• Journal of the Mineralogical Society of Korea
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• v.4 no.1
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• pp.32-42
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• 1991

Illite 'crystallinity' and chlorite chemistry are applied to the evaluation of the thermal grade of the Jigunsan Formation. Illite 'crystallinity' value of the formation has the range fom 4.48 to 32.5 in Weaver index (W.I.) and from $0.14{{\Delta}{\circ}}\;2{\Theta}$to $0.03{{\Delta}{\circ}}\;2{\Theta}$in kubler index (K.I). Most of illite 'crystallinity' values in this formation belong to the epizone field (K.I.<$0.21{{\Delta}{\circ}}\;2{\Theta}$). The chemistry and calculated temperature of chlorites from this formation (Fe/(Fe+Mg)=0.45, Tet. Al/Octa. Al=0.84, the calculated temperature=250-270${\circ}C$) are similar to those of epizone chlorites in the literature. The results of this work show that the metamorphic grade of the Jigunsan Formation belongs to the epizone and the formation is believed to have been reached paleotemperatures of at least 300${\circ}C$.

• The Korean Journal of Petroleum Geology
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• v.5 no.1_2 s.6
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• pp.16-26
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• 1997

The Yemi Breccia has been reported as a separate formation near Yemi area, Kangwondo. This formation overlies the Maggot Formation of the Joseon Supergroup unconformably, and is overlain by the Goseong Shale conformably. Based on the field observation and textural examination of the Yemi Breccia, the breccia beds are interpreted as soluton-collapse breccia beds, which were formed by the dissolution of the pre-existing evaporites. The evaporites were precipitated during the deposition of the upper part of the Maggot Formation. Therefore, the Yemi Breccia should not be regarded as a separate formation, instead, it should be considered to be a upper part of the Maggot Formation. This implies that the overlying Goseong Shale and Goseong Limestone can be correlated with the Jigunsan and Duwibong Formtions, respectively.

• Journal of the Mineralogical Society of Korea
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• v.25 no.3
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• pp.143-153
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• 2012

Mica-type minerals (illites) in the shales of the Jigunsan formation at Seokgaejae in Samchuk-City, Gangwon-do are studied using electron probe micro analysis (EPMA). The average chemical formula of the mica-type mineral obtained from the quantitative analysis is $(K_{1.17}Na_{0.04}Ca_{0.01})(Al_{2.80}Mg_{1.17}Fe_{0.78})(Si_{6.34}Al_{1.66})O_{20}(OH)_4$, which shows a chemical formula within the range of illite. These illites so called can be considered as mixed-phases among muscovite, pyrophyllite and chlorite due to the low contents of interlayer cations and high Mg, Fe. The formula of illite is separated into those three minerals and the method for the separation is newly formulated and proposed in this study. From the formula of illite, the content of muscovite is estimated from K (Na and Ca included), the content of chlorite by Mg+Fe, and the rest remains as pyrophyllite. The chemical formula of muscovite can be calculated by subtracting the compositions of pyrophyllite and chlorite from the analyzed composition of illite using an ideal formula for pyrophyllite and analyzed average formula for chlorite. The calculated formula of muscovite is supposed to be stoichiometric in principle. The result of the separation of analyzed illite is 61% muscovite, 27.3% chlorite and 11.7% pyrophyllite and the calculated formula of muscovite after separation is $(K,Na,Ca)_{2.00}Al_{3.69}(Si_{6.75}Al_{1.25})O_{20}(OH)_4$. The calculated formula of muscovite slightly low in Al content can be considered to be reasonable in general when the low content of Al in the rock and the uncertainties of chlorite compositions used in the calculation are counted. This supports that the method of separation proposed in this study is also applicable.