Four relatively intense and broad absorption bands centred at ~12500, ~19500, ~21500 and ~23000 cm−1 were recorded in polarized electronic single-crystal spectra of natural, optically uniaxial and pleochroic hydrogarnets with henritermierite contents ranging from 35 to 97 mol.%. These absorption bands arise from spin-allowed electronic d-d transitions in trivalent Mn located at the axially distorted six-coordinated site of the tetragonal hydrogarnet structure.
The crystal field stabilization energy (CFSE) for trivalent Mn at the Mn site, as derived from band energies, is ~185 kJ/mol. This considerably higher CFSE for Mn3+ in tetragonal hydrogarnets as compared to cubic garnets (130–145 kJ/mol) explains the natural occurrence of close to end-member tetragonal Mn3+-hydrogarnets while only limited Mn3+-substitution is observed in natural cubic garnets.
The fact that incorporation of Mn3+ at intermediate concentrations stabilizes the tetragonal hydrogarnets indicates the potential natural existence of a number of new, partially Mn3+-substituted, hydrogarnets, e.g. tetragonal Mn3+-bearing ‘hydroandradite’.