Spectroscopic and Magnetic Studies of Ferric lron Hydroxy Sulfates: Intensificatin of Color in Ferric Iron Clusters Bridged by a Single Hydroxide Ion

George R. Rossman
 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.

 ehnanced spectrum

Optical absorption spectrum of magnesiocopiaptte showing enhanced intensity in the gamma polarization.

last updated: 26-Jul-2023