Ankerite, a dolomite altered by isomorphous cation substitution, is the primary carbonate mineral in Mahogany Zone oil shales of the Green River Formation in Colorado and Utah. X-ray diffraction spacings determined on ten composite samples are shifted from those of unsubstituted dolomite to spacings very similar to an ankerite. Substitution of Fe++ for Mg++ in these samples was insufficient to account for the large shifts detected. Isomorphous substitution of Sr++ for Ca++, suggested as contributing to the shift in spacing, was strongly supported by correlation significant to more than 99 percent between d 104 spacing shift and the proportions of substitution of Fe++ for Mg++ and Sr++ for Ca++ in these samples.

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