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 L*a*b* 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 Fe³⁺ without clear Fe²⁺-Fe³⁺ 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 Fe³⁺ replacing Al³⁺ in the crystal structure. Since determination of physicochemical states of Fe such as valence and coordination states (Fe²⁺ or Fe³⁺, 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.