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Purplish-red almandine garnets with alexandrite-like effect: causes of colors and color-enhancing treatments

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Abstract

Fine gem-quality, purplish-red garnets from the Tocantins State, Brazil, were investigated for their crystal chemistry and optical properties by several spectroscopic techniques, including electron microprobe analysis, Mössbauer, Raman spectroscopy and optical absorption. Although most garnets are purplish-red, some specimens show color zoning, with deep red color in the core and purple in the outer parts. Electron microprobe analysis showed that these garnets are principally almandine–pyrope solid solution at the rim. However, at the red core, they contain also up to 7 % of spessartine. Mössbauer spectroscopy reveals that the iron content is predominantly Fe2+ (>99 %) in the natural garnets. The optical absorption spectra are dominated by spin-allowed and unusual high-intense spin-forbidden transitions from eightfold coordinated Fe(II) in the near infrared and visible spectral region, respectively. For the red core, in addition, three sharp bands centered in the blue part of the visible spectral range and a broad charge transfer band in the near-UV region are observed. All garnets with purplish colors show also a remarkable color-changing effect from purple in daylight light to red in incandescent light called alexandrite-like effect. Heat treatments in the 700–900 °C temperature range in oxidizing and reducing atmospheres lead to reversible and irreversible color changes which are discussed based on the microscopic changes in the Fe ion coordination and valence states.

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Acknowledgments

The authors acknowledge financial support from the Brazilian agencies CNPq and FAPEMIG. Special thanks also to the Brazilian company VALE and theColorgems company, Osório and Eloy Neto.

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Authors and Affiliations

  1. Departmento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil

    K. Krambrock, F. S. Guimarães, M. V. B. Pinheiro, R. Paniago, A. Righi & A. I. C. Persiano

  2. Departamento de Geologia, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil

    J. Karfunkel

  3. Stone Group Labs, Springfield, MO, USA

    D. B. Hoover

Authors
  1. K. Krambrock
  2. F. S. Guimarães
  3. M. V. B. Pinheiro
  4. R. Paniago
  5. A. Righi
  6. A. I. C. Persiano
  7. J. Karfunkel
  8. D. B. Hoover

Corresponding author

Correspondence toK. Krambrock.

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Krambrock, K., Guimarães, F.S., Pinheiro, M.V.B.et al. Purplish-red almandine garnets with alexandrite-like effect: causes of colors and color-enhancing treatments.Phys Chem Minerals40, 555–562 (2013). https://doi.org/10.1007/s00269-013-0592-6

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