Color of Mn-bearing gahnite: A first example of electronic transitions in heterovalent exchange coupled IVMn2+-VIMn 3+ pairs in minerals

A natural yellow and transparent crystal of Mn-bearing, Mg-rich gahnite (ZnAl2O4) from Nordmark (Sweden) was studied by electron microprobe, single-crystal X-ray diffraction and optical absorption spectroscopy. The empirical structural formula of the crystal, T(Zn 0.52Mg0.34Mn2+ 0.08Al 0.06)Σ1.00 M(Al1.88Mn 3+ 0.06Fe3+ 0.01Mg 0.04Mn2+ 0.01)Σ2.00 O 4, shows that Mn2+ and Mn3+ are almost completely ordered at the T and M sites, respectively. The electronic absorption spectrum of the spinel shows, in addition to a strong UV-absorption edge (O2--Mn3+ and O2--Fe3+ ligand-metal charge transfers) and two broad VIMn3+-bands, a set of relatively narrow absorption bands at energies comparable to those caused by spin-forbidden electron transitions in tetrahedrally coordinated Mn2+ in oxide spinel. However, the set of bands in the present spectrum is shifted to lower energies and they are intensified by approximately an order of magnitude compared to those recorded for spin-forbidden IVMn 2+-bands in spinel. These characteristic differences in combination with the determined cation distribution in the present gahnite demonstrates that electronic transitions in exchange coupled IVMn2+- VIMn3+ pairs cause its color. © 2014 by Walter de Gruyter GmbH & Co. 2014.