Chemistry Reference
In-Depth Information
Fig. 3.36 Mössbauer
spectrum at 80 K of a natural
Mn-substituted heterosite
species
(unpublished results). Clearly the spectra consist of an outer doublet component
and an inner much broader doublet, and are most adequately analyzed with a
superposition of two quadrupole-splitting distributions. The maximum-probability
D values at RT were calculated to be 1.63 mm/s and 0.40 mm/s, respectively. On
the basis of the isomer shifts both doublets are attributable to ferric cations.
A value of 1.63 mm/s for the quadrupole splitting is unusually high for Fe
3+
species and is ascribed to a strong deformation of the M2 octahedra as a result of
the presence of Mn
3+
cations, which are Jahn-Teller active. The appearance of two
distinct spectral components is not consistent with the availability of only one type
of site for the cations. Yamada and Chung [
265
] reported MS at RT for synthetic
(Mn
y
Fe
1-y
)PO
4
compounds, showing indeed only one doublet with D increasing
with increasing Mn content from 1.53 mm/s for y = 0, to 1.65 mm/s for y = 0.5.
Therefore, it is likely that the inner doublet in the MS of the Buranga heterosite is
due to a second Fe-bearing phase. This suggestion is corroborated by the obser-
vation that the inner doublet remains to be present at temperatures as low as 15 K,
at which the heterosite is magnetically ordered and produces a well-resolved sextet
(unpublished results).
Considering the substantial fraction of total iron that is present in this second
phase (*40 %), it is puzzling that this phase was not detected in the X-ray
diffractogram. A possible explanation could be that the involved impurity concerns
an amorphous iron phosphate. It should be noted that recently Fehr et al. [
266
]
reported the Mössbauer spectrum of a naturally occurring Mn-substituted heterosite
(Sandamab, Namibia). In addition to the heterosite doublet with D = 1.63 mm/s, the
authors found an inner doublet having D = 0.69 mm/s. They concluded that this
inner doublet is due to an associated iron-phosphate mineral.
3.6.8 Leucophosphite
Leucophosphite is an iron-potassium phosphate with ideal composition given as
KFe
2
(PO
4
)
2
(OH).2H
2
O. It is often found in guano deposits, in pegmatite deposits
and in lateritic crusts. The crystal structure of leucophosphite is triclinic. The
atomic arrangement is based on an octahedral tetramer involving an edge-sharing
