Geoscience Reference
In-Depth Information
Ferric iron hydroxides (such as those composing ferromanganese nodules) are reduced to
soluble Fe
2
+
, which then precipitates with S
2
−
to form the pyrite Fe
2
S commonly found
in reduced sediments.
13.8 Sulfur
Most common forms: S
0
,S
2
−
, and SO
2
4
Ionic radius: 0.31 Å (tetrahedral), 0.29 Å (octahedral), and 1.84 Å (S
2
−
)
Stable isotopes: 32 (95.02%), 33 (0.75%), 34 (4.21%), 36 (0.02%)
Atomic weight: 32.07
Condensation temperature: 648 K
Dissociation of H
2
Sinwater:
HS
−
+
H
+
(
H
2
S
⇔
−
log
K
=
7.02)
HS
−
⇔
S
2
−
+
H
+
(
−
=
log
K
13.9)
Reactions limiting solubility in water:
Ca
2
+
+
SO
2
4
CaSO
4
.2H
2
O
⇔
+
2H
2
O(
−
log
K
=
4.62) (gypsum)
Fe
2
+
+
S
2
−
(
FeS
⇔
−
log
K
=
18.1) (pyrrhotite)
Residence time in seawater: not known but oceans are well mixed for sulfate
Sulfur is strongly chalcophile and volatile. It has been repeatedly suggested that very large
quantities of this element are dissolved in the core and contribute to the relatively low
seismic velocities of the core with respect to those of pure iron. Sulfur does not readily
dissolve in silicates. Terrestrial sulfur is therefore stored in sulfides. At high temperatures,
solid solutions of Ni and Fe dominate (monosulfide solid solution, MSS). At ambient tem-
perature, sulfur enters a variety of sulfides. The major repository of sulfur in sediments is
sulfides, notably pyrite. Because sulfates, the oxidized form of sulfur, are relatively solu-
ble, these minerals play a minor role in the making of continental crust, with the exception
of gypsum and anhydrite in evaporites, and barite in hydrothermal veins. In seawater, river,
and rain water, in which substantial amounts of dissolved oxygen are present, the stable
form of sulfur is the oxidized form, sulfate SO
2
4
, which is the third most abundant ion of
seawater.
In magmas, sulfur is present as sulfide and also as sulfate in more oxidized granites. Dur-
ing mantle melting and magma differentiation, sulfur has a compatible behavior controlled
by the exsolution and gravitational segregation of sulfide blebs (the melting point of sul-
fides is lower than average magma temperature) that are occasionally observed as mineral
inclusions. Sulfides are the most abundant minerals of inclusions in diamond. The high-
temperature form of sulfides precipitated out of mafic magmas are characteristically rich
in nickel (pentlandite). The high-temperature forms of igneous sulfides are very unstable
with respect to low-temperature alteration.
