The Origin of Sillimanite in Glen Clova, Angus

At the sillimanite isograd in Glen Clova, sillimanite appears to have formed within biotite, rather than in kyanite. Biotite is thought to have been a nucleating agent, the trigonally arranged oxygen octahedra and tetrahedra in the alternate mica layers acting as nuclei for the growth of the octahedral Al–O and the tetrahedral (Al, Si)–O chains that constitute the sillimanite structure. Nucleation seems to have been dominantly epitaxial; no permanent breakdown of biotite was involved, and it is suggested that Al and Si for sillimanite growth was mainly derived from the solution of unstable kyanite.