Polarized absorption spectra of natural piemontite (Ca1.802Mn
2+0.178
Mg0.025) (Mn
3+0.829
Fe
3+0.346
Al1.825) (Si2.992Al0.008) O12OH], viridine (Al1.945Mn
3+0.033
Fe
3+0.063
Mg0.003) O|Si0.970 O4], and kanonaite (Al1.291Mn
3+0.682
Fe
3+0.019
) O|Si1.006 O4] were measured at 295 and ca. 100 K. For piemontite, lowering the temperature resulted in a sharpening of broad bands in the 10 000–25 000 cm?1 region supporting their assignment to single ion Mn3+ in M3 non-centrosymmetric sites. Alternatively, in kanonaite, temperature behaviour pointed to a slightly stronger influence of vibronic coupling on strong bands near 16 000 and 22 000 cm?1, which supported an interpretation of Mn3+ in nearly centrosymmetric M1 sites. Measurements at ca. 100 K show pronounced fine structure in the viridine spectra which is attributed to Fe3+. The ? values for Mn3+ spin-allowed bands in the three minerals lie in the range 18 to 227 1·g-atom?1·cm?1]. For the same band and polarisation, ? values in Mn3+-bearing andalusite-type minerals viridine and kanonaite are the same, which indicates an absence of strong magnetic coupling effects between Mn3+ ions in the andalusite type structure down to ca. 100 K. In silicates, the high ? values for Mn3+ spin-allowed bands, in comparison to those obtained for Fe2+ spin-allowed bands from sites of “similar distortion”, is attributed to a higher degree of covalency in the Mn3+-O bonds compared to the Fe2+-O bonds, as a result of the higher valence state of manganese. |