Abstract

The yellow color of beryl has been related to charge-transfer between Fe3+ ions substituting for octahedral Al3+ ions in the crystal and the surrounding oxygen ions. This assumption is contradicted by results from Electron Paramagnetic Resonance (EPR) measurements. A strong EPR signal from octahedral Fe3+ ions can be found in beryl of all colors and in colorless beryl. An EPR signal unique to yellow beryl comes from Fe3+ ions at a tetrahedral site. I propose that the yellow color of beryl comes from charge-transfer of tetrahedral Fe3+ ions. A simple model involving electrons trapped in the crystal structure explains the creation and the decay of the yellow color. The absence of trapped electrons in dark red beryl can explain the high temperature stability of its color. There is no evidence that iron ions are located in the channels of the beryl structure, which has often been assumed.

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