Natural versus synthetic quasicrystals: analogies and differences in the optical behavior of icosahedral and decagonal quasicrystals

The optical behavior of both synthetic and natural icosahedral and decagonal quasicrystals in the range 400–700 nm has been explored by repeated measurements of the reflectance values on crystallographically oriented fragments. We found differences in the optical behavior between natural and synthetic quasicrystals, and discuss the possible reasons to justify this feature. Natural quasicrystals, both icosahedral and decagonal, exhibit an almost identical chemical composition to their synthetic analogs, but seem to show a higher degree of structural perfection, as indicated from their electron and X-ray diffraction patterns. Such perfection is mainly due to the very low phason strain shown by the minerals with respect to the synthetic compounds and substantiates the differences in the measured reflectance values. The new finding constitutes sig- nificant new support for the original proposal that icosahedral and decagonal quasicrystals can be energetically stable states of matter, on the same footing as crystals.