Crystal field spectra of trivalent manganese in synthetic and natural (Na+-Mn3+)-substituted diopside
Hålenius, Ulf; Skogby, Henrik
European Journal of Mineralogy Volume 8 Number 6 (1997), p. 1231 - 1240
published: Jan 8, 1997
manuscript accepted: Jul 16, 1996
manuscript received: Mar 28, 1996
ArtNo. ESP147050806001, Price: 29.00 €
Abstract The optical absorption spectra of Mn3+-bearing natural and synthetic diopside are characterised by four spin-allowed d-d absorption bands at 20 350, 18 600, 16 800 and 7900 cm-1. The 10Dq and CFSE for Mn3+ (d4-configuration) at the Ml site is calculated according to the barycentre method to be 14630cm-1 and 152 kJ/g-ion, respectively. The recorded band polarisation is inconsistent with the scheme predicted by the point charge model applying the oxygen ligand arrangement at the Ml site (C2 symmetry). Possible causes for this anomaly include charge contributions from next-nearest ions and vibronic coupling effects. Based on the assumption that Mn3+ = Mntot, the molar extinction coefficients calculated for the various Mn3+ bands in the natural sample vary from ~ 5 to ~ 1001/molcm, which consequently represent minimum values. However, data for the synthetic samples indicate slightly higher e-values ranging from ~ 10 to ~ 1701/mol-cm, which are also comparable to those found for corresponding bands in spectra of other Mn3+-bearing silicates. A relatively sharp and weak absorption band observed in spectra of the synthetic samples is on the basis of its intensity dependence on sample cation chemistry assigned to a Mn2+-Mn3+ pair transition.