Co2+-doped diopside: crystal structure and optical properties
Synthetic clinopyroxenes along the CaMgSi2O6–CaCoSi2O6 join were investigated by a combined chemical-structural-spectroscopic approach. Single crystals were synthesized by flux growth methods, both from Ca-saturated and Ca-deficient starting compositions. Single crystal structure refinements show that the incorporation of Co2+ at the octahedrally coordinated cation sites of diopside, increases the unit-cell as well as the M1 and the M2 polyhedral volumes. Spectroscopic investigations (UV–VIS–NIR) of the Ca-rich samples reveal three main optical absorption bands, i.e. 4 T 1g → 4 T 2g(F), 4 T 1g → 4 A 2g(F) and 4 T 1g → 4 T 1g(P) as expected for Co2+ at a six-coordinated site. The bands arising from the 4 T 1g → 4 T 2g(F) and the 4 T 1g → 4 T 1g(P) electronic transitions, are each split into two components, due to the distortions of the M1 polyhedron from ideal Oh-symmetry. In spectra of both types, a band in the NIR range at ca 5000 cm−1 is caused by the 4 A 2g → 4 T 1g(F) electronic transition in Co2+ in a cubic field in the M2 site. Furthermore, an additional component to a band system at 14,000 cm−1, due to electronic transitions in Co2+ at the M2 site, is recorded in absorption spectra of Ca-deficient samples. No variations in Dq and Racah B parameters for Co2+ at the M1 site in response to compositional changes, were demonstrated, suggesting complete relaxation of the M1 polyhedron within the CaMgSi2O6–CaCoSi2O6 solid solution.
KeywordsPyroxenes Crystal structure Cobalt UV–VIS–NIR Crystal field
We thank Michael Taran and an anonymous reviewer for comments that helped to improve the paper.
- Burns RG (1993) Mineralogical applications of crystal field theory. Cambridge University PressGoogle Scholar
- Ghose S, Wan C, Okamura FP (1987) Crystal structures of CaNiSi2O6 and CaCoSi2O6 and some crystal-chemical relations in C2/c clinopyroxenes. Am Miner 72:375–381Google Scholar
- Hugh-Jones DA, Woodland AB, Angel RJ (1994) The structure of high-pressure C2/c ferrosilite and crystal chemistry of high-pressure C2/c pyroxenes. Am Miner 79:1032–1041Google Scholar
- Sheldrick GM (1996) SADABS, program for empirical absorption correction of area detector data. University of Göttingen, GöttingenGoogle Scholar
- Sheldrick GM (1997) SHELXL-97, program for crystal structure analysis. University of Göttingen, GöttingenGoogle Scholar
- Tribaudino M, Ohashi H (2011) High temperature structure and thermal expansion of Co3Al2Si3O12 garnet. Periodico di Mineralogia 80:135–144Google Scholar
- White BW, McCarthy GJ, Scheetz BE (1971) Optical spectra of chromium, nickel, and cobalt-containing pyroxenes. Am Mineral 56:72–89Google Scholar