AN INTERSTRATIFIED MINERAL OF CHLORITE AND MONTMORILLONITE FROM THE GREEN TUFF IN THE YAMAKATA DISTRICT IBARAGI PREFECTURE JAPAN

An interstratified chlorite-montmorillonite mineral was found as an alteration mineral of tuff and tuffaceous sediments in the Neogene Tertiary “Green Tuff” near the Yamakata-machi, Kuji-Gun, Ibaragi Prefecture. The <2μ fraction obtained by the sedimentation method contains about 3.2% quartz as an impurity. Chemical composition: SiO2 37.82%; TiO2 0.22%; Al2O3 14.72%; Fe2O3 6.47%; FeO 8.90%; MgO 14.36%; MnO 0.37%; CaO 1.81%; Na2O 1.10%; K2O 1.10%; H2O (+) 7.10%; H2O (-) 6.44%; total 100.41%. After subtracted 3.2% quartz from the chemical analysis, the structural formula was obtained as (Ca0.357Na0.391K0.258) (Al1.563Fe+30.895Fe+21.369Mg3.938) (Si6.371Al1.629) O20 (OH)10. nH2O. X-ray examination of the natural material shows a reflection at about 31A and its higher orders, but they make an irrational series. The montmorillonite-like layers expand with ethylene glycol and contract with heat treatment similar to montmorillonite. X-ray analysis revealed that the interstratification is produced by two component stacking of about 60% chlorite-like layers and about 40% montmorillonite-like layers. DTA and I. R. data are also described in this paper.