Mineralogy and Genesis of Bentonites from the Tertiary Formations in Geumgwangdong Area, Korea

제(第)3기층(紀層)에 부존(賦存)하는 점토광물(粘土鑛物)에 대(對)한 광물학적(鑛物學的) 및 성인적(成因的) 연구(硏究)

  • Kim, Soo Jin (Department of Geological Sciences, Seoul National University) ;
  • Noh, Jin Hwan (Department of Geology, Kangweon National University) ;
  • Yu, Jae Young (Department of Geological Sciences, Seoul National University)
  • 김수진 (서울대학교 자연과학대학 지질과학과) ;
  • 노진환 (강원대학교 자연과학대학 지질학과) ;
  • 유재영 (서울대학교 자연과학대학 지질과학과)
  • Published : 1985.08.31


Bentonites from the Janggi Group of the Lower Miocene age from the Geumgwangdong area, Korea, have been studied for mineralogical and genetic characterization. The Janggi Group is subdivided, in ascending order, into the Janggi Conglomerate, the Nuldaeri Tuff, the Geumgwangdong Shale, the Lower Coal-bearing Formation, the Basaltic Tuff, and the Upper Coalbearing Formation. Bentonites occur as thin or thick beds in all sedimentary units of the Janggi Group, except for the Janggi Conglomerate. Significant bentonite deposits are found in the Nuldaeri Tuff, the Lower Coal-bearing Formation and the Basaltic Tuff. Bentonites consist mainly of smectite (mainly montmorillonite), with minor quartz, cristobalite, opal-CT and feldspar. Occasionally, kaolinite, clinoptilolite or gypsum is associated with bentonites. Bentonites were studied by the methods of petrographic microscopy, X-ray diffraction, thermal analysis (DT A and TG), infrared absorption spectroscopic analysis, SEM, intercalation reaction, and chemical analysis. Smectites commonly occur as irregular boxwork-like masses with characteristic curled thin edges, but occasionally as smoothly curved to nearly flat thin flakes. Most of smectites have layer charge of 0.25-0.42, indicating typical montmorillonite. Crystal-chemical relations suggest that Fe is the dominant substituent for Al in the octahedral layer and there are generally no significant substituents for Si in the tetrahedral layer. Ca is the dominant interlayer cation in montmorillonite. Therefore, montmorillonite from the study area is dioctahedral Ca-montmorillonite. Occurrence and fabrics of bentonites suggest that smectites as well as cristobalite, opal-CT and zeolites have been formed diagenetically from tuffaceous materials. The precursor of smectites is trachytic or basaltic tuff. Smectites derived from the former contain relatively more $Al_2O$ a and less $Fe_2O_3$ than those from the latter.