Carbonaceous Material in Some Metamorphic Rocks of New England and Other Areas

Carbonaceous material in regionally metamorphosed coal, black slate, graphite schist, and mica schist shows the following continuous changes with increasing metamorphic grade: increase in particle size; loss of hydrogen, nitrogen, and oxygen ; increase in the carbon content; a decrease in the peak width of the (002) reflection; and a shift of the (002) reflection to a higher 2θ position. These changes are greatest in the chlorite zone, where distinct (111) lines replace an unmodulated (111) band. In this study, only carbonaceous material from the staurolite and sillimanite zones is pure carbon. A large extent of layer ordering is acquired in the chlorite and biotite zones, but well-ordered graphite, comparable with Ceylon graphite, is limited to the sillimanite zone. There is little growth in the carbonaceous particles until the staurolite zone; in lower-grade zones, they rarely exceed 0.02 mm. The temperature at which layer ordering first appears in regionally metamorphosed rock is roughly 300°-500° (P = 3 kbar or more) ; in the laboratory it is 1,500°-2,000° (P ~ 1 bar). Well-ordered graphite probably formed at 660°-690° and 4.5-5.0 kbar in the Narragansett basin, Rhode Island. Complete graphitization of heat-treated carbons in the laboratory usually requires temperatures of 2,500°-3,600° (P ~ 1 bar). The relatively low temperatures of graphitization in metamorphic rocks, compared with the temperatures required for the process in the laboratory, might be due to the greater hydrostatic pressure, to deformation, to duration of heat treatment, to silicate minerals acting as catalysts, and to an ambient fluid phase under pressure.