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Stabilization of trivalent Mn in natural tetragonal hydrogarnets on the join ‘hydrogrossular’–henritermierite, Ca₃Mn³⁺₂ [SiO₄]₂[H₄O₄]

Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden

^* E-mail: ulf.halenius{at}nrm.se

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’.

KEYWORDS: chemical analysis, electronic spectra, trivalent manganese, henritermierite, ‘hydrogrossular’

This article has been cited by other articles:

				U. HALENIUS, U. HAUSSERMANN, and H. HARRYSON Holtstamite, Ca3(Al,Mn3+)2(SiO4)3-x(H4O4)x, a new tetragonal hydrogarnet from Wessels Mine, South Africa European Journal of Mineralogy, April 1, 2005; 17(2): 375 - 382. [Abstract] [Full Text] [PDF]