The modulated structures of feldspars

The modulated structures of plagioclase feldspars have been extensively studied by the X‐ray diffraction method and high resolution electron microscopy. The results on bytownite (An73) and labradorite (An52) indicate that the modulated structure of the e‐plagioclase consist of the periodic antiphase domains of the anorthite‐like structure separated by the antiphase boundaries which form bands of albite‐like structure. Orientation and width of the antiphase domains change with composition of the e‐plagioclase. This results in the movement of the e satellites for the e‐plagioclase.The modulated structures in the e‐plagioclase are considered to appear as a possible precursor to unmixing to two stable phases of anorthite and albite in the process of ordering of substituting atoms in the high‐temperature form. The periodicity of the antiphase boundaries observed in the e‐plagioclase becomes irregular as the Na content of plagioclass becomes less than An75, resulting in the b‐type antiphase boundaries.