A NEW OCCURRENCE OF FERRIFEROUS SEPIOLITE FROM THE AKAN-CHO, EASTERN HOKKAIDO, JAPAN

Abstract

A new occurence of sepiolite found as a probable hydrothermal alteration product of plagioclase porphyrite, which intrudes into tuffaceous sandstone of Palaeogene age, is described.
Although X-ray powder data and thermographs of the mineral completely coincide with previously reported sepiolite, chemical analysis indicates that the mineral is rich, in Fe³⁺ and Fe²⁺. The structural formula based on Brauner-Preisinger's model is calculated to be (Si_11.48Ti_0.01Al_0.51) (Al_0.26Fe_3+1.54Fe²⁺_0.30Mg_4.45Mn_0.15) O₃₂Ca_0.19K_0.69Na_0.77 Accordingly this mineral may be called as ferriferous sepiolite.
The ferriferous sepiolite mass is found in a dyke form of average 30cm, up to 150cm wide, cutting Nuibetsu formation (Oligocene) in Akan-cho, eastern Hokkaido, Japan.