174
J. W.Morse
open, and no attempt will made to review the massive literature on the topic here. Instead, the major points on this topic in the review by Rickard et al. (1995) are briefly
summarized and the reader should refer to this article for further details and referencing. The major problem in pyrite formation is how to produce the disulphide ligand
and cause the Fe(II) to change from high spin in FeS to the low spin state found in
pyrite. Different pathways have been proposed and demonstrated in laboratory experiments. These are:
1. The FeS oxidation pathway. Amorphous-FeS ages to mackinawite. Then under slightly
oxidizing conditions, mackinawite converts to griegite and finally pyrite. This is the
most often cited mechanism in the literature.
2. The polysulphide pathway. FeS (or FeSH+) reacts with polysulphides forming a complex that then breaks down producing FeS2' This reaction is relatively slow and does
not necessarily involve a solid phase. It is a more realistic pathway than the classically cited schematic solid-solid reaction FeS + SO ~ FeS2'
3. The H 2 S pathway. FeS reacts with hydrogen sulphide producing hydrogen gas
(FeS + H 2 S ~ FeS2 + H 2 ). Because this reaction involves H 2 S, it is favoured at lower
pH values and can be relatively rapid compared to the other pathways.
Pyrite occurs with different morphologies in sediments. Framboidal pyrite is most
common and generally is assumed to be a relatively rapidly-formed early diagenetic
product, whereas euhedral pyrite is usually presumed to form more slowly over long
time periods during later stages of diagenesis.
Because of the large uncertainties in the value of the second dissociation constant
for hydrogen sulphide, a special approach to representing metal sulphide solubility
has been developed. This approach incorporates H+ in the reaction, resulting in the
formation of bisulphide (Eq. 7.30) and is therefore a pH dependent solubility constant
(*Ks' see Stumm and Morgan 1996 for discussion and examples).
MeS + H+ ~ Me 2 + + HSKs=(aMe2+aHS-) / aH+
(7·30)
The pKs values for amorphous-FeS, mackinawite, griegite and pyrite are (using SO
in the griegite and pyrite reactions), respectively, 2.95,3.6, 4.4, 16.4 (Davison 1991). Many
of the preceding concepts have been incorporated into constructing Fig. 7.5. In this
figure, the activity of HS- has been plotted vs. pH for a pore water of S = 35 at 25°C
containing 20 ~M total H2S over a pH range typical of most anoxic marine sediments
of 6.5 to 8. This was accomplished using Eq. 7.31. Then the activity of Fe 2 + was calculated assuming
[ ( r ) ]
-1
a
_
Y HS - a +
HS- - Y HS _LH2S -
2L +1
YH 2 S KI
(7.31)
equilibrium with mackinawite according to Eq. 7.32 and also plotted against pH.
aFe2+ = * Ks(aH+ / aHS-)
(7.32)
J. W.Morse
open, and no attempt will made to review the massive literature on the topic here. Instead, the major points on this topic in the review by Rickard et al. (1995) are briefly
summarized and the reader should refer to this article for further details and referencing. The major problem in pyrite formation is how to produce the disulphide ligand
and cause the Fe(II) to change from high spin in FeS to the low spin state found in
pyrite. Different pathways have been proposed and demonstrated in laboratory experiments. These are:
1. The FeS oxidation pathway. Amorphous-FeS ages to mackinawite. Then under slightly
oxidizing conditions, mackinawite converts to griegite and finally pyrite. This is the
most often cited mechanism in the literature.
2. The polysulphide pathway. FeS (or FeSH+) reacts with polysulphides forming a complex that then breaks down producing FeS2' This reaction is relatively slow and does
not necessarily involve a solid phase. It is a more realistic pathway than the classically cited schematic solid-solid reaction FeS + SO ~ FeS2'
3. The H 2 S pathway. FeS reacts with hydrogen sulphide producing hydrogen gas
(FeS + H 2 S ~ FeS2 + H 2 ). Because this reaction involves H 2 S, it is favoured at lower
pH values and can be relatively rapid compared to the other pathways.
Pyrite occurs with different morphologies in sediments. Framboidal pyrite is most
common and generally is assumed to be a relatively rapidly-formed early diagenetic
product, whereas euhedral pyrite is usually presumed to form more slowly over long
time periods during later stages of diagenesis.
Because of the large uncertainties in the value of the second dissociation constant
for hydrogen sulphide, a special approach to representing metal sulphide solubility
has been developed. This approach incorporates H+ in the reaction, resulting in the
formation of bisulphide (Eq. 7.30) and is therefore a pH dependent solubility constant
(*Ks' see Stumm and Morgan 1996 for discussion and examples).
MeS + H+ ~ Me 2 + + HSKs=(aMe2+aHS-) / aH+
(7·30)
The pKs values for amorphous-FeS, mackinawite, griegite and pyrite are (using SO
in the griegite and pyrite reactions), respectively, 2.95,3.6, 4.4, 16.4 (Davison 1991). Many
of the preceding concepts have been incorporated into constructing Fig. 7.5. In this
figure, the activity of HS- has been plotted vs. pH for a pore water of S = 35 at 25°C
containing 20 ~M total H2S over a pH range typical of most anoxic marine sediments
of 6.5 to 8. This was accomplished using Eq. 7.31. Then the activity of Fe 2 + was calculated assuming
[ ( r ) ]
-1
a
_
Y HS - a +
HS- - Y HS _LH2S -
2L +1
YH 2 S KI
(7.31)
equilibrium with mackinawite according to Eq. 7.32 and also plotted against pH.
aFe2+ = * Ks(aH+ / aHS-)
(7.32)