Ca
2+ has an ionic radius close to 1 A ˚ which is intermediate between small and large cations and near the
limit of sixfold co-ordination. Thus CaCO 3 is dimorphous forming either rhombohedral or orthorhombic
structures.
Cations smaller than 1 A ˚ such as Fe
2+
, Mn
2+
, Zn
2+
and Mg
2+ can be incorporated in the calcite lattice.
These metals all have an ionic radius of about
0.6–0.7 A ˚ , and therefore calcite can contain considerable concentrations of these cations. The calcite group
of minerals, such as siderite (FeCO 3 ), rhodochrosite
(MnCO 3 ), smithsonite (ZnCO 3 ) and magnesite
(MgCO 3 ), all have the same crystal structure as calcite.
Two types of calcite are recognised, depending
on the magnesium content: low-Mg calcite
ð<4 mol% MgCO 3 Þ
and
high-Mg
calcite
ð>4 mol% MgCO 3 Þ. Much of the biologically secreted
calcite typically ranges between 11 and 19 mol%
MgCO 3 . Some organisms like coccoliths are however
composed of low-Mg calcite (or simply called calcite).
Low-Mg calcite is more stable than high-Mg calcite,
and fossil fragments originally composed of high-Mg
calcite are converted to low-Mg calcite during diagenesis. The alteration of high-Mg calcite to low-Mg
calcite takes place by a process of leaching of Mg
2+
ions, which leaves the microarchitecture of the grain
unaffected. The exsolved Mg
2+ may form microdolomite rhombs that are sometimes seen as inclusions
in calcitised high-Mg calcites (quite common in
fragments of echinoderms and calcareous red algae).
The orthorhombic lattice has an arrangement of
CO
2À
3 anions where cations larger than 1 A ˚ (such as
Sr
2+ , Ba
2+ and Pb
2+ ) are preferred. Analogous with
this aragonite crystal structure are strontianite
(SrCO 3 ), witherite (BaCO 3 ) and cerrusite (PbCO 3 ).
Sr in particular is an important trace element in aragonite. Aragonite crystals forming in marine
environments today contain 5,000–10,000 ppm Sr.
Aragonite is unstable and after some time will be
replaced by calcite which still retains relatively high
concentrations of strontium. Aragonite may occasionally be preserved, particularly in dense shales, even in
Mesozoic rocks.
Iron is only very weakly soluble in the oxidised
state, forming hydroxides Fe(OH) 3 and oxides
(Fe 2 O 3 ), but in the reduced state it occurs as soluble
Fe
2+ . Reduction of iron normally takes place within
the microbial sulphate reduction zone where high
concentrations of sulphur will cause available Fe
2+ to
be precipitated as sulphides (pyrite, FeS 2 ), so that very
little is available to enter the calcite structure. The
principal environment in which Fe
2+ can enter the
calcite lattice to form ferroan calcite is thus in the
reducing porewater below the sulphate reduction
zone. The ferroan calcite may contain a few thousand
ppm of iron.
Dolomite (CaMg(CO 3 Þ 2 Þ is a carbonate mineral in
which layers of CaCO 3 alternate with layers of
MgCO 3 : Fe
2þ is commonly found substituting for
Mg
2+ in dolomite, and a complete series extends to
ankerite (Ca(Fe,Mg)(CO 3 Þ 2 Þ. Dolomite formed early
in diagenesis is fine-grained and can often have a
magnesium deficit in relation to calcium [e.g.
Ca 55 Mg 45 ðCO 3 Þ 100 ]. This is called protodolomite
which during burial may be transformed into a regular
dolomite.
5.2
Carbonate – CO 2 Systems in the Sea
Even if most carbonate precipitation occurs biologically it is important to understand the chemical
constraints on carbonate reactions. Carbon dioxide
concentration is the factor which has the greatest
influence on pH and the solubility of carbonates in
water. CO 2 dissolves in water to form carbonic acid
(H 2 CO 3 ), which dissociates into bicarbonate ðHCO
À
3 Þ
and carbonate ions ðCO
2À
3 Þ.
CO 2 þ H 2 O ¼ H 2 CO 3 ¼ H + þ HCO
À
3
¼ CO
2À
3 þ 2H +
Table 5.1 Mineralogy of the most common carbonate
minerals
Carbonate sediments formed in normal marine environments
consist of three main minerals:
Low-Mg calcite CaCO 3 ð<4% MgCO 3 Þ (hexagonal)
High-Mg calcite ðCa,Mg) CO 3 ð>4% MgCO 3 Þ (hexagonal)
Aragonite (CaCO 3 ) (orthorhombic)
Other common carbonate minerals are:
Siderite FeCO 3
Magnesite MgCO 3
Strontianite SrCO 3
Rhodochrosite MnCO 3
Smithsonite ZnCO 3
Ankerite Ca(Mg,Fe)(CO 3 Þ 2
Dolomite CaMg(CO 3 Þ 2
152
N.-M. Hanken et al.
