Determination of Plagioclase Feldspars in Random Sections

using the method of Nieuwenkamp (1948)

by Christian de Capitani

This demo for plutonic (low temperature) plagioclases and albite twin law only.

Input are the readings on the rotation stage for the four positions illustrated above.

(1): trace of twin plane is in N-S direction	degrees
(2): extinction of lamella 1	degrees
(3): extinction of llamella 2	degrees
(4): both lamellae show equal illumination	degrees

W. Nieuwenkamp (1948): Stereograms for the Determination of Plagioclase Feldspars in Random Sections.
Spectrum Publishers, New York - Utrecht - Brussels, MCMXLVIII

some info on the method