Abstract
The models of the modulated structure of the plagioclase feldspar based on X-ray diffraction and electron microscopy have been reviewed. Important models so far presented can be divided into two categories due to difference in evaluation of the second harmonic of the (Ca, Na) modulation wave. The first type (TF***, HTKJ**** and KHNE***** models), where the second harmonic is assumed to be negligible, gives the ordered arrangement of Na and Ca among the neighbouring subcells. On the other hand, in the second type (KM** model) the second harmonic is strong enough to give the alternating bands of the albite-like and anorthite-like structures. Because of the weak intensities of the second satellite reflections corresponding to the second harmonic, the final answer between the model types has not been obtained using only the X-ray diffraction method. However, the fine textures with the antiphase domain boundaries and crystal-chemical considerations of the plagioclase feldspars support the existence of the second harmonic, or, of the alternating bands of the albite-like and anorthite-like structures.
