Chemistry Reference
In-Depth Information
Fig. 3.19 RT spectrum of a
natural greigite-smythite
sample
Smythite is a hexagonal iron sulfide with composition close to Fe
9
S
11
(x = 0.182). There is still considerable doubt about the exact composition and
whether it should be included as a separate phase in the iron-sulfur system. The
Mössbauer spectra taken for some natural samples, containing other sulfides as
well, reveal more or less three sextets [
148
] for the smythite phase, which are quite
similar to three of the four sextets of pyrrhotite. Hoffmann et al. [
149
] also found
three to four sextets but the one with the highest hyperfine field and nearly zero
quadruple shift might be a sextet of greigite because the samples were from the
same origin.
Greigite (Fe
3
S
4
, x = 0.250) is the sulfide analogue of magnetite and crystallizes
in the cubic spinel structure. The RT spectrum exhibits an asymmetric sextet
(Fig.
3.19
) which is composed of two sextets: one arising from the A sites and one
from the B sites with nearly equal hyperfine fields but different isomer shifts
[
148
,
150
] (Table
3.9
). In contrast to magnetite, greigite does not show a Verwey
transition [
148
,
151
]. Because at higher temperature greigite transforms to pyrrhotite
and FeS
2
the magnetic transition temperature cannot be determined, but lies above
480 K [
148
]. However, the magnetic properties are strongly dependent on the grain
size [
152
].
Mackinawite, Fe
1+x
S, with (x = 0.01-0.07) is a tetragonal iron sulfide with
excess of iron. Morice et al. [
153
] reported a complex Mössbauer spectrum con-
sisting of at least three sextets with hyperfine fields 29.8, 26.2 and 22.8 T and small
quadrupole shifts of about 0.09, 0.06 and 0.09 mm/s respectively. On the other
hand, only a singlet spectrum, even down to 4 K, has been observed by Vaughan
and Ridout [
154
]. Probably the concentration of Co and Ni found to be present in
the involved natural samples is decisive for the different magnetic behavior.
Pentlandite, (Fe,Ni,Co)
9
S
8
, has a face-centered cubic structure with iron and other
metal atoms such as Ni and Co distributed among tetrahedral and octahedral sites. At
RT the Mössbauer spectrum appears as an asymmetric doublet consisting of a
quadrupole doublet with d
Fe
= 0.36 and D = 0.30-0.37 mm/s and an additional
singlet with d
Fe
& 0.6 mm/s, which is responsible for the asymmetry [
155
,
156
].
