Modulated structure of a plagioclase An52: Theory and structure determination

Abstract

AbstractA modulated structure of an An52(Labrador, Canada) was determined according to an extended theory of satellites. The theory explains the whole extinction rules of satellite reflections of plagioclase feldspars. The superstructure, which is interpreted as the periodic antiphase domains, consists of two centrosymmetric structure elementsϱ1andϱ2with M2T8O16(M: Ca, Na and T: Al, Si),c∼ 7 Å. The structure-factors for main and satellite reflections are expressed asFmain∝F1+F2andFsat∝F1−F2, respectively, whereF1andF2are the Fourier transforms ofϱ1andϱ2. Based on the model,ϱ1andϱ2were refined. TheR-factor attained 8% for 4910 reflections (Ra= 6.6% for 1640areflections andRe= 18.2% for 3270ereflections), including unobserved reflections. Al/Si-distributions in the region of An100–An50are discussed using the results of the refinement.