Geoscience Reference
In-Depth Information
In silicates, Mg ions occupy octahedral sites. Many of its properties are better understood
by observing that Fe and Mg have identical ionic charges and similar ionic radii. In sedi-
mentary and igneous carbonates, Mg substitutes for Ca. During mantle melting and magma
differentiation, Mg is strongly compatible. Primary basalts are rich in Mg and rapidly
lose most of it by gravitational removal of olivine and clinopyroxene. The most common
indices of magmatic differentiation involve Mg, either as the MgO content, the FeO/MgO
ratio, or the fraction known as the mg# and which is the atom ratio Mg
/
(Fe
+
Mg)
≈
MgO). Relatively undifferentiated basalts may contain 8-16 wt %
MgO with FeO/MgO ratios close to unity and mg# in excess of 65. Typical granitic melts
only contain 2-4 wt % MgO. Probably the most striking property of Mg in hydrous fluids
is its strong shift in solubility between ambient temperature and about 70
◦
C: seawater,
cold groundwater, and run-off contain substantial amounts of Mg, while all hydrothermal
and warm diagenetic fluids are Mg-free. A number of processes help explain this shift:
precipitation of magnesium-calcium carbonate in coastal environments (dolomitization),
etc. The removal of seawater Mg in the ridge-crest hydrothermal systems approximately
balances out riverine input of Mg to the ocean.
Magnesium is liberated from silicates by weathering
(7.30)
and transported to the sea
in the dissolved load. Its abundance is not limited by a low-solubility mineral phase and
is essentially constant down the water column. Partitioning of Mg into calcium carbonate
seems to be dependent on the temperature conditions of precipitation: the Mg content of
carbonates can therefore be used as a thermometer of ancient marine environments.
100
/
(1
+
0.56 FeO
/
13.6 Calcium
Most common form: Ca
2
+
Ionic radius: 1.00 Å (octahedral)
Stable isotopes: 40 (96.94%), 42 (0.65%), 43 (0.14%), 44 (2.09%), 46 (0.004%), 48
(0.19%)
Atomic weight: 40.078
Condensation temperature: 1518 K
Complexes in water: hydroxides, carbonates, sulfates
Reactions limiting solubility in water:
Ca
2
+
+
CO
2
3
CaCO
3
⇔
(
−
log
K
=
8.22) (aragonite)
Ca
2
+
+
CO
2
3
CaCO
3
⇔
(
−
log
K
=
8.22) (calcite)
Ca
2
+
+
SO
2
4
CaSO
4
.2H
2
O
⇔
+
2H
2
O(
−
log
K
=
4.62) (gypsum)
10
6
years
Residence time in seawater: 1.1
×
Calcium is a refractory and lithophile alkaline-earth element. The abundance of the iso-
tope 40 produced by radioactive decay of
40
K is occasionally used as a chronometer. The
relative abundances of non-radiogenic isotopes have also been used as a tracer of cer-
tain biological processes. Just like Mg, Ca does not enter core composition in substantial
