A QUANTITATIVE DESCRIPTION OF THE CAUSES OF COLOR IN CORUNDUM

We developed a quantitative description of the six major chromophores in natural corundum and determined their relative absorption strengths from weakest to strongest - Fe³⁺< V³⁺< Cr³⁺< Fe²⁺-Ti⁴⁺< h•-Fe³⁺< h•-Cr³⁺- and present predictive depth of color based upon the determined absorption cross section information of each.

A combination of SIMS chemical analyses and quantitative visible absorption spectra on singly doped synthetic corundum samples and natural corundum samples were used to develop absorption cross section curves for each chromophore.

Singly-doped crystallographically oriented wafers from synthetic corundum were used to study each chromophore with two exceptions. Natural samples were required to study Fe²⁺-Ti⁴⁺ pairs in conjunction with synthetic samples. Natural sapphire samples from Yogo Gulch, Montana were carefully screened for uniformity of color and trace element distribution. These were used to determine the absolute amplitudes of the chromophore absorption spectra curves determined from the synthetic samples. Similarly, in studying the Fe³⁺ chromophore, natural samples from various locations were needed as synthetic corundum growth methods have not been able to produce crystals with adequate iron concentrations. Polarized spectra were collected and corrected for multiple reflections between the two polished surfaces. Trace element concentration data for the samples were determined with SIMS using validated relative sensitivity factors developed from ion implants. Absorption coefficient and absorption cross section plots were then determined for each chromophore.

The calculated absorption cross section plots provided the opportunity for direct comparison of the relative chromophore strengths of the six naturally occurring corundum chromophores. The absorption cross section data further enabled us to predict the color and its depth produced by any of these chromophores along the c-axis and perpendicular to the c-axis given the chromophore concentration, wafer path length, and illuminant. To appreciate the variation in strength of these chromophores, we compared the ppma concentrations required to produce a similar color saturation – we found that ppma concentrations for the chromophores varied by over three orders of magnitude when generating comparable depth of color.

The process of determining absorption cross sections to study color in corundum could be applied to other allochromatic minerals in the quest to better understand the nature of any present absorbing species.