The microstructure of hydrobiotite, interstratified biotite/vermiculite, was examined to demonstrate an atomic scale investigation of weathering of minerals by high-resolution transmission electron microscopy (HRTEM). Vermiculite basal spacing was collapsed to about 0.95 nm, compared to the unchanged biotite 1.0 nm spacing, by dehydration probably during the ion-milling sample preparation. Atom-resolving structure images with 0.2 nm point resolution show a distinct difference between K-containing and K-depleted interlayers, corresponding to biotite and vermiculite respectively. Thus, the difference in image enables us to identify vermiculite layer positions in hydrobiotite unambiguously. Although the vermiculite layers are randomly formed in the original biotite, there is a tendency to form 1:1 regular biotite/vermiculite interstratification locally. In some interlayers the contrast of potassium columns changed gradually, which suggests that biotite is directly transformed to vermiculite in the weathering process. These observations demonstrate atom-resolving HRTEM is a useful method for structure analyses of such water containing secondary minerals as vermiculite in spite of spacing changes caused during sample preparation or observation in TEM.