Are nanotubes and carbon nanostructures the precursors of coexisting graphite and micro-diamonds in UHP rocks?

• Carbon nanosized particles have been found in garnet from diamond-grade rocks.
• They include nanotubes and graphite + diamond polyhedral and spherical nanoparticles.
• They are interpreted as formed through an exsolution process.
• Diamond formed in the cores of the polyhedral and spherical particles
• Nanosized particles are interpreted as the precursor of microsized graphite and diamond.

A transmission electron microscopy study of garnet from diamond-grade gneisses of the Betic Cordillera (Spain) has revealed the presence of abundant, previously unrecognized, nanosized carbonaceous grains, coexisting with micrometer-sized graphite and diamond. The nanosized particles occur as multiwall nanotubes, and as polyhedral and quasi-spherical graphite + diamond nanoparticles, whereas larger graphite particles appear as rods and as tabular crystals. The topotactic relationships between graphite in nanoparticles and in micrometer-sized particles and the host garnet suggest that carbon nano- and microparticles precipitated from an originally homogeneous solid solution of carbon in the garnet. Based on orientation relationships and on experimental data it is suggested that the three main types of nanosized particles (nanospheres, polyhedral particles and nanotubes) were the precursor of the three main types of larger carbon phases (diamond, tabular and rod-shaped graphite particles, respectively). It is interpreted, as in the case of diamond-graphite nanocomposites, that diamond formation in the core of the nanoparticles is due to an increase of the cross-links between the layers, and then, to the collapse, at a certain point, of the whole graphite structure into diamond. This finding opens a new door for explaining the origin of some metamorphic diamonds and of coexisting graphite and diamond in ultrahigh pressure rocks.

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