Abstract: Layer charge and cation-exchange characteristics of weathered biotite isolated from a Gray Luvisol in Saskatchewan showed that this product was a high-charge vermiculite. Cation-exchange capacity (CEC) of the sand-size (100–250 µm) weathered biotite particles from different horizons, as determined by a 30-min Ca/Mg exchange, was low (23–71 meq/100 g). Increasing the exchange period to 24 hr increased the CEC to 33–95 meq/100 g for samples from the solum horizons and to 155–163 meq/100 g for samples from C horizons. The free oxide coatings on the mineral particles in the solum horizon samples apparently prevented the exchange cations from entering the interlayers. Removal of free oxides followed by 24-hr exchange sharply increased the CEC of these samples to 105–155 meq/100 g. Oxidation and loss of structural iron resulted in lower octahedral-cation occupancy (2.4–2.6 per half unit cell), suggesting a transition of the trioctahedral biotite to dioctahedral vermiculite. X-ray powder diffraction and high-resolution transmission electron microscopic analyses of alkylammonium-exchanged samples from the sand fractions of all horizons and magnetic separates of the coarse clays showed a linear increase in the d-value of the vermiculite with increasing chain length of the cations. The linear relationships suggest a paraffin-type arrangement having a uniform and high layer charge (0.70–0.76 per half unit cell in the sand-size particles; 0.80–0.84 in the clay fraction). The empirical relationship between tilt angle (α) and the layer charge density for paraffin-type structures, as suggested by Lagaly and Weiss, cannot be used for accurate layer-charge determination. Based on experimental evidence, a straight line relationship between the tilt angle and layer charge is suggested. The layer charge values show no marked variation within the profile and agree well with those calculated from the chemical composition. The repotassified layers of vermiculite in the sand-size weathered biotite particles did not respond to the Ca/Mg exchange, but were expanded by the alkylammonium cations; in contrast, a pure biotite standard (Bancroft, Ontario) of same particle size was unaffected by either treatment.