Four nearly pure MgAl2O4 spinels, of both natural and synthetic occurrence, have been studied by means of X-ray single
crystal diffraction and FTIR spectroscopy in order to detect their potential OH content. Absorption bands that can be
assigned to OH incorporated in the spinel structure were only observed in spectra of a non-stoichiometric synthetic sample.
The absorption intensity of two bands occurring at 3350 and 3548 cm1 indicate an OH content of 90 ppm H2O. Based on
correlations of OH vibrational frequencies and O.H O distances, the observed absorption bands correspond to O.H O
distances of 2.77 and 2.99A ¢a , respectively, which is close to the values obtained by the structure refinements for VIO.Ounsh
(2.825A ¢a ) and IVO.O (3.001A ¢a ). This indicates that one probable local position for hydrogen incorporation is the oxygens
coordinating a vacant tetrahedral site. The present spectra demonstrate that the detection limit for OH in Fe-free spinels
is in the range 10.20 ppm H2O. However, at appreciable Fe2+ levels, the detection of OH bands becomes hampered due
to overlap with strong absorption bands caused by electronic d.d transitions in Fe2+ in the tetrahedral position.
