The characteristic altered biotites are observed in the weathered granite at eastern part of Tottori prefecture, Japan,
From the mineral assemblage, they are classified into following types:
(1) Kaolinite+interstratified Chlorite-Vermiculite
(2) Kaolinite+Vermiculite + interstratified Chlorite-Vermiculite
(3) Kaolinite+Chlorite+Vermiculite+ interstratified Chlorite-Vermiculite.
The hydroxy-aluminium interlayer may form in the all of discrete vermiculite and interstratified vermiculite.
They are different from the mineral assemblages of weathered biotite.
Judging from the mode of occurrence, they were formed from the altered chloritic biotite under the weathering condition.
They gives a evidence of hydrothermal alteration before weathering of the parent rock.