HPHT Conditions Research Articles

Effect of fluid concentration on the formation of diamond in the CO 2–H 2O–graphite system under HP–HT conditions

By changing the amount of fluid source to that of graphite by the way of loading both materials, the effect of fluid concentration on the formation of diamond was investigated at 7.7 GPa and 1500°C for 24 h in the system of graphite and CO 2–H 2O fluid, where anhydrous oxalic acid was used as a fluid source. In the samples where powders of oxalic acid and graphite were loaded separately into a platinum-sealed capsule, diamond yield was greatly affected by the fluid concentration. Namely, when the molar ratio of fluid to graphite was about 0.1, only a small amount of graphite of less than 20% was transformed into diamond, but the diamond yield increased to about 90% when the molar ratio of fluid increased to one. When powders of oxalic acid and graphite were mixed before being loaded, the transformation reaction became accelerated remarkably. In the samples where the molar ratio of fluid to graphite was about 0.6, for example, diamond yield reached 100% when a pre-mixed starting sample was used, while only about 40% of diamond was obtained without pre-mixing.

The Ntui-Betamba high-grade gneisses: a northward extension of the Pan-African Yaoundé gneisses in Cameroon

The Ntui-Bétamba area (southern Cameroon) is composed of high-grade migmatitic gneisses in which two lithological units are distinguished: (i) a metasedimentary unit (kyanite-biotite-garnet gneisses, biotite-muscovite-garnet gneisses, calc-silicate rocks and quartzites) interpreted as a continental margin sedimentary series; and (ii) meta-igneous rocks comprising alkaline ultramafic to mafic pyroxene gneisses and amphibolites and amphibole-bearing alkaline orthogneisses. These units recrystallised under HP-HT conditions (T=750–800 °C, P≥0.9–1.3 GPa) and were deformed in relation to a major tangential tectonic event with a north-northeast-south-southwest kinematic direction. This lithological association and its tectono-metamorphic evolution show striking similarities with the Yaoundé gneisses, suggesting that the extensional depositional environment envisaged for this formation can be extended farther north, towards the Adamawa Shear Zone (Lorn series). The contrasted metamorphic evolution between areas located to the south of the Adamawa (high pressure: Yaoundé, Ntui-Betamba), and those located to the north (low pressure: Banyo), along with widespread remains of a Palaeoproterozoic crust, suggest important crustal thickening during tangential tectonics in southern Cameroon. As a consequence, the Adamawa Shear Zone is not simply a late Pan-African transcurrent or transpressive shear zone but appears to have been formerly a major (possibly intracontinental) thrust zone.

Reaction between carbon and water under diamond-stable high pressure and high temperature conditions

Reactions between graphite and water were investigated at high pressures of 5.5 and 7.7 GPa and high temperatures from 1200 to 1500°C for a duration of 24 h in a platinum sealed capsule. At temperatures above 1200°C, at both pressures, graphite was recrystallized into well-crystallized flaky crystals. At 7.7 GPa, graphite partly transformed to diamond at 1400°C and almost completely transformed at 1500°C. At 5.5 GPa, no diamond with spontaneous nucleation was formed throughout the temperatures, but growth of diamond was observed on a diamond seed crystal at temperatures above 1300°C. Fluids coexisting with solid carbon were analyzed by a mass spectrometer, and a small amount of CO2 was found to be present with H2O, showing that H2O–CO2 fluid was formed in the above HP-HT conditions by the dissolution of graphite into water.

Formation of diamond in the system of Ag 2CO 3 and graphite at high pressure and high temperatures

Diamonds were reproductively formed from graphite in the presence of Ag 2CO 3 and Ag 2O at 7.7 GPa and 1500–2000°C for 0.5–27 h. The complete conversion from graphite to diamond was confirmed in the samples obtained at temperatures above 1800°C for 0.5–2 h, but no diamond was detected in the system of Ag and graphite under these conditions. To make clear the mechanism of diamond formation from graphite–Ag 2CO 3 system, decomposition temperatures of Ag 2CO 3 and Ag 2O were investigated at 7.7 GPa and it was found that they were 1000 and 1200°C, respectively. Then, the decomposition products of Ag, CO 2 and O 2 were present instead of the carbonate in the diamond-forming condition. Since O 2 should react with graphite to form CO 2 in the HP–HT condition, our experimental results strongly suggest that fluid CO 2 has a strong catalytic action for the formation of diamond from graphite.

Growth of HPHT diamonds in alkali halides: Possible effects of oxygen contamination

Abstract We carried out experiments of diamond growth in several alkali halides under HPHT conditions of 6 GPa and 1400–1620 °C for 5 h in order to determine whether the halides are active agents in the conversion of graphite to diamond. Growth layers with an order of 1 μm in thickness were detected on diamond seed crystals embedded in the halides. The diamond growth layers were formed even in solid halides, as well as in molten halides. When NaCIO 3 was added to the halides, the growth rate increased drastically. This result indicates that oxygen which contaminates the halides affects the diamond growth, even in the case without intentionally introducing oxygen into the halides, and we propose a model of the diamond growth in the carbon-halides systems that carbon atoms are transported as oxide in the halide media and precipitate on the seed crystal.