Natural Mg-rich lucchesiite was thermally treated in air and hydrogen atmosphere up to 800 °C to study potential changes in Fe-, Mg- and Al ordering over the octahedrally coordinated Y-  and Z -sites, and to explore possible applications to intracrystalline geothermometry based on tourmaline. Overall, the experimental data (structural refinement, Mössbauer, infrared and optical absorption spectroscopy) show that thermal treatment of lucchesiite results in an increase of Fe_tot contents at Z balanced by an increase of Mg and Al at Y . This process is accompanied by a significant deprotonation of the O3 anion site. The Fe order–disorder reaction depends more on temperature, than on redox conditions. During heat treatment in H₂ ,reduction of Fe³⁺ to Fe²⁺ was not observed despite strongly reducing conditions, indicating that the f_O2  conditions do not exclusively control the Fe oxidation state at the present experimental conditions. On the basis of this and previous studies, the intersite order–disorder process induced by thermal treatment indicates that Fe redistribution is an important factor for Fe–Mg–Al-exchange and is significant at temperatures around 800 °C. As a result, Fe–Mg–Al intersite order–disorder is sensitive to temperature variations, whereas geothermometers based solely on Mg–Al order–disorder appear insensitive and involve large uncertainties. The presented findings are important for interpretation of the post-crystallization history of both tourmaline and tourmaline host rocks, and indicate that successful tourmaline geothermometers may be developed by thermal calibration of the Fe-Mg–Al order–disorder reaction.

Six natural, blue colored spinel crystals were studied chemically by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) techniques and optically by UV–VIS–NIR–MIR spectroscopy in the range 30,000–2,000 cm−1  to investigate the causes of their blue color hues. The positions of the absorption bands vary only marginally with the principal composition of the samples (gahnite vs. spinel s.s .). Although blue colors in spinels are frequently the result of various electronic processes in Fe cations, we demonstrate by comparison with synthetic Co-bearing samplesthat Co acts as an important chromophore also in natural spinels. Already at concentration levels of a few ppm (e.g.,>10 ppm), cobalt gives rise to absorption bands at ~18,000, 17,000 and 16,000 cm−1  that result in distinct blue coloration. In spinels with insignificant Co contents, different shades of paler blue (from purplish to greenish blue) colors are caused by electronic transitions in TFe2+, MFe2+, MFe3+ and Fe2+–Fe3+  cation pairs.

Synthetic clinopyroxenes along the CaMgSi₂O₆– CaCoSi₂O₆ join were investigated by a combined chemical-structuralspectroscopic 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. ⁴T_1g → ⁴T_2g(F), ⁴T_1g → ⁴A_2g(F) and ⁴T_1g → ⁴T^_1g(P) as expected for Co²⁺ at a six-coordinated site. The bands arising from the ⁴T_1g → ⁴T_2g(F) and the ⁴T_1g → ⁴T_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 ⁴A_2g → ⁴T_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.