Influence of Hematite on the Color of Red Beds

We have studied and compared the color characteristics of mixtures of synthetic hematites and kaolinite with the color of red beds from different geologic ages and locations. The color of synthetic mixtures depends on the particle size of hematites; increasing size results in more purplish hues. In contrast, the color of the red beds is not related to the size of the hematites. We hypothesize that the presence of clusters of hematite crystals having an optical behavior different from that of individual particles could be responsible for hue differences between red bed samples. This is supported by the fact that, after prolonged grinding, implying, presumably, cluster destruction, the hue of all red beds becomes yellowish red, even when the hue of the unground red beds ranges from purp ish red to yellowish red.