We present characteristics of hydrothermal chlorite and its interstratification with 7-a mineral phase that occur in the propylitic alteration zone of the Bobae sericite deposit formed in rhyodacitic tuff. Chlorite is found as disseminated fine-grained aggregate or replacement materials of precursor minerals such as Fe-oxides and amphibole. Based on X-ray diffraction(XRD), all chlorites belong to IIb polytype and the (060) reflections averaging 1.53~1.54a indicate a trioctahedral structure. Chemical compositions of chlorite show that the Fe/(Fe+Mg) values are mostly in the range of 0.44~0.53, and cation deficiencies in octahedral sites range from 0.06 to 0.37. Under scanning electron microscope(SEM) chlorite occurs as well-crystallized aggregates and is subparallely stacked in interstices or between grain boundaries of associated minerals. transmission electron microscopic(TEM) images reveal that chlorite shows regular layers with 14-a spacings, locally interstratified with 7-a or 21-a periodicities. The 21- a periodicity corresponds to the sum of the d001 values of chlorite and 7-a phase. The chlorite packet coexisting with 7-a layers displays abundant defects such as edge dislocations and layer terminations. Selected-area electron diffraction(SAED) indicates that chlorite and 7-a phase are randomly interstratified in the mixed-layer areas. We propose a lateral change of layers for the polymorphic transition of 7-a phase to chlorite.e.

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