Division of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA 91125
Optical and infrared absorption spectroscopic data and magnetic susceptibility data are reported for the
ferric hydroxy-sulphate minerals magnesiocopiapite and botryogen and
the related mineral coquimbite. The physicsl properties of intensity of
color, pleochroism, and magnetic susceptiblity associated with the hydroxobridged Fe3+ dimer of magnesicoiapite and with the hydrosobridged Fe3+ infinite chain of botryogen are compared to thiose of isolated Fe3+.
The hydroxobridged minerals show strong pleochroism with intensity
enhancement of the spin-forbidden ligand field absorption bands
occurring when the electric vector of the incident light is oriented n
the direction of the Fe-O-Fe axis. The instensity enhancement and
pleochroism are associated with antiferromagnetic exchange coupling of
the Fe3+ ions.