We present characteristics of hydrothermal chlorite and its interstratification with 7-$AA$ 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 $1.53~1.54AA$ 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 $14-AA$ spacings, locally interstratified with $7-AA$ or $21-AA$ periodicities. The $21- AA$ periodicity corresponds to the sum of the $d_{001}$ values of chlorite and $7-AA$ phase. The chlorite packet coexisting with 7-$AA$ layers displays abundant defects such as edge dislocations and layer terminations. Selected-area electron diffraction(SAED) indicates that chlorite and $7-AA$ phase are randomly interstratified in the mixed-layer areas. We propose a lateral change of layers for the polymorphic transition of $7-AA$ phase to chlorite.e.