Geoscience Reference
In-Depth Information
volcanic areas, are also rich in siliceous sediments. The older waters from the southern
oceans being rich in silica, it is not surprising to see a substantial fraction of the sea floor
of the southern hemisphere covered with diatom-rich oozes.
13.2 Aluminum
Most common form: Al
3
+
Ionic radius: 0.39 Å (tetrahedral) and 0.54 Å (octahedral)
Stable isotope: 27 (100%)
Atomic weight: 26.982
Condensation temperature: 1650 K
Complexes in water: hydroxides
Reactions limiting solubility in water:
1
2
Al
2
Si
2
O
5
(OH)
4
+
7
2
H
2
O
Al(OH)
4
H
+
(
⇔
H
4
SiO
4
+
+
−
log
K
=
19.5)
(kaolinite)
Residence time in seawater: 620 years
Aluminum is the sixth most abundant element in the Earth. It is a highly refractory
lithophile element. The radioactive isotope
26
Al quickly decayed into
26
Mg in the first
millions of years of the Solar System's evolution. It provided substantial heating to the
early planetary bodies, and the isotopic composition of Mg is one of the most widely used
extinct radioactivity chronometers. It is unlikely that Al enters in large proportions in the
core, but it is a major constituent of many major minerals at any depth in the mantle and
in the crust. In the mantle, it enters plagioclase up to pressures of about 1 GPa, spinel to
2 GPa, and garnet beyond. At these high pressures, Al also enters clinopyroxene in large
proportions: garnet and clinopyroxene dissolve into each other to form majorite, an essen-
tial mineral phase of the mantle above the 660 km discontinuity. At higher pressure, Al
is hosted in a perovskite structure, but its precise behavior is still largely unknown. The
major mineral that hosts Al in igneous rocks is feldspar: only plagioclase occurs in basalts,
while plagioclase and alkali feldspar may occur together in felsic rocks. Biotite mica may
occur in both types of rocks but normally accounts for only a small part of the Al inven-
tory. In sedimentary rocks, Al is hosted in clay minerals such as kaolinite and illite and,
occasionally, in detrital feldspars. In metamorphic gneisses and schists, Al largely resides
in feldspars and micas.
Aluminum can be tetrahedrally coordinated and in this coordination it replaces Si in the
center of oxygen tetrahedra. It can also be octahedrally coordinated and form solid solu-
tions with elements such as Ca, Mg, and Fe. During melting, the Al-rich minerals (feldspar,
spinel, garnet) quickly dissolve into the melt and Al therefore behaves as a moderately
incompatible element. During low-pressure fractionation of basalts, Al is removed by pla-
gioclase precipitation. At higher pressure, plagioclase solubility in silicate melts increases,
