The optical orientation of the plagioclase

Forelesninger holdt ved Advanced Study Institute of Feldspar - Oslo 1962
Lectures held at Advanced Study Institute of Feldspar - Oslo 1962

Much interest attaches to a knowledge of the connections between chemical composition and optical orientation of the plagioclases. In the first place the plagioclases are one of the very few isomorphic series of triclinic symmetry about which accurate data are available. Secondly, the relations between An-content and optical orientation constitute an essential foundation for the microscopic determination of plagioclases in rocks. It is, therefore, not surprising that much attention has been devoted to this topic over the past hundred years and that it is at the present still subject of active research. It is impossible to review in this paper the interesting historical development of the insight gained into this topic or to mention the contributions made by individual scientists. Reference must be made, however, to the achievements of F. Becke and his numerous pupils. Their accurate determinations constituted for man y years practically the only reliable foundation on which to construct the diagrams for plagioclase determinations. In the twenties of the present century work on the subject seemed to be more or less concluded, the results achieved being contained in the well-known stereograms for plagioclase determinations by L. Duparc and M. Reinhard (1924) or M. Reinhard (1931). As in all previous work the plagioclases were assumed by these authors to constitute a binary system albite-anorthite. Accessory small amounts of a potash feldspar component (usually called "Or") were neglected as was a suspected content in carnegeite, efforts to detect their influence having remained unsuccessful. In this respect the situation remains practically unchanged even today.