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 Fe
3+ and Fe
2+. The structural formula based on Brauner-Preisinger's model is calculated to be (Si
11.48Ti
0.01Al
0.51) (Al
0.26Fe
3+1.54Fe
2+0.30Mg
4.45Mn
0.15) O
32Ca
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.
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