Describes a modification of Howell and Dawson's method of optical identification of Fe-bearing dolomites which results in a curve, rather than a straight line, to show the relationship of the angle of rotation of the microscope stage and the molecular percentage ofFe in (Mg, Fe) position. In practice, the error of about + or - 15 percent Fe over the central part of the range invalidates the results too much, and the best available method still seems to be the lengthy determination of omega on cleavage fragments.

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