David B. Slemmons, 1962. "Determination of Volcanic and Plutonic Plagioclases Using a Three- or Four-Axis Universal Stage: Revision of Turner method", Determination of Volcanic and Plutonic Plagioclases Using a Three- or Four-Axis Universal Stage, David B. Slemmons
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Procedures are outlined for measuring optical and crystallographic directions in twinned plagioclase with a universal stage having at least three axes in order to determine twinning law, anorthite percentage, and type of structure (volcanic, transitional, or plutonic).
Turner’s earlier determinative curves, based mainly on plutonic or “low-temperature” plagioclase, are replaced by new determinative curves utilizing optical data from 27 analyzed plagioclases, with measurements by Becke (5), Game (1), Grosspietsch (2), Nikitin (2), and Slemmons (17). The volcanic plagioclase curves are based on optical data from analyzed plagioclases with optical determinations by Game (1), Köhler (3), and Laves and Ghaisson (1) with additional optical data from Kano, Muir, and van der Kaaden on unanalyzed plagioclases.
The optical curves indicate that the variation in properties is a function of anorthite percentage, and volcanic or plutonic modes of origin. Most of the volcanic curves are displaced in position and somewhat different in shape from those of plutonic plagioclase, thereby generally making it possible to distinguish between these structural types and determine more accurately the composition of the plagioclase. Transitional structural types are represented on the determinative charts by points falling approximately midway between the curves for the volcanic and plutonic types.
Ambiguous results may be obtained for certain optical orientations and compositions. The determinative charts are labelled to indicate possible areas or points of ambiguity. Tests and procedures are given for avoiding or resolving these ambiguities.