B. Singh et al., Structural chemistry of Fe, Mn, and Ni in synthetic hematites as determined by extended X-ray absorption fine structure spectroscopy, CLAY CLAY M, 48(5), 2000, pp. 521-527
The incorporation of transition metals into hematite may limit the aqueous
concentration and bioavailability of several important nutrients and toxic
heavy metals. Before predicting how hematite controls metal-cation solubili
ty, we must understand the mechanisms by which metal cations are incorporat
ed into hematite. Thus, we have studied the mechanism for Ni2+ and Mn3+ upt
ake into hematite using extended X-ray absorption fine structures (EXAFS) s
pectroscopy. EXAFS measurements show that the coordination environment of N
i2+ in hematite corresponds to that resulting from Ni2+ replacing Fe3+. No
evidence for NiO or Ni(OH), was found. The infrared spectrum of Ni-substitu
ted hematite shows an OH-stretch band at 3168 cm(-1) and Fe-OH bending mode
s at 892 and 796 cm(-1). These vibrational bands are similar to those found
in goethite. The results suggest that the substitution of Ni2+ for Fe3+ is
coupled with the protonation of one of the hematite oxygen atoms to mainta
in charge balance.
The solubility of Mn3+ in hematite is much less extensive than that of Ni2 because of the strong Jahn-Teller distortion of Mn3+ in six-fold coordinat
ion. Structural evidence of Mn3+ substituting for Fe3+ in hematite was foun
d for a composition of 3.3 mole % Mn2O3. However a sample with nominally 6.
6 mole % Mn2O3 was found to consist of two phases: hematite and ramsdellite
(MnO2). The results indicate that for cations, such as Mn3+ showing a stro
ng Jahn-Teller effect, there is limited substitution in hematite.