Crystallographic preferred orientations may develop in nanocrystalline materials on fault planes due to surface energy interactions

A layer of substantially noncrystalline material, composed of partially annealed nanopowder with local melt, was experimentally generated by comminution during ∼1.5 mm total slip at ∼2.5 × 10−6 m s−1, Pconf ∼ 0.5 GPa, and 450°C or 600°C, on saw cut surfaces in novaculite. The partially annealed nanopowder comprises angular grains mostly 5–200 nm diameter in a variably dense packing arrangement. A sharp transition from wall rock to partially annealed nanopowder illustrates that the nanopowder effectively localizes shear, consistent with generation of nanoparticles during initial fragmentation, not by progressive grain size reduction. Dislocation densities in nanopowder grains or immediate wall rock are not significantly high, but there are planar plastic defects spaced at 5–200 nm parallel to the host quartz grain's basal plane. We propose these plastic defects developed into through‐going fractures to generate nanocrystals. The partially annealed nanopowder has a crystallographic preferred orientation (CPO) that we hypothesize developed due to surface energy interactions to maximize coincident site lattices (CSL) during annealing. This mechanism may also have generated CPOs recently described in micro/nanocrystalline calcite fault gouges