Coloration of natural beryl by iron ion implantation

Abstract

Natural colorless crystals of Ural beryl were implanted at room temperature with 40 keV Fe+ ions with fluences in the range of 0.5-1.5 x 10(7) ion/cm(2). As-implanted samples show dark-grey tone due to radiation damage of beryl crystal. Subsequent thermal annealing of irradiated crystals in oxygen at 600 C for 30 min results in the color change, to yellowish or yellow-orange tones with golden luster, depending on value of iron fluence. The nature of beryl coloration was studied by optical absorption, Mossbauer and Rutherford backscattering (RBS) spectroscopes. It was established that the thermal treatment of iron-irradiated beryl lead to inward diffusive redistribution of iron ions. An appearance of optical absorption bands connected with charge-transfers O2- --> Fe-VI(3+) and O2- --> Fe-IV(2+), Fe-IV(3+) determine the yellow tone in colored beryls. Most of implanted iron ions are founded in both tetrahedral Fe-IV(2+) and octahedral Fe-VI(3+) sites where they may substitute beryllium and aluminum host ions by isomorphic way. (C) 2003 Elsevier Science B.V. All rights reserved.