Abstract

A study is reported of the polarized optical absorption spectra of a specimen of andalusite from Brazil. The weakly visible pleochroism of relatively thick a-b sections is thought to be the result of variation in the intensity of a u.v.-centered band due to O (super 2-) Fe (super 3+) charge transfer. Arguments are presented to show why absorption is more intense in the E b spectrum. Intense pleochroism of andalusite sections containing the c-axis is considered to be the result of Ti (super 3+) Ti (super 4+) charge transfer via overlapping t 2g orbital lobes of adjacent lobes of adjacent ions located in the chains of octahedra that parallel the c-axis. Absorption is at a maximum when polarized light vibrates along the c-axis, the direction in which t 2g orbital lap occurs.

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