Combined influences of iron-oxides and micropores on reddish coloration of alkali feldspars in granitic rocks

The international color parameters (L*, a*, and b*) of four granite alkali feldspars, determined by a new method using a scanner and personal computor system show a successive variation of reddish colors. Small reddish particles and whitish micropores were observed using a binocular microscope. Raman spectroscopy showed that the common reddish particles are hematite in all four feldspar samples, and showed maghemite in the G175 feldspar. Distributions of Fe-oxide particles and micropores were assessed by electron microprobe mapping, and their area percentages in the feldspars were obtained using image processing. The results suggest that the size distribution and number density of both hematite and micropores control the reddish color variation in the alkali feldspars, with some additional influencing factors such as the presence of maghemite.