Characteristic patterns of plagioclase twinning

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

Pattems of plagioclase twinning show characteristic differences in the following groups of rocks:
rocks in greenschistfacies: simple Gorai A- and Gorai C-twins, all with (010) as composition plane;
rocks in almandine amphibolite facies: lamellar Gorai A-twins, acline or pericline law more frequent than al bite law;
thermometamorphic rocks: lamellar Gorai A-twins, albite law more frequent than acline or pericline law;
magmatic rocks: lamellar Gm·ai A- with simple Gorai C-twins.

For a short characterization of the twinning pattem it is suggested to use not only the amount of Gorai C-twins, but also the (010) twin ratio.