Calcite phases with different amounts of ferrous iron (determined by microprobe analysis) are distinguishable by study of stained thin sections and stained acetate peels. We have defined four such phases, they are (a) non-ferroan calcite (0.0% FeO; red stain), (h) ferroan calcite I (0.5% to 1.5% FeO; red-purple stai,), (e) ferroan calcite II (1.5% to 2.5% FeO; purple stain), and (d) ferroan calcite III (2.5% to 3.5% FeO; purple-blue stain). Several factors, including acidity and temperature of staining solution, affect the color (hue) of the stain, indicating that a standard procedure must be followed if colorimetric techniques are to be used in the determination of ferrous iron content in calcite.

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