CHROMOPHORIC DIVALENT IRON IN OPTICALLY ANISOTROPIC MAGNUSSONITE

Results from new optical studies of natural magnussonite, including ob served weak birefringence and distinct pleochroism, indicate that the symmetry of the mineral structure may be lower than cubic. The optical absorption spectra of natural and synthetic Fe-doped magnussonite cry stals are dominated by an absorption band at 15,300 cm(-1). The band, which is distinctly polarised (E > O), displays a Beer law relation to sample Fe-content. On the basis of Fe-concentration dependence, band width (omega(1/2) approximate to 2,500 cm(-1)), pressure dependence (D elta approximate to 100 cm(-1)/GPa) and extinction coefficient, this b and is assigned to a spin-allowed electronic d-d transition in Fe2+. A n additional absorption band at 14,250 cm(-1) assignable to Cu2+ is ob served as a relatively weak feature in Cu-rich natural samples. The mo lar extinction coefficient of the Fe band is approximately 20 times th at of the Cu-band. The presence of a second unresolved Fe2+ d-d band a t approximately 3,100 cm(-1) is inferred from IR-spectra of a Fe-rich and a Fe-poor magnussonite crystal. A Mossbauer Fe-57 spectrum of magn ussonite recorded at room temperature reveals a dominating quadrupole doubler, CS = 0.79 and QS = 0.68 mm/s, assignable to ferrous iron in a four-fold planar coordination. The major cause for the colour and ple ochroism of green magnussonite is the presence of ferrous iron in comp aratively low concentrations. Weaker colours in greenish hues may be p roduced by the presence of Cu.