Annealing studies have been done on iron-bearing natural samples of red ochre and yellow ochre. The amounts of iron, silicon and aluminium in these samples have been determined by chemical analysis. It has been concluded that red ochre contains iron mainly in the form of alpha -Fe 2 O 3 and in alumina-type octahedra, while yellow ochre has iron chiefly in the form of alpha -FeOOH. The temperature of transformation of alpha -FeOOH to alpha -Fe 2 O 3 was found to be about 400 degrees C. Red ochre was found to exhibit superparamagnetism. Infrared and differential thermal analyses were also carried out to supplement the results obtained through the Mossbauer technique.
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Singh et al. (1978) studied this question.
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