Combined Microspectroscopic Characterization of a Red-Colored Granite Rock Sample.

Combined microspectroscopic mapping have been conducted on a red-colored Tenzan granite sample by using an original visible-fluorescence-Raman microspectrometer together with a low vacuum scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) without coating. Visible darkfield reflectance spectra were converted to Lab* color values and Kubelka-Munk (KM) spectra. Large a* value (red) positions correspond to large band areas at 500-560 nm, possibly due to hematite-like iron oxide, while large b* value (yellow) positions to large band areas at 450-500 nm, due to epidote-like mineral. Scanning electron microscopy-energy dispersive spectrometry analyses indicated that the reddish parts are Na and K-feldspars with low Fe contents (<0.5 wt%). Raman microspectroscopy could not detect hematite-like minerals. Since some hematite-like minerals were only identified by transmission electron microscope, they are considered to be submicron microcrystals disseminated in feldspar matrices. The KM spectra for prehnite-like minerals show a weak broad band around 430 nm due possibly to a ligand field band of Fe without clear Fe-Fe inter-valence charge transfer (IVCT) bands around 720 nm. Therefore, Fe in prehnite is not considered to be present as hematite-like iron oxide, but can be mainly present as Fe replacing Al in the crystal structure. Since determination of physicochemical states of Fe such as valence and coordination states (Fe or Fe, oxide or in crystal lattice, etc.) and their distributions are extremely difficult, especially in complex colored materials such as rocks, the combined microspectroscopic methods are useful for their nondestructive characterization.