Chemistry Reference
In-Depth Information
3.4.2 Sulfates
Sulfates are known in nature as a series of minerals that are formed by oxidation of
sulfides. Depending on the environment of formation they possess a different stage
of hydration. Iron sulfates often accompany coal and are indicative of the
weathering of the latter. Recently, a renewed interest for these minerals has been
evoked in connection with the mineralogy on Mars, prospected by the Mars
Exploration Rovers (MERs).
In recent years, the structural classification of the sulfates has been well
established [
157
]. Sulfates consist predominantly of a polymerization of MO
6
octahedra, where M represents divalent or trivalent cations such as Fe
2+
,Fe
3+
and
Mg
2+
,Mn
2+
,Zn
2+
,Al
3+
, etc.) and TO
4
tetrahedra where T stands for hexavalent or
pentavalent cations such as S
6+
and Mo
6+
,P
5+
,V
5+
, etc.
The most common iron sulfate is szomolnokite, FeSO
4
.H
2
O. Other sulfates are
the ferrous species rozenite, FeSO
4
.4H
2
O, and melanterite, FeSO
4
.7H
2
O, and the
ferric species kornelite, Fe
2
SO
ð Þ
3
:
7H
2
O
;
and coquimbite Fe
2
SO
ð Þ
3
:
9H
2
O
:
Sulfates of mixed valence between ferrous melanterite and ferric kornelite are known
as römerite. Another well-known mixed valence sulfate group is voltaite with for-
mula K
2
Fe
2
5
Fe
3
3
Al SO
ð Þ
12
:
18H
2
O, but containing a large variety of substituting
elements. A diversity of anhydrated sulfate minerals exists with general formula
MFe
3
(SO
4
)
3
(OH)
6
where M = Na
+
,K
+
, … being catalogued as jarosites.
Distortions of polyhedra are a common phenomenon in sulfates and result in
different stereographic environments. Moreover, natural samples contain a diver-
sity of cations other than iron, which also influences the local environment of the
latter. Therefore, the Mössbauer spectra of iron sulfates usually exhibit one or
more predominant doublets and some additional smaller doublets. Such spectra
can
either
be
fitted
with
discrete
doublets
or
by
quadrupole
distributions
(Fig.
3.20
).
Fig. 3.20 RT spectra of some sulfates: szomolnokite (adapted from Van Alboom et al. 2009
[
162
]) and voltaite (adapted from Ertl et al. [
161
])
