Metastable hydrocarbon formation during diamond-to- graphite transformation in presence of fluid

Minerals of native carbon (diamond and graphite) play an important role for understanding origins of deep carbon reservoirs, global carbon cycling, deep subduction of continental and oceanic slabs, oxidation states and PT-conditions of their host rocks. Though the stability fields of both graphite and diamond are well studied experimentally, the mechanisms of diamond-to-graphite and graphite-to-diamond transformation are not well constrained yet. Observations from UHPM terranes show that diamond reaction to graphite was accompanied by the formation of disordered graphite which later, during exhumation, was recrystallized to “normal” graphite. The disordered graphite is characterized by d₀₀₂ =0.346, = 0.373 and =0.447 nm interplanar and lattice fringe spacings. Because the structures of diamond and graphite are very dissimilar, it is unlikely that a direct phase transformation would occur during the graphitization process. At least three different mechanisms of diamond-to-graphite transformation were proposed (Evans, 1964; Butenko et al., 2000; Pantea et al., 2002). We conducted a series of anhydrous experiments in a piston-cylinder apparatus at P=1 GPa and T=1300^oC using synthetic diamonds. Mg(OH)₂ was added to the starting material as the H₂O supply. Run products were studied with SEM, Raman spectroscopy combined with SEM, and FIB assisted TEM. The hydrous experiments show the formation of spheroidal (20 nm) carbon particles, and tiny flakes of graphite at the {100} and {111} diamond surfaces. Experiments show that diamond-to-graphite transformation at P = 1GPa T = 1300^oC in the presence of fluid is accomplished through a multi-stage process as follows:

Stage 1: Diamond reacts with a supercritical H₂O producing an intermediate 200-500 nm size

“spheroidal carbon” phase. Stage 2: The linear carbon chains of polyyne decompose under the formation of sp²-bonded structures. Stage 3: “Normal” & “disordered” graphites are produced by the decomposition of the sp-hybridized carbon chains which are re-organized into sp2 bonds.

Our experiments showed that intermediate metastable phase (a hydrocarbon) is required for transformation from sp³ C-bonds of diamond into sp² C-bonds of graphite, which fits well to Ostwald’s Rule operating, as we see, also in a high-pressure environment.