Abstract

Schumann's method for determination of the orientation of the C-axis of uniaxial minerals using an ordinary petrographic microscope, described nearly thirty years ago, apparently has been generally overlooked in this country. It is simple, faster, and in certain cases more accurate than determinations using a universal stage.As an additional determination, since in dolomite the pole to the twin plane graphic lies at graphic to the C-axis, the orientation of the twin plane can be found if the attitude of the C-axis and the 'strike' of the twin lamella when the thin section is in a reference position are known. The method is not successful with calcite because graphic is only 26°, leading to ambiguous answers for most orientations.

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