Intergrowth and Interlayering of Muscovite, Chlorite, and Biotite in a Garnet Zone Metamorphic Rock of the Ogcheon Belt, South Korea

Muscovite, chlorite and biotite in metapelites of the Ogcheon Hetamorphic Belt are studied using electron probe microanalysis (EPMA), backscattered electron images (BEI) of scanning electron microscopy (SEM) and lattice fringe images of transmission electron microscopy (TEM). These minerals are observed to be intergrown under a polarized light microscope and are apparently interlayered below EPMA resolution; EPMA data often indicate mixtures of phyllosilicates such as muscovite/chlorite (M/C), biotite/chlorite (B/C), muscovite/pyrophyllite/chlorite (M/P/C). biotite/pyrophyllite/chlorite (B/P/C) or biotite/muscovite/chlorite (B/M/C). BEI observations show that the three minerals (muscovite, chlorite and biotite) are mixed at various scales in a grain through the garnet zone, and the interlayering of the three minerals are observed from TEM lattice fringe images and selected area electron diffraction patterns. The result of TEM observations reveals that 7- layers (serpentine, precursor of chlorite) are interlayered within 10- layers (muscovite) at 100~200 scale as well as M/C in the chlorite zone. The 7- layers become smaller in size and less frequent in the biotite tone, and 10- layers are interlayered with chlorite (14 ) at an individual layer scale. The 7- layers are no longer observed in the garnet zone, and 10- layers (biotite) are interlayered with chlorite (B/C) at 50~100 scale. Relatively large scale (1000~2000 ) of intergrowth is also frequently observed from the garnet zone samples. However, rocks from all three metamorphic zones show interlayering of a few units of 7-, 10- and 14- layers with each other at TEM observations. The result of this study implies that metamorphic minerals such as muscovite, chlorite and biotite form through disequilibrum mineral reactions resulting in inhomogenious phases.