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 (Si11.48Ti0.01Al0.51) (Al0.26Fe3+1.54Fe2+0.30Mg4.45Mn0.15) O32Ca0.19K0.69Na0.77 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.
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