SUPERGENE VERMICULITIZATION OF PHLOGOPITE AND BIOTITE IN ULTRAMAFIC AND MAFIC ROCKS, CENTRAL KOREA

An X-ray diffraction study of vermiculitized micas in ultramafic and m afic intrusive rocks from Cheongyang, Korea, shows the following weath ering sequence: mica --> ordered mica/vermiculite interstratification --> vermiculite. Electron microprobe analyses show the general trends of K leaching and Ca enrichment with increased weathering. The vermicu litization of phlogopite from ultramafic rocks proceeds by means of a continuous decrease in Al-for-Si tetrahedral substitutions and a progr essive increase in Al-for-(Fe2+ + Mg) octahedral substitutions in the early stage of weathering. These substitutions occur to compensate for the excess of negative charge in the mica-like layer, in agreement wi th currently accepted vermiculitization mechanisms. They change to a s light increase of Al-for-Si tetrahedral substitutions in the late stag e of the vermiculitization of phlogopite, owing to the oxidation of Fe despite its low content. However, the behavior of Fe in the late stag e of the transformation of biotite into vermiculite is significantly d ifferent; that is, Fe increases substantially. The reason for this Fe increase in the late stage remains unresolved. Recalculations of the s tructural formulas on the basis of several assumptions indicate that t he oxidation of Fe is necessary for the vermiculite derived from bioti te to form the reasonable structural formulas.