Geology Reference
In-Depth Information
Oxygen fugacity
f
o
2
is analogous to
activity
in describ-
ing the 'effective concentration' of oxygen in these
non-ideal conditions.
Consider the reaction between iron-rich olivine crystal-
lizing from a magma and oxygen dissolved in the melt:
boundary. Because this is a univariant equilibrium (cf.
Figure 2.2), one would need to estimate the temperature
of crystallization of the phenocrysts by some other
means before a numerical value of
f
o
2
could be worked
out. Nonetheless, the univariant boundary is a useful
reference line, and the assemblage quartz-fayalite-mag-
netite provides a means of regulating (or
buffering
)
redox conditions in high-temperature phase equilibrium
experiments; it is often referred to as the 'QFM buffer'.
Under more oxidizing conditions, as shown by the
upper equilibrium boundary in Figure 9.5, the Fe
2+
in
magnetite becomes oxidized, causing it to recrystallize
as the mineral hematite (Fe
2
O
3
), in which the iron is
entirely ferric.
The isotope geochemistry of oxygen is discussed in
Chapter 10.
(
)
3
Fe SiOO FeOFeO
+
2
.
+
3
SiO
2
4
2
2
3
2
(9.4)
quar
tz
fayalite
olivine
melt
magnetite
(
)
Fayalite is a ferrous (Fe
2+
) compound, whose crystal
structure tolerates only a minute amount of Fe
3+
. If suf-
ficient oxygen is present in the system to cause signif-
icant oxidation of Fe
2+
in olivine to Fe
3+
, the olivine
breaks down to a mixture of magnetite (Fe
3
O
4
= FeO.
Fe
2
O
3
), which contains both Fe
2+
and Fe
3+
, and quartz.
The reaction between fayalite and oxygen occurs at
specific
f
o
2
-
T
conditions, as shown in Figure 9.5
(a kind of phase diagram). The lower curve is a univari-
ant equilibrium boundary separating fields where
fayalite + oxygen (below) and magnetite + quartz (above)
are the stable assemblages. The reaction can proceed in
either direction, and the coexistence of all four phases -
as recorded by coexisting phenocrysts of fayalite,
magnetite and quartz in a fine-grained volcanic rock,
for instance - suggests that crystallization occurred
under conditions lying somewhere on the equilibrium
Sulfur
Native
sulfur (oxidation state 0) forms yellow encrust-
ations around volcanic vents and fumaroles, where it
crystallizes as a
sublimate
. It can also be deposited from
hot springs rich in H
2
S or SO
2
and can occur in sedimen-
tary rocks as a result of bacterial reduction of sulfate.
Sulfur can form compounds either with elements
less electronegative than itself (hydrogen and the met-
als) or with oxygen, which is more electronegative. It is
useful to distinguish these two tendencies as 'reduced
sulfur' and 'oxidized sulfur' respectively.
0
Oxidising
(no Fe
2+
)
Fe
3+
+ Fe
2+
-5
Reduced sulfur compounds
-10
Sulfur is an essential nutrient element for living things,
and organosulfur compounds therefore have consider-
able biochemical importance. They produce the dis-
tinctively pungent flavour and odour of onions and
garlic. Hydrogen sulfide (H
2
S), familiar as the smell of
rotten eggs, is produced by decay of organic matter in
anaerobic conditions, for example in stagnant water.
Significant amounts of organosulfur compounds are
present in oil, natural gas and coal; oxidation of these
compounds to SO
2
during fuel combustion is the main
anthropogenic cause of acid rain.
H
2
S is also a significant constituent of volcanic
gases. This suggests that
sulfide
(oxidation state -II) is
the predominant form of sulfur in the Earth's interior.
-15
Reducing
(No Fe
3+
)
-20
400
700
Temperature/°C
1000
Figure 9.5
f
O
2
-T
equilibrium diagram showing the experi-
mentally determined reaction boundaries for the magnetite
(Mt)-hematite (Hm) and fayalite (Fa)-magnetite-quartz (Q)
reactions.
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