A new perspective of so called illite is proposed in this thesis. The illite has been introduced as a single mineral that is formed by the diagenesis of smectite a few decades ago. However, in this thesis, the illite is proposed as a mixed phase of muscovite/pyrophyllite/chlorite. In the study of illite, various approaches through EPMA quantitative analysis, observations of TEM lattice fringe images of the mixed phase suggest that the illite is not a single mineral. And the separation of illite chemical formula into the three component minerals also supports that the illite is a mixed-phase among muscovite, pyrophyllite, and chlorite.
The low content of interlayer cation (K) and the high Si in the EPMA analysis of muscovite in the rocks from the Sadong formation at Taebaek, Gobangsan formation at Munkyung, Baekunsa formation at Buyeo, and Jigunsan formation at Samchuk show the typical chemical compositions of illite. This deviations from the stoichiometric compositions of muscovite seem to be caused by the mixing of muscovite, pyrophyllite, and chlorite within a single grain.
Observations of TEM lattice fringe images of illite, reveal interlayer and intralayer mixing between muscovite (10 Å) and pyrophyllite (9.3 Å) in a single layer scale. The chemical formula of mixed-phase muscovite (illite) are separated into individual minerals of muscovite, pyrophyllite, and chlorite quantitatively using the method propose in this thesis. In this method, the contents of the minerals in the mixed-phase muscovite are calculated on the bases of K content for muscovite and Mg+Fe contents for chlorite from the chemical formula obtained by quantitative analysis, and then the rest is regarded as pyrophyllite. After this calculation, the chemical formula of muscovite is then calculated by subtracting the contents of pyrophyllite and chlorite assuming the ideal formula for pyrophyllite and the average formula analysed in the rock for chlorite. The calculated formula of muscovite, then, can be compared with an ideal muscovite as a measure of accuracy and reliability of this separation method. The result of this recalculation of muscovite composition turns out to be reasonably close to an ideal muscovite which supports that the method we used is reliable, and illite in real is a mixed-phase muscovite.
Chloritoid and staurolite are observed in shales of the Gobangsan formation from Munkyung and the Baekunsa formation from Buyeo, respectively with mixed-phase muscovite. At low temperature metamorphism, chloritoid and staurolite are formed by reactions of pyrophyllite + chlorite and pyrophyllite + chloritoid, respectively. It is inferred that the pyrophyllite and chlorite in these reactions are provided from the mixed-phase muscovite. Therefore, mixed-phase muscovite plays an important role as a supplier of necessary minerals (pyrophyllite and chlorite) to the formation of Al-rich metamorphic minerals (chloritoid and staurolite) at low temperature metamorphism.