SINGLE CRYSTAL XRD ANALYSES OF INCOMMENSURATELY MODULATED STRUCTURE OF LABRADORITE

Plagioclase feldspars [Na_1-xCa_x(Si_3-xAl_1+xO₈)] that commonly occur in igneous and metamorphic rocks are the most abundant minerals in the earth’s crust. Although crystal structures for end members of albite and anorthite are relatively simple, the structures for intermediate plagioclase feldspars with incommensurate modulations are very complicated at low temperature, and are not simple mixtures of albite and anorthite subunits. The crystal structures and formation mechanisms of the incommensurately modulated structure in intermediate plagioclase, or e-plagioclase (from ~An₂₅ to ~An₇₅) have been an enigma for decades beginning with their first discovery in 1940. Very different structure models were even proposed based on exactly the same set of experimental data.

Single crystal X-ray diffraction data were collected on two labradorite samples formed under different cooling conditions. Two samples has similar composition of about An55, and both of them showed sharp satellite diffractions (so called e-diffraction) around the absent b reflections (h+k=odd; l=odd). The labradorite in a troctolite from Duluth complex, Duluth, Minnesota, is homogeneous small crystal with no exsolution lamellae. XRD result shows only very weak second order satellite diffractions (f-reflections) around main a reflections (h+k=even; l=even). The second crystal from Nain, Labrador, Canada shows red iridescent color generated by nearly periodic exsolution lamellae. TEM images show similar modulation directions and periodicities in Ca-rich and Ca-less lamellae. Sharp and relatively stronger second order satellite diffractions were detected in this sample. Both structures were solved and refined with JANA2006 software, as the satellite diffractions are treated as the projection of reflections from higher dimension. The sub cells in the result structure do not have inversion center (i.e., I1 symmetry), corresponding to the direct observation by Xu (2015). The displacement modulation appears similar in two samples, but evident density modulation occurs only in the Labrador sample with extremely slow cooling.