Lattice parameter determination of exsolution structures in labradorite feldspars

Plagioclase feldspars with chemical formula Na_0.5Ca_0.5Al_1.5Si_2.5O₈ for their bulk composition consist of lamellar structures on the 1000 Å scale at low temperatures. The lamellae have different compositions as a result of exsolution. In electron-diffraction patterns certain of the Kikuchi lines from these lamellae are doubled. These have been utilized for lattice-parameter determination by a method which is based on measurements of the differences in the orientation of the atomic planes in the lamellae. The factors determining the accuracy are discussed. For two specimens it is found that there is a difference of about 0.005 Å between the a axes and about 0.15° between the γ angles in the two lamellae.