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

Abstract

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.

 

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