Chemistry Reference
In-Depth Information
Mn octahedra share opposite O-O edges, while the more regular Al octahedra share
opposite H
2
O corners. The two types of chains are linked to one another by sharing
their OH corners, thus forming parallel sheets that are held together by phosphorous
species in a tetrahedral O
4
co-ordination. Eosphorite is isomorphous with childrenite,
FeAlPO
4
(OH)
2
.H
2
O, and the two mentioned minerals form the end members of a
complete solid-solution series. Naturally occurring eosphotites usually exhibit a
substantial Fe-for-Mn substitution. It is generally accepted in the literature that only
Fe
2+
is present in the structure of iron-substituted eosphorite, (Mn,Fe)Al-
PO
4
(OH)
2
.H
2
O. This belief is corroborated by its Mössbauer spectrum, which
consists of a relatively narrow ferrous doublet at temperatures as low as *35 K. At
RT the relevant hyperfine parameters are as listed in Table
3.19
[
253
].
3.6.4 Strunzite
Three distinct variants of strunzite are known to occur in nature: manganostrunzite,
Mn
2
þ
Fe
3
þ
2
;
and ferrostrunzite Fe
2
þ
Fe
3
2
PO
ð Þ
2
O
ð
2
6H
2
ð Þ;
respectively. Ferristrunzite may
be regarded as the fully oxidized form of ferrostrunzite. The strunzite structure is
triclinic and consists of infinite chains of octahedral ferric sites along the c axis which
are linked one another by sharing hydroxyl groups and by PO
4
tetrahedra. The latter
also bind adjacent chains, thus forming slabs that are connected to each other by Mn
octahedra between remaining PO
4
vertices. Within the chains two ferric sites Fe(1)
and Fe(2) alternate, both having an octahedral O
3
(OH)
2
(OH
2
) coordination. The
Fe(1) is somewhat more distorted than Fe(2) as indicated by a slight difference in
average Fe-O bond length and average O-Fe-O bond angle. The crystallographic
unit cell contains two iron octahedra of each type and two Mn octahedra with
coordination O
2
(OH
2
)
4
. In ferristrunzite the manganese is substituted by Fe
3+
,in
ferrostrunzite by Fe
2+
. In the first case the charge balance is re-established by sub-
stitution of OH by H
2
O at a non-bridging vertex of the Mn octahedron. Mössbauer
spectra (MS), both for these three mineral species have been reported [
254
-
256
]. The
spectrum of manganostrunzite recorded at*60 K is reproduced in Fig.
3.33
a. It was
decomposed into three ferric quadrupole doublets. This model was imposed by the
results at 4.2 K at which temperature the sample is magnetically ordered and clearly
three distinct sextet components are recognized. Obviously, there is no indication
whatsoever that Fe
2+
would be present in the structure of this manganostrunzite
species. The doublet hyperfine parameters observed at RT are included in
Table
3.19
. The presence of three spectral components implies that the manganese
sublattice is partly substituted by Fe
3+
and from the relative spectral area of the
corresponding doublet (D = 0.61 mm/s), i.e. *12 %, it is inferred that the Fe-for-
Mn substitution is around 25 %, which is in accordance with the results of the
chemical analysis [
255
]. The two other ferric doublet components are assigned to the
Fe(1) and Fe(2) sites on the basis that the former sites exhibit a somewhat higher
distortion and hence are expected to produce the larger quadrupole splitting. The
Þ;
ferristrunzite, Fe
3
þ
3
ð
PO
4
Þ
2
O
ð
2
6H
2
O
ð
ð
PO
4
Þ
2
O
ð
2
ð
H
2
O
Þ
5
O
ðÞ
