Ferric Iron in Tourmaline

Stephanie M. Mattson and George R. Rossman
Division of Geological and Planetary Sciences, 170-25, California Institute of Technology,
Pasadena, California 91125, U.S.A.


Red Fe3+-rich and black Fe3+, Fe2+-rich tourmalines have been studied by optical and Mössbauer spectroscopies to determine the optical characteristics of Fe3+ in tourmaline. Prominent optical absorption features at 485 and 540 nm are assigned to transitions of multiple exchange-coupled Fe3+ pairs in several site combinations. These transitions are more intense than those of isolated Fe3+ and are polarized along the vector between the interacting ions, thus permitting site assignments. The 485 nm band occurs at an unusually low energy for Fe3+ in silicate minerals. Similar behavior has been observed in the spectrum of coalingite, Mg10Fe3+2(OH)24CO3·2H20, in which Fe3+ occurs in related pairs in edge-shared sheets. These lower energies are proposed to result from magnetic exchange in edge-shared geometries. Antiferromagnetic exchange has been confirmed by a variable temperature magnetic susceptibility study of a Kenyan dravite with 3.36 wt percent Fe. The Mössbauer spectrum of this sample is unusual in that it shows a pronounced decrease in width of component peaks from 298 K to 5 K.