Abstract
Trioctahedral illites were found from two talc mines, Zeniya and Yugan, in southwestern Hokkaido. They were examined by optical methods, X-ray diffraction and DTA. The stuctural formula of the Yugan illite derived from chemical analysis was (K0.51Na0.05)(Mg1.87Fe0.37Al0.63)(Si3.05 Al0.95) O10 (OH) 2. These analyses were also carried out for the chlorite associated with the Yugan illite. The illites were considered to have formed in the early Cretaceous period on the basis of K-Ar age.
