The transformation of diamond to graphite: Experiments reveal the presence of an intermediate linear carbon phase

Natural diamonds that have been partially replaced by graphite have been observed to occur in natural rocks. While the graphite-to-diamond phase transition has been extensively studied the opposite of this (diamond to graphite) remains poorly understood. We performed high-pressure and temperature hydrous and anhydrous experiments up to 1.0 GPa and 1300 degrees C using Amplex premium virgin synthetic diamonds (20-40 mu m size) as the starting material mixed with Mg(OH)(2) as a source of H2O for the hydrous experiments. The experiments revealed that the diamond-to-graphite transformation at P = 1.0 GPa and T = 1300 degrees C was triggered by the presence of H2O and was accomplished through a three-stage process. Stage 1: diamond reacts with a supercritical H2O producing an intermediate 200-500 nm size globular carbon phase. This phase is a linear carbon chain; i.e. a polyyne or carbyne. Stage 2: the linear carbon chains are unstable and highly reactive, and they decompose by zigzagging and cross-linking to form sp(2)-hybridized structures. Stage 3: normal, disordered, and onion-like graphite is produced by the decomposition of the sp-hybridized carbon chains which are re-organized into sp(2) bonds. Our experiments show that there is no direct transformation from sp(3) C-bonds into sp(2) C-bonds. Our hydrous high-pressure and high-temperature experiments show that the diamond-to-graphite transformation requires an intermediate metastable phase of a linear hydrocarbon. This process also provides a simple mechanism for the substitution of other elements into the graphite structure (e.g. H, S, O).