Optical absorption of the d4ion Fe4+in pleochroic amethyst quartz

Abstract

Optical absorption spectra have been taken at 300K for samples of amethyst quartz that show anomalous pleochroism instead of dichroism. The transition moment of the 945 nm band is parallel to a lattice C₂ axis, called the 'optical C₂ axis'. The strong visible band which produces the purple colour can be decomposed into three overlapping components B₀, B₁, B₂ at 527, 538.5 and 599.5 nm respectively. B₀ is polarised parallel to the optical C₂ axis whereas B₁, B₂ are polarised perpendicular to this axis at theta =+53 degrees , -57 degrees to C₃ respectively. The absorption coefficient in the visible was proportional to the concentration of a recently discovered C₂ symmetry S=2 centre through a series of annealing and re-irradiation experiments. This work establishes that the S=2 centre produces the optical absorption and that it is an Fe⁴⁺ ion on a silicon site, verifying Lehmann and Moore's model (1966) of the colour centre.