High-pressure Cr3+ R-line luminescence of zoisite and kyanite: a probe of octahedral site distortion

Abstract

The Cr³⁺ luminescence spectra of zoisite and kyanite, two geologically important minerals, were studied up to 40 and 20 GPa, respectively, in various pressure media. Cr³⁺ substitutes into the octahedral aluminum sites in both minerals and the R-line luminescence is a particularly sensitive site-specific probe of the octahedral Al site. Unlike many previous studies where Cr³⁺ luminescence was utilized, both these minerals have multiple highly distorted octahedral sites resulting in very large splitting of their R-lines, ~ 300 cm⁻¹ in zoisite and ~ 360 cm⁻¹ in kyanite (for reference, ruby is 29 cm⁻¹). For zoisite, the R-line splitting increases as pressure increases and more than triples from its ambient value by 40 GPa, while the R-line splitting in kyanite from the M1 and M2 sites does not change when compressed in a Ne pressure medium up to 20 GPa. We do not observe evidence of any phase transitions in either zoisite or kyanite across the pressure range of these new luminescence measurements. We present some high-pressure luminescence results where kyanite was known to be bridged between the diamond anvils and show how these spectra illustrate the different effect of uniaxial relative to hydrostatic stress on luminescence spectra.