Particle size fractions of biotite (0.5–2; 2–5; 5–20; 20–50 μ) were gradually transformed to vermiculite by exchanging the interlayer potassium with barium. In a subsequent exchange procedure, interlayer barium was removed by n-alkylammonium ions in order to preserve the expanded structure under the conditions in the electron microscope. Ultramicrotome sections cut perpendicular to the plane of silicate layers were prepared from samples representing different states of mica-vermiculite transformation.