Chemistry Reference
In-Depth Information
Fig. 3.33
Mössbauer spectra of manganostrunzite at 60 K (a) and of ferrostrunzite at 50 K (b)
paramagnetic spectra of the ferristrunzite variant are very similar to those of
manganostrunzite, the major distinction being the higher relative contribution of the
first doublet (D = 0.64 mm/s), i.e. *30 %, which is due to Fe
3+
cations on the Mn
sites. The ideal value for a complete Fe-for-Mn substitution would be 33 %. The
small deviation is ascribed to the presence of Al cations in the structure to an amount
of approximately 0.1 p.f.u., as derived from the chemical analysis. A spectrum of
ferrostrunzite is shown in Fig.
3.33
b and refers to a temperature of *60 K. Clearly
two ferrous components are present in addition to a dominant ferric doublet. The
latter was decomposed into two contributions which were assigned to Fe(1) and Fe(2)
sites, respectively (see Table
3.19
). Rationally, the two ferrous doublets were
attributed to the Mn sites. However, the reason why two distinct Mn sites appear has
remained unexplained. Moreover, the ferrous-to-ferric concentration range was
found to be 0.36, which is significantly smaller than the value 0.5 for ideal
ferrostrunzite.
3.6.5 Vivianite
Ideally, the chemical formula of vivianite is Fe
2
þ
3
PO
ð Þ
2
:
8H
2
O
:
As such the
mineral is colorless. In soils vivianite species are commonly fine-grained and
easily oxidize in the air, the color becoming pale blue. There are no structural
changes observed upon oxidation and oxidized vivianite can be represented as
Fe
2
þ
ð
3
q
Þ
Fe
3
þ
ð Þ
H
2
Oq O
ð½ :
The unit cell of (oxidized) vivianite is
monoclinic. The structure consists of single and paired octahedra bound together
by PO
4
tetrahedra, thus forming infinite sheets parallel to the (010) plane. The
single octahedron has a O
2
(OH
2
)
4
coordination with the oxygens in trans position,
and is denoted as the Fe(1) site. The paired octahedra form the group O
6
(OH
2
)
4
and share two O
2-
ð
PO
4
Þ
2
:
8
q
q
anions along their common edge. For both octahedra of the
