A mica-like mineral with 12.5 Å basal spacing found in the soils of western Hopei was identified as biotite-vermiculite interstratified mineral.Treatment with N KCl at room temperature changed this mineral into 10 Å biotite readily,while the N MgCl₂ treatment at 100℃ gradually converted it into 14Å vermiculite.39 mini-equivalents of potassium per 100℃ grams of sample were fixed during the KCl treatment,and consequently the ration exchange capacity of the mineral was reduced in about the same amount.This interstratified mineral seems to be a weathering product of biotite.