A Soil Beidellite

X‐ray diffraction analysis of the < 2 µ fraction of the main horizons of a Deer Park sand, a Podzol soil in northern Michigan, shows that montmorillonite is the dominant layer silicate in the A2 horizon of this soil. This montmorillonite appears to have weathered from illite and mica and chlorite interstratified with vermiculite and montmorillonite which are present in the underlying horizons. Chemical and X‐ray data indicate that the montmorillonite in the fine fraction (< 0.2 µ) of the A2 horizon contains some randomly interstratified mica. The Greene‐Kelly test, X‐ray powder diffraction, differential thermal, infrared, and chemical data show that the expanding component of this fine fraction is a beidellite of formula [Al 1.62 Fe 0.18 Mg 0.28 ][Al 0.45 Si 2.55 ]O 10 [OH] 2 with exchange capacity = 0.49 M + per half unit cell. This beidellite has a high layer charge, most of which arises from tetrahedral substitution, and shows a strong tendency to fix potassium.