Spectroscopic and magnetic studies of ferric iron hydroxy sulfates: Intensification of color in ferric iron clusters bridged by a single hydroxide ion

Abstract

Optical and infrared spectroscopic data and magnetic susceptibility data are reported for the ferric hydroxy-sulfate minerals magnesiocopiapite and botryogen and the related mineral coquimbite. The physical properties of intensity of color, pleochroism, and magnetic susceptibility associated with the hydroxobridged Fe^(3+) dimer of magnesiocopiapite and with the hydroxobridged Fe^(3+) infinite chain of botryogen are compared to those of isolated Fe^(3+). 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 in the direction of the Fe-OH-Fe axis. The intensity enhancement and pleochroism are associated with antiferromagnetic exchange coupling of the Fe^(3+) ions.

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