Edward A. Cross III

cross eye catching photoIn granulite-facies metagabbros of the Adirondack Mountains, New York, olivine and plagioclase reacted to form coronas of garnet, plagioclase, clinopyroxene, andorthopyroxene, in which both reactant and product minerals are preserved. Electron backscatter diffraction analysis reveals a systematic crystallographic relationship between the garnet and the precursor plagioclase from 50 different coronas.

One single orientation of garnet in relationship to plagioclase predominates. These data provide direct evidencefor epitaxial nucleation of garnet on plagioclase. Four crystallographic relationships occur: (011)Grt || (001)Pl , (101)Grt || (110)Pl , [111]Grt || [111]Pl , and (100)Grt 15°subparallel to (010)Pl. Comparing unit-cell misfits at these garnet-plagioclase interfaces sets a minimum lattice misfit of 25%, but domain-matching epitaxy can explain this relationship: periodic displacements at the interface, which die out within a distance of only a few atomic layers from the interface itself, may allow this garnet-plagioclase relationship to produce a low-energy interface, and permit epitaxy to govern the nucleation of garnet in these rocks.

Advisor: William Carlson