Geoscience Reference
In-Depth Information
quantities. In the mantle, Ca is stored in clinopyroxene and in its high-pressure equivalent
Ca-perovskite. In igneous rocks, as in the crust in general, calcic plagioclase (anorthite)
and amphibole are major hosts for calcium. There is no major Ca-rich clay mineral. The
major low-temperature Ca-rich phase is calcium carbonate in its two forms of calcite and
aragonite. Calcium phosphates are a ubiquitous form of Ca storage in igneous (fluorapatite)
and sedimentary rocks (carbonate-apatite). Apatite is the essential ingredient of vertebrate
hard parts (bones and teeth). Calcium sulfates (gypsum and anhydrite) are an essential
component of evaporitic sequences.
In silicates, Ca ions occupy octahedral sites. During mantle melting, Ca is slightly
incompatible. Its behavior during magmatic differentiation is controlled by the stability of
plagioclase and clinopyroxene (see
Section 11.1
). High-pressure magmatic differentiation
takes place in the presence of clinopyroxene but in the absence of plagioclase and there-
fore decreases the Ca/Al ratio in the residual melt. Low-pressure fractionation of basalts in
the stability field of clinopyroxene and plagioclase leaves the Ca/Al ratio essentially con-
stant. Although the Ca content of olivine is low (a fraction of a percent), it increases with
decreasing temperature. Olivine phenocrysts in basalts may contain up to 0.5 wt % CaO,
while mantle peridotites normally are far more depleted. Basaltic melts typically contain
10-12 wt % CaO and a granitic melt 2-4 wt % CaO.
Calcium is a mobile element during water-rock interaction at any temperature. Pla-
gioclase, clinopyroxene, and amphibole are easily weathered by water equilibrated with
atmospheric CO
2
and by seawater alteration of submarine basalts. The Ca
2
+
ion goes into
solution and is transported to the sea by run-off, while Ca-free mineral phases, silica and
clay minerals, are left behind. Carbonate, sulfate, and phosphate complexes of Ca are
strong. Calcium is the third most concentrated cation of seawater. The Ca concentration
level in fresh water and seawater is controlled by the solubility of calcite and aragonite.
Calcite is less soluble than aragonite. Surface oceanic water is saturated in calcite, while
deep water is undersaturated. This results from the combined effect of temperature and
pressure on the solubility product of calcium carbonate and the dissociation constants of
carbonic acid together with the
CO
2
content of local seawater. Removal of calcium car-
bonate from the ocean by biogenic carbonate precipitation is the most important control of
seawater alkalinity; therefore it is the crux of the response of the ocean-atmosphere system
to CO
2
fluctuations resulting from changes in volcanic activity, biological productivity, and
erosion patterns.
In brines from landlocked and lagoonar environments, precipitation of calcium sulfate
may occur as either hydrated gypsum at temperatures below about 35
◦
C or anhydrous
anhydrite at higher temperatures. Gypsum normally converts to anhydrite during burial.
13.7 Iron
Most common forms: Fe
0
,Fe
2
+
, and Fe
3
+
Ionic radius: 0.61 Å for Fe
2
+
and 0.55 Å for Fe
3
+
(octahedral)
Stable isotopes: 54 (5.90%), 56 (91.72%), 57 (2.10%), 58 (0.28%)
