THE weathering of biotite in the soil frequently leads to replacement of interlayer K+ by hydrated Mg2+, with accompanying structural expansion along the c crystallographic axis from 10 to 14 A (ref. 1). Usually, the 14 A vermiculite phase becomes established without the development of well-defined intermediate spacings on the X-ray diffraction pattern. Biotite, however, sometimes weathers to hydrobiotite2 in which there is a regular alternation of 10 A biotite and 14 A vermiculite units yielding distinct X-ray reflexions at 12 and 24 A. Hydrobiotite may also originate, perhaps more frequently3, through hydrothermal alteration. The vermiculitization of biotite in the laboratory has been extensively studied without observation of an intermediate regularly layered phase4–6, although in one study7 hydrobiotite was detected as a minor component. Complete regular interstratification has only been achieved by reversing the reaction so that vermiculite is converted to a mica-like phase by controlled sorption of K+ and other ions8–10. We have now accomplished the transformation of biotite to hydrobiotite in conditions which throw some light on the mechanism involved.