Geoscience Reference
In-Depth Information
Table 8.3
Crustal minerals
Range of Crustal
Mineral
Composition
Abundances (vol. pct.)
Plagioclase
31--41
Anorthite Ca(Al
2
Si
2
)O
8
Albite Na(Al,Si
3
)O
8
Orthoclase
9--21
K-feldspar
K(Al,Si
3
)O
8
Quartz
SiO
2
12--24
Amphibole
NaCa
2
(Mg,Fe,Al)
5
[(Al,Si)
4
O
11
]
2
(OH)
2
0--6
Biotite
K(Mg,Fe
2+
)
3
(Al,Si
3
)O
10
(OH,F)
2
4--11
Muscovite
KAl
2
(Al,Si
3
)O
10
(OH)
0--8
(Mg,Fe
2+
)
5
Al(Al,Si
3
)O
10
(OH)
8
Chlorite
0--3
Pyroxene
Hypersthene
(Mg,Fe
2+
)SiO
9
0--11
Ca(Mg,Fe
2+
)(SiO
3
)
2
Augite
(Mg