Optical properties of graphite

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Abstract
SUMMARY: Reflectances of natural surfaces of fresh graphite peels parallel to the (0001) plane have been measured throughout the visible spectrum using only slightly modified microphotometric equipment and measuring procedures. There is satisfactory agreement between the level and trends of reflectances of graphites from different metamorphic environments throughout the spectrum, the maximum relative difference in the most difficult conditions in the blue region being approximately 4%. There is less good agreement between the dispersion curves of derived refractive and absorptive indices for which the maximum relative difference is as high as 12% in the red region. At 546 nm, the wavelength at which optical data in organic petrology are principally reported, relative errors below 1% for all the optical parameters were attained between samples. Values of Rair = 31.40%, Roil = 17.85%, n = 2.49 and k = 0.61 compare well with other recent estimates and although there is still not complete agreement between laboratories, the true constants for graphite at this wavelength must lie close to these values. The use of semi‐graphites as metamorphic indicators in a scale with graphite as the end‐point is considered, as are the implications on such a scale of the current use of natural surfaces for reflectance measurement, because of an inability of many laboratories to produce specularly polished surfaces of semi‐graphites and graphites of high uniform quality.