Geoscience Reference
In-Depth Information
Weathering oxidizes sulfides into soluble sulfate and all the sulfur from the crust is
transported to the sea, dissolved in the run-off. In landlocked and coastal environments,
evaporation concentrates natural waters eventually to the point of gypsum and anhy-
drite saturation: these minerals are the most abundant form of evaporites. In seawater,
BaSO
4
saturation is often reached, but most of it is redissolved shortly after deposition.
Microbial activity during early diagenesis turns marine SO
2
4
from interstitial fluids into
sedimentary pyrite. Marine SO
2
4
introduced into submarine hydrothermal systems (black
smokers) is first precipitated as anhydrite whose solubility decreases significantly with
temperature, reduced by ferrous iron from ambient basalt, and mixed with sulfide leached
from the basalts. The reduced forms H
2
S and HS
−
are present in hydrothermal fluids,
largely leached from the basalts, in proportions that vary largely with the pH. Upon cool-
ing, hydrothermal sulfur commonly precipitates as sulfides of Cu and Fe (chalcopyrite,
pyrrhotite) at temperatures in excess of 300
◦
C, and of Zn (sphalerite) at lower tempera-
tures. Sulfur is abundant in volcanic fumaroles and is released as SO
2
in large quantities
into the atmosphere by volcanic eruptions. An important aspect of sulfur atmospheric
chemistry is the production of gaseous dimethyl-sulfide (DMS) generated in vast quantities
by phytoplankton.
13.9 Phosphorus
Most common form: PO
3
4
Ionic radius: 0.17 Å (tetrahedral)
Stable isotope: 31 (100%)
Atomic weight: 30.974
Condensation temperature: 1230 K
Dissociation of H
3
PO
4
in water:
H
2
PO
4
H
+
(
H
3
PO
4
⇔
+
−
log
K
=
2.15)
H
2
PO
4
HPO
2
4
H
+
(
⇔
+
−
log
K
=
7.20)
HPO
2
4
PO
3
4
H
+
(
⇔
+
−
log
K
=
12.35)
Reaction limiting solubility in water:
5Ca
2
+
+
3PO
3
4
Ca
5
(PO
4
)
3
OH
⇔
OH
−
(
+
−
log
K
=
55.6) (hydroxylapatite)
Residence time in seawater: 70 000 years
Phosphorus is a lithophile and moderately siderophile element. Substantial amounts of
this element are probably dissolved in the liquid core. It is almost exclusively hosted in
calcium phosphate (apatite) (see
Section 13.6
). Apatite may be of igneous origin. Although
apatite is certainly present in the upper mantle, P repository in the deep mantle is not
well understood. Biogenic (fish teeth and bones) and diagenetic apatites are the essential
repositories of sedimentary phosphorus. They occasionally form huge deposits, as in West
Africa, that are actively mined to provide agricultural fertilizer. Some of these deposits,
