CRYSTALLOGRAPHIC RELATIONSHIPS: THE KEY TO UNDERSTANDING GARNET NUCLEATION?

Garnet is a useful mineral for understanding tectono-metamorphic processes, but despite its utility, the atomic scale processes that control the position and rate at which it crystallizes are poorly constrained. However, microstructural data can be utilized to interpret some of these controls, as shown here for samples from the Nelson contact aureole, British Columbia and the Mosher’s Island formation, Nova Scotia. We present electron back scattered diffraction data that reveals crystallographic relationships between the orientation of garnet crystals and the orientation of the foliation (defined by chlorite and muscovite) in each sample. Specifically, garnet is preferentially oriented such that either the [-116]_garnet, [079]_garnet, or [-779]_garnet crystal directions are parallel to the foliation. This is interpreted as the result of garnet templating on the crystal structure of pre-existing muscovite, potentially providing a preferential pathway for garnet nucleation and growth. Muscovite contains Al octahedra- Si tetrahedra bonds with lengths of 3.23Å (Richardson and Richardson, 1982), almost identical to the 3.22Å bond length between the Al octahedra and Si tetrahedra along the crystal planes of interest in garnet (Novak & Gibbs 1971). Thus, garnet in these samples may nucleate by templating onto the Al octahedra-Si tetrahedra structure in muscovite. We propose that this templating lowered the energetic barrier for nucleation and provided a viable pathway for nucleation and growth.