Origin of the color in cobalt-doped quartz

Synthetic Co-doped quartz was grown hydrothermally in steel autoclaves at the Technological Center of Minas Gerais (CETEC), Brazil. The quartz samples, originally yellow in the as-grown state acquired blue coloration after prolonged heat treatment times at 500°C near the alpha–beta transition temperature. UV–VIS–NIR absorption spectroscopy shows the characteristic spectra of Co3+ before heat treatment. After heat treatment, the optical absorption spectrum is dominated by two split-triplet bands the first in the near infrared region centered at about 6,700 cm−1 (1,490 nm) and the second in the visible spectral range at about 16,900 cm−1 (590 nm). Both split-triplet bands are typical for Co2+ ions in tetrahedral coordination environments. From the absence of electron paramagnetic resonance (EPR) spectra, we conclude that the Co2+ found in the optical absorption spectra of the blue quartz is not due to an isolated structural site in the quartz lattice. Instead, the blue color is associated with electronic transitions of Co2+ in small inclusions in which the Co site has tetrahedral symmetry. The non-observation of polarization-depend optical absorption spectra is also in agreement with this model. The results for Co2+ in quartz are different from Co-bearing spinel and staurolite and other silicates like orthopyroxene, olivine, and beryls. The formation process of the color center is discussed.