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

The mineralogical and petrographic work of Barth, Kano, Köhler, Muir, J. R. Smith, J. V. Smith, Tertsch, van der Kaaden and others have clearly demonstrated that there are optical differences between the structurally ordered types of plagioclase ("low-temperature", or plutonic and metamorphic), disordered types ("high-temperature", or volcanic and synthetic), and intermediate types (intermediate, "transitional", or hypabyssal). These optical differences include changes in optical orientation, which are well expressed for certain twin laws, and differences in the optic angle, 2V. These relationships can be measured quickly and easily with the universal stage, a tool available to most petrographers and mineralogists. The chief disadvantage of these methods is the lack of sensitivity of the twin-axis method for bytownite and anorthite and for certain twin laws, and the lack of sensitivity of the optic angle, 2V, in plagioclases more calcic than An₄₀.