Mariner 10 UV and MDIS 430 nm filter bandpasses vs. the UV spectral reflectance of carbon phases. The UV spectral reflectance of diamond and lampblack powders (amorphous carbon) is shown by the blue and red dotted lines, respectively. Grain sizes of samples are listed on the figure legend in microns. * Graphite sample GRP102, labeled “amorphous” by the manufacturer, in fact exhibits crystallinity under both spectral reflectance (shown here) and X-ray diffraction (Mineral and Rock Sample Database, Planetary Spectrophotometer Lab, University of Winnipeg; E. Cloutis et al. 2006), as is the case for many industrial synthetic graphite samples, contributing to the ambiguity around amorphous carbon nomenclature (Figure 1).