2+ has an ionic radius close to 1 A ˚ which is intermediate between small and large cations and near the
limit of sixfold co-ordination. Thus CaCO 3 is dimorphous forming either rhombohedral or orthorhombic
structures.
Cations smaller than 1 A ˚ such as Fe
2+
, Mn
2+
, Zn
2+
and Mg
2+ can be incorporated in the calcite lattice.
These metals all have an ionic radius of about
0.6–0.7 A ˚ , and therefore calcite can contain considerable concentrations of these cations. The calcite group
of minerals, such as siderite (FeCO 3 ), rhodochrosite
(MnCO 3 ), smithsonite (ZnCO 3 ) and magnesite
(MgCO 3 ), all have the same crystal structure as calcite.
Two types of calcite are recognised, depending
on the magnesium content: low-Mg calcite
ð<4 mol% MgCO 3 Þ
and
high-Mg
calcite
ð>4 mol% MgCO 3 Þ. Much of the biologically secreted
calcite typically ranges between 11 and 19 mol%
MgCO 3 . Some organisms like coccoliths are however
composed of low-Mg calcite (or simply called calcite).
Low-Mg calcite is more stable than high-Mg calcite,
and fossil fragments originally composed of high-Mg
calcite are converted to low-Mg calcite during diagenesis. The alteration of high-Mg calcite to low-Mg
calcite takes place by a process of leaching of Mg
2+
ions, which leaves the microarchitecture of the grain
unaffected. The exsolved Mg
2+ may form microdolomite rhombs that are sometimes seen as inclusions
in calcitised high-Mg calcites (quite common in
fragments of echinoderms and calcareous red algae).
The orthorhombic lattice has an arrangement of
CO
2À
3 anions where cations larger than 1 A ˚ (such as
Sr
2+ , Ba
2+ and Pb
2+ ) are preferred. Analogous with
this aragonite crystal structure are strontianite
(SrCO 3 ), witherite (BaCO 3 ) and cerrusite (PbCO 3 ).
Sr in particular is an important trace element in aragonite. Aragonite crystals forming in marine
environments today contain 5,000–10,000 ppm Sr.
Aragonite is unstable and after some time will be
replaced by calcite which still retains relatively high
concentrations of strontium. Aragonite may occasionally be preserved, particularly in dense shales, even in
Mesozoic rocks.
Iron is only very weakly soluble in the oxidised
state, forming hydroxides Fe(OH) 3 and oxides
(Fe 2 O 3 ), but in the reduced state it occurs as soluble
Fe
2+ . Reduction of iron normally takes place within
the microbial sulphate reduction zone where high
concentrations of sulphur will cause available Fe
2+ to
be precipitated as sulphides (pyrite, FeS 2 ), so that very
little is available to enter the calcite structure. The
principal environment in which Fe
2+ can enter the
calcite lattice to form ferroan calcite is thus in the
reducing porewater below the sulphate reduction
zone. The ferroan calcite may contain a few thousand
ppm of iron.
Dolomite (CaMg(CO 3 Þ 2 Þ is a carbonate mineral in
which layers of CaCO 3 alternate with layers of
MgCO 3 : Fe
2þ is commonly found substituting for
Mg
2+ in dolomite, and a complete series extends to
ankerite (Ca(Fe,Mg)(CO 3 Þ 2 Þ. Dolomite formed early
in diagenesis is fine-grained and can often have a
magnesium deficit in relation to calcium [e.g.
Ca 55 Mg 45 ðCO 3 Þ 100 ]. This is called protodolomite
which during burial may be transformed into a regular
dolomite.
5.2
Carbonate – CO 2 Systems in the Sea
Even if most carbonate precipitation occurs biologically it is important to understand the chemical
constraints on carbonate reactions. Carbon dioxide
concentration is the factor which has the greatest
influence on pH and the solubility of carbonates in
water. CO 2 dissolves in water to form carbonic acid
(H 2 CO 3 ), which dissociates into bicarbonate ðHCO
À
3 Þ
and carbonate ions ðCO
2À
3 Þ.
CO 2 þ H 2 O ¼ H 2 CO 3 ¼ H + þ HCO
À
3
¼ CO
2À
3 þ 2H +
Table 5.1 Mineralogy of the most common carbonate
minerals
Carbonate sediments formed in normal marine environments
consist of three main minerals:
Low-Mg calcite CaCO 3 ð<4% MgCO 3 Þ (hexagonal)
High-Mg calcite ðCa,Mg) CO 3 ð>4% MgCO 3 Þ (hexagonal)
Aragonite (CaCO 3 ) (orthorhombic)
Other common carbonate minerals are:
Siderite FeCO 3
Magnesite MgCO 3
Strontianite SrCO 3
Rhodochrosite MnCO 3
Smithsonite ZnCO 3
Ankerite Ca(Mg,Fe)(CO 3 Þ 2
Dolomite CaMg(CO 3 Þ 2
152
N.-M. Hanken et al.