Weathered Biotite from Strathdon, Aberdeenshire

Abstract

WALKER¹ showed that biotite weathered to vermiculite in the soil. This transformation involved the complete replacement of interlayer potassium ions by water molecules and exchangeable magnesium ions together with various changes in the silicate layers such as oxidation of ferrous iron and the substitution of octahedral iron by magnesium. MacEwan², however, found that in poorly drained soils biotite broke down into a macroscopic trioctahedral montmorillonite. Recent work, summarized by Jackson³, reveals that the interlayer positions of expanded 2 : 1 layer silicates in soil clays may be incompletely filled with positively charged hydroxy aluminium, such as Al₆(OH)₁₂(OH₂)⁶⁺₁₂. Thus the expanded 2 : 1 layer silicates are effectively converted to a 2:2 intergrade in which the interlayer positions are partially occupied by hydroxy aluminium islands. These intergrades which usually show a 14 Å c-spacing have been referred to as ‘;dioctahedral vermiculite’ or ‘chlorite-like’ and possess properties intermediate between homogeneous 2 : 1 or 2 : 2 layer silicates.