Stacking structure in disordered talc: Interpretation of its X-ray diffraction pattern by using pattern simulation and high-resolution transmission electron microscopy

Abstract

Stacking structure in disordered talc, Mg3Si4O10(OH)2, has been investigated by using highresolution transmission electron microscopy (HRTEM) and by comparison between experimental and simulated powder X-ray diffraction (XRD) patterns. The talc specimen investigated was massive aggregates of Þ ne platy crystals from the Shirakashi mine, Nagasaki-Prefecture, Japan, formed from regional metamorphism. HRTEM observations revealed that the orientation of the 2:1 layer, which is described by the direction of the lateral shift of a/3 from the lower tetrahedral sheet to the upper tetrahedral sheet within the 2:1 layer (intralayer shift), is almost completely disordered. In contrast, lateral displacement between the adjacent tetrahedral sheets across the interlayer region (interlayer displacement) is relatively ordered. The interlayer displacement parallel to the intralayer shift in lower or upper 2:1 layers tends to be avoided. A GandolÞ camera was used to record XRD patterns from a small fragment of the aggregates, to avoid preferred orientation and artifacts in stacking sequence by grinding. XRD patterns were simulated using the DIFFaX program and compared with experimental patterns. The experimental pattern from hk = 02, 11, and 1 –lattice rows is explained by a mixture of the stacking sequence with almost random directions of intralayer shift and a small amount of 1A dominant stacking sequence. The peak proÞ les for the res ections indexed with 20l, 13l, or 1 –l are reproduced by considering their