Graphite pseudomorphs after diamonds: An experimental study of graphite morphology and the role of H2O in the graphitisation process

Experiments at 1 atmosphere and 2.0-2.5 GPa over a range of temperatures of 1400-2100 degrees C have been carried out to investigate the diamond-to-graphite transformation in dry and wet systems. Internal and external morphologies of graphite pseudomorphs after diamonds were studied by Raman spectroscopy, reflected light microscopy and scanning electron microscopy. In a dry system, the results show that at 2.0-2.5 GPa, graphite pseudomorphs preserve the finest details of external morphology of original diamond crystals, whereas at 1 atmosphere only the general outline of diamond crystals can be recognised on these pseudomorphs. In all experimental runs at P = 2.0-2.5 GPa under various temperatures, the growth of oriented graphite crystallites was observed only on the {111} diamond faces, while randomly oriented graphite crystals were observed on the {100} and especially (110) diamond faces. In a wet system, we were unable to reproduce graphite pseudomorphs after diamond. However, newly formed large graphite crystals were found on the partly dissolved diamonds. The diamond crystal form has been changed from that of a sharp octahedron to octahedron having rounded edges and corners. Flat-bottomed negatively oriented trigons were formed on the octahedral {111} faces. Large graphite crystals tend to be concentrated at {100} and (110) surfaces. Rare single euhedral graphite flakes occur on the {111} faces. Visible growth spirals on {001} faces of graphite crystals appear on all crystals. The {111} faces of diamond crystal are partly covered by translucent graphite coat The diamond-to-graphite transformation in the wet system occurs via coupled dissolution-precipitation processes. All morphological features (e.g., negatively oriented trigons, rounded edges) described for partly dissolved diamonds are easily recognised. None of these features have been detected so far for partly graphitised metamorphic diamond crystals. Our results suggest that in geological environments, the presence of volatiles (in particular, H2O) definitely cannot be considered as favourable for the diamond-to-graphite transformation and, thus, show that this hypothesis seems highly unlikely. (C) 2015 Elsevier B.V. All rights reserved.