SMALL-ANGLE NEUTRON-SCATTERING BY OPALS

Micro- and non-crystalline opals, chalcedony and flint show diffuse sm all angle neutron scattering (SANS). Precious opals give rise to two a dditional intensity maxima at very small scattering angles which are d ue to Bragg reflections from the closest packed non-crystalline silica spheres. A small angle texture diagram reveals that the closest packi ng is faulty, Synthetic non-crystalline opals yield much less intense small angle scattering due to lower contrast between silica spheres an d interstitial cement or particles; in this case intensity maxima were not observed. The outer part of the scattering curves of opal-CT and microcrystalline quartz deviates from Pored's law. The specific surfac e of natural non-crystalline opals ranges from 0.006 to 0.018 nm(-1). In microcrystalline opals, the specific surface is about 10 times larg er than in non-crystalline opals.