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Clays and Clay Minerals; December 2004; v. 52; no. 6; p. 749-759; DOI: 10.1346/CCMN.2004.0520610
© 2004 Clay Minerals Society
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DICKITE AND OTHER KAOLIN POLYMORPHS FROM AN Al-RICH CLAY DEPOSIT FORMED IN VOLCANIC TUFF, SOUTHEASTERN KOREA

Chang Oh Choo*,{dagger} and Soo Jin Kim

Department of Geological Sciences, Seoul National University, Seoul 151-742, Republic of Korea

* E-mail address of corresponding author: chooco{at}hanmir.com

Kaolins from an Al-rich clay deposit formed in volcanic tuff, southeastern Korea contain predominant dickite and quartz with accessory diaspore, pyrophyllite, alunite, illite, sulfides and calcite. Intergrowth of blocky crystals with thin ones within stacks suggests that blocky crystals gradually replaced thin ones within the same stacks. Numerous dickite plates with pseudo-hexagonal outline are placed in rows eventually forming a U-shaped stack and a vermiform stack. Individual crystals within the stack are of pseudo-hexagonal form and vermiform, accompanying slips across basal planes. The euhedral dickite with blocky habit has basal planes (001) and edge planes (110) perpendicular to each other, and it shows pseudo-hexagonal crystallites elongated in one direction. Fourier transform infrared spectra at OH-stretching and far-infrared regions show that the dickitic mass contains a small proportion of kaolinite or nacrite, indicating interstratification or at least mixtures of kaolin polytypes. The formation of mixtures of various polytypes that are of kaolinite-dickite, dickite and mixtures of dickite-nacrite could be attributed to hydrothermal processes ranging over a broad spectrum of temperature and fluid chemistry.

Key Words: Blocky Crystal • Dickite • Kaolinite • Nacrite • Polytypes • Vermiform Stack






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