 h there was spontaneous crystallization of diamond in the CO2–H2O–C system. For the CH4–H2–C system, at 1300–1420°C no diamond synthesis has been established, only insignificant growth on seeds was observed. Diamond octahedra form from the C–O–H fluids at all temperature ranges under investigation. Diamond formation from the fluids at 5.7 GPa and 1200–1420°C was accompanied with the active recrystallization of metastable graphite.

Introduction

Investigation of diamond crystallization in non-metallic systems in the presence of C–O–H fluids and from the C–O–H fluids under the diamond stable high-pressure and high-temperature conditions started approximately 10 years ago [1], [2]. Recently, experimental studies of diamond crystallization have been performed in alkaline carbonate-fluid–carbon systems [3] and directly in C–O–H fluids of variable composition [4], [5], [6], [7], [8].

Diamond nucleation (and growth) in non-metallic systems starts after a period of induction [3], [9], [10], [11], whose duration depends on P, T parameters and composition of a system. At 7.7 GPa and 2000°C diamond can be synthesized from graphite in the presence of hydroxides or fluid for several tens of minutes [1], [5]. The period of induction can reach tens or even hundred of hours at 5.7–7.7 GPa and 1150–1500°C in carbonate-fluid–carbon and fluid–carbon systems [3], [4], [5], [6], [7]. The lowest P, T parameters of diamond nucleation were established in the alkaline carbonate–CO2–H2O–C system at 5.7 GPa, 1150°C [3], from C–O–H fluids at 7.7 GPa, 1300–1500°C [5], [6], [7], and at 5.7 GPa, 1300°C [8].

The molecular composition of the fluid which catalyzes diamond crystallization under HP-HT conditions has not yet been adequately studied. The main components in equilibrium with solid carbon under the diamond stable high-pressure and high-temperature conditions are CO2, H2O and CH4 [8], [12], [13]. The H2O–CO2 fluid is stable in the high oxygen fugacity region and CH4–H2O fluid in the low oxygen fugacity region. The study of the interaction of isotopically pure graphite (13C) with C–O–H fluid showed that the fluid actively reacts with solid carbon [6]. Post-experiment gas composition revealed several compositional trends of H2O and H2O–CO2 fluids. In the course of experimental runs the H2O–C system accumulated CO2 and the H2O–CO2–C system was enriched in H2O [12], [14].

Fluid systems are of great interest because they help to study general features of diamond nucleation and growth. Moreover, these studies provide a base for development of the models for the natural medium of diamond formation [15], [16]. We analyzed the available experimental results and concluded that the main problem to be solved is the influence of fluid molecular composition on the processes of diamond nucleation and growth at pressures and temperatures lower than 7.7 GPa and 1500°C. In the present study, diamond and graphite crystallization were studied in the CO2, H2O–CO2, H2O and CH4–H2 fluids with graphite at 5.7 GPa and 1200–1420°C.

Section snippets

Experimental technique

For diamond crystallization in fluid-carbon systems, we applied the multi-anvil high-pressure apparatus of a ‘split sphere’ type [17], which was maintained at 5.7 GPa and 1200–1420°C. High pressure cells in the form of tetragonal prisms (18.4×18.4×21.8 mm) were made from materials that were stable within the given range of experimental conditions (Fig. 1). Pressure calibration was done by standard procedures based on the variation of Bi (2.55 GPa) and PbSe (4.0 and 6.8 GPa) conductivities at

Thermodynamic calculations

We estimated the fluid composition from thermodynamic calculations for the C–O–H system at 1100–1500°C and 5.7 GPa. Compositional modeling of the fluid in equilibrium with diamond (graphite) was carried out via the minimization of Gibbs free energy using ‘SELEKTOR’ code [18]. The fluid is approximated by an ideal mixture of real gasses — H2, O2, H2O, CO2, CO, CH4. Thermodynamic properties for gases were taken from the ‘SELEKTOR’ complementary database G_REID and diamond, graphite constants from

Experimental results

Table 2 shows the results of diamond and graphite crystallization from the C–O–H fluids at 5.7 GPa and 1200–1420°C.

Discussion

We used several fluid-generating substances and obtained CO2, H2O–CO2, H2O and CH4–H2 fluids, which were in equilibrium with graphite/diamond (Table 1). The intensities of diamond spontaneous nucleation and growth on seeds were close in all fluids, except for the methane one. As the temperature decreases from 1420 to 1200°C, the size of spontaneous crystals and the thickness of overgrown layers on seeds decrease as well. At a temperature as low as 1200°C for 136 h, there was spontaneous

Conclusions

Joint crystallization of diamond and metastable graphite was observed in all compositionally variable fluids at 5.7 GPa and 1200–1420°C. As the temperature decreases from 1420 to 1200°C, the intensity of diamond nucleation and growth decreases as well. At 1200°C and 136 h, diamond nucleation and growth was established only in the CO2–H2O–C system.

Catalytic ability of CO2 and H2O fluids is essentially higher than that of oxygen-free, i.e. CH4–H2, fluids. In the C–O–H fluid there was spontaneous

Acknowledgements

The authors are grateful to Prof. N.V. Sobolev for fruitful discussions. The project was financially supported by the Russian Foundation for Basic Research (N 00-05-65462).

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