BLUE-GREEN PYROPE-SPESSARINE GARNET WITHOUT A COLOR CHANGE PHENOMENON: THE ROLE OF VANADIUM IN THE COLOR OF GARNETS

Garnets hold a unique position in the gem world as they display a wider range of colors than perhaps any other gem species. Color change garnets are also known that appear violet, blue, or green in daylight and orange, red, or purple in incandescent light. Recently, a new type of pyrope-spessartine garnet which remains blue or blue-green in any lighting condition entered the market in small quantities. We present here a chemical and spectroscopic analysis of these purely blue garnets.

The main chromophore in color change pyrope-spessartine garnets is V³⁺. Octahedral V³⁺ has two spin-allowed electron transitions that cause visible light absorption bands around 568 – 625 nm and 408 – 444 nm depending on the garnet composition (C. A. Geiger, et al. 2000). The former band absorbs orange and yellow light to form two transmission windows in the green-blue and red portions of the visible spectrum. The strength of these two transmission windows depends on the concentration of V³⁺ relative to other chromophores (Mn²⁺, Fe²⁺ and Cr³⁺).

The blue garnets are chemically very similar to other color change garnets except for their much higher concentration of vanadium. Visible light spectroscopy confirms that the increased V³⁺ concentration in these garnets leads to the red transmission window being closed which is why these higher vanadium garnets are only blue in any lighting condition. In this study, we were also able to isolate the V³⁺ absorption spectrum between 380 nm and 780 nm by subtracting the visible absorbance spectra of one high vanadium pyrope-spessartine garnet from that of another low vanadium pyrope-spessartine garnet (but with almost equal amounts of other chromophores such as Mn²⁺, Fe²⁺ and Cr³⁺. This V³⁺ visible spectrum can then added to or subtracted from pyrope-spessartine visible light spectra in order to synthetically produce visible light spectra for a wide range of garnet chemistry. The color can be further calculated from these spectra in order to fully explore the wide range of colors possible in pyrope-spessartine garnets.

Geiger, C. A., et al. (2000) E. J. Min., 12, 259-271