Oriented growth of sillimanite in andalusite, Placitas – Juan Tabo area, New Mexico, U.S.A.

Abstract

In metapelitic schists near the Sandia granite in the Placitas – Juan Tabo area, New Mexico, nucleation sites for sillimanite are very variable, but growth of sillimanite in andalusite is common, either as bundles of fibrous ("fibrolitic") sillimanite crystals or as coarser grained parallel prisms or dendritic crystals of sillimanite. Optical relationships indicate a coaxial relationship between the andalusite and the sillimanite but with a and b axes interchanged. Stacking faults formed by dissociated screw dislocations in the andalusite may have provided nucleation sites for sillimanite. Some large sillimanite grains appear to have formed by coarsening of fibrous sillimanite, but others show no evidence of such coarsening. Some fibrous sillimanite occurs in folia in zones of inferred noncoaxial strain accumulation. Some occurs in radiating or crystallographically controlled aggregates in low-strain zones between sillimanite folia. These aggregates do not necessarily indicate postdeformation growth, except on a very local scale, and many of the fibrous sillimanite aggregates have been deformed. Hence, the sillimanite is a product of syndeformational rather than post-deformational metamorphism, as previously suggested, although regional metamorphism and granite emplacement may have been broadly synchronous. The andalusite–sillimanite microstructural relationships indicate a prograde P–T–time path similar to those occurring in other low-pressure (andalusite–sillimanite) metamorphic areas.