Experimental Conversion of Biotite to Hydrobiotite

Abstract

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 Å (ref. 1). Usually, the 14 Å 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 Å biotite and 14 Å vermiculite units yielding distinct X-ray reflexions at 12 and 24 Å. 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 phase^4–6, 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 ions^8–10. We have now accomplished the transformation of biotite to hydrobiotite in conditions which throw some light on the mechanism involved.