Effect of early nucleation conditions on synthetic muscovite polytypism as seen by high resolution transmission electron microscopy

Abstract

Three isobaric (PH2O=1 kb) and isothermal (TN=355, 502 and 630° C) nucleation experiments of OH-muscovite have been carefully studied by powder X-ray diffraction and high resolution transmission electron microscopy. The intimate stacking sequences of layers within individual spiral-free crystallites are determined by lattice (or structure)-imaging techniques. At TN=355° C, disordered stacking involving local 1M and 2M1 order, largely dominates. A few nuclei with 1M structure coexist with disordered ones. At TN=502° C, most of micas are of more or less heavily faulted 1M structure but coexist with heavily faulted 2M1 nuclei. At TN=630° C, almost all the population is of 2M1 structure with rare stacking faults often located in the middle of the sequence. The nature of intrinsic and extrinsic stacking faults within 1M and 2M1 matrices is described. Decreasing temperature and/or increasing supersaturation are found to promote disorder in muscovite. The role of the style of packing of the two early first condensed layers (and related specific distortions of the single layer structure) on the selection of the basic structure developed through further 2D-nucleation, as suggested by Takeda and Ross (1975), is shown to be counterbalanced by environmental parameters like T and supersaturation. These high resolution transmission electron microscopy (HRTEM) observations on screw dislocation-free crystallites give experimental support to the basic assumptions of a recent faulted matrix model which has been proposed to explain the origin of complex polytypes of mica according to a subsequent spiral growth mechanism.