Conversion of carbonaceous material to graphite during metamorphism

Using high-resolution transmission electron microscopy (HRTEM), we have charted some of the structural changes that occur as non-crystalline organic matter in low-grade metamorphic rocks becomes ordered, eventually to form well-crystallized graphite. It has long been known from X-ray studies that the state of crystallization of carbonaceous matter increases with metamorphic grade. Images obtained by HRTEM of samples from a range of metamorphic terranes provide considerable additional detail regarding the process of graphite development. The process is considerably more complex than is suggested by light-optical microscopy and powder X-ray diffraction measurements. In low-grade metamorphic rocks, the carbon layers are relatively few in number, short in length, and rather contorted. Electron-diffraction patterns show only one or two rings, and they are diffuse. As metamorphic grade increases, the carbon layers progressively increase in length and in number, the number of layers in the crystallite stacks increases, the planarity of the layers increases, and the number of layer terminations decreases. Electron-diffraction patterns show complementary increasing numbers of rings, decreasing diffuseness, and, for well-crystallized graphite, discrete spots. Well-crystallized graphite crystals appear in the chlorite zone but, in contrast to most X-ray measurements, a range of structural order is commonly observed among the various crystallites within a given rock sample. These presumably reflect the chemical and structural character of the carbonaceous precursors, possibly their sedimentary sources, as well as the degree of internal equilibration. The results of our electron microscope observations also have implications for interpreting measurements of carbon-isotope fractionations and the apparent lack of equilibrium reported in many such measurements.