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not contain iron, it is often a pain-staking task to collect a sufficient amount of
uncontaminated sample to derive a sound and diagnostic Mössbauer-spectroscopic
fingerprint for that given Fe phosphate. This drawback is possibly one of the
reasons why in the past Fe-bearing phosphates, compared to silicates, have less
frequently been studied by the Mössbauer effect. In what follows a few Fe
phosphates are selected and their Mössbauer characteristics as observed from their
paramagnetic spectra (mostly room temperature) are presented.
3.6.2 Anapaite
The ideal chemical formula of anapaite is Ca 2 Fe 2+ (PO 4 ) 2 4H 2 O. It has the triclinic
structure. The Ca and Fe cations exhibit an eight- and a six-fold co-ordination
respectively. The iron is bound to four basal water molecules and two apical
oxygen atoms, which belong to a phosphate tetrahedron. The Fe octahedron shares
two edges with the Ca dodecahedron. Hence, there is only one type of octahedral
site available for the Fe 2+ cations with coordination O 2 (OH 2 ) 4 . Consequently, the
Mössbauer spectrum (see Fig. 3.32 a) consists of a sharp quadrupole doublet with
parameters as indicated in Table 3.19 . The mineral remains paramagnetic down to
a temperature as low as 4.2 K. From the temperature dependence of the quadru-
pole splitting, combined with external-field spectra, it is concluded that the Fe
octahedron is subjected to a tetragonal compression [ 252 ]. Oxidation treatments in
solutions with various H 2 O 2 concentrations up to 20 % and subsequent Mössbauer
experiments at room temperature, have revealed that the anapaite structure is
unusually highly resistant against oxidation since eventually only a small amount
of Fe 2+
(*6.5 %) is converted into Fe 3+
(see Fig. 3.32 b). The Fe 2+
doublet
parameters are not affected by this partial oxidation.
Fig. 3.32
Mössbauer spectra of anapaite at 11 K (a) and of oxidized anapaite at 80 K (b)
 
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