Chemistry Reference
In-Depth Information
Fig. 3.7 Mössbauer spectrum of ferrihydrite at RT fitted with two doublets (left) and at 4 K fitted
with a d-correlated hyperfine field distribution (right)
Fig. 3.8 Quadruple
distribution in a poorly
crystalline sample of goethite
and lepidocrocite, and in
ferrihydrite
distribution of quadrupole splittings (Fig.
3.8
), they are usually satisfactorily
described by two doublets (Fig.
3.7
) with parameters of about d
Fe
= 0.37 mm/s,
D = 0.6 mm/s (FWHM C = 0.3 mm/s), and d
Fe
= 0.38 mm/s, D = 0.9 mm/s
(FWHM C = 0.4 mm/s). The intensity ratio of these doublets is strongly depen-
dent on the crystallinity and varies from 70/30 to about 30/70 for ''better'' to
poorly crystallized ferrihydrite, respectively.
Such a broad doublet is not only characteristic for ferrihydrite, but also for other
Fe
3+
bearing minerals. Paramagnetic akaganéite, for instance, shows also a broad
doublet spectrum at RT, although it is in fact composed of two discrete spectral
components. Moreover, akaganéite is readily recognized by the appearance of a
magnetically split spectrum at temperatures far above the transition temperature of
ferrihydrite. Another compound showing the typically broad doublet at RT
with similar distribution is fully oxidized vivianite, often called oxykerchenite.
