Occurrence of chlorite and interstratified 7-A phase in rhyodacitic tuff, southeastern Korea, and implications for hydrothermal alteration conditions
Choo, C.O.; Kim, S.J.
published: Feb 8, 2002
Hydrothermal chlorite occurs in the propylitic alteration zone of the Bobae sericite mine formed in rhyodacitic tuff, Korea. Chlorite is found as disseminated fine-grained aggregate or replacement materials of precursor minerals such as Fe-oxides and amphibole. Based on XRD, all chlorites belong to IIb polytype and the (060) reflections averaging 1.53-1.54Å indicate a trioctahedral structure. Chemical compositions of chlorite show that the Fe/(Fe+Mg) values are in the range of 0.44-0.74. Under SEM, chlorite occurs as well-crystallized aggregates and is sub-parallelly stacked in interstices or between grain boundaries of associated minerals. TEM images reveal that chlorite shows regular layers with 14Å, locally interstratified with 7-Å or 21-Å periodicities. The chlorite packet coexisting with 7-Å layers displays abundant defects such as edge dislocations and layer terminations. Selected-area electron diffraction (SAED) indicates that chlorite and 7-Å phase are randomly interstratified in the mixed-layer areas. We propose a lateral change of layers for the polymorphic transition of 7-Å phase to chlorite. Chlorite geothermometry based on chemical data obtained from EMPA, which has been widely used in diagenesis and paleogeothermal systems, should be applied with caution, together with TEM data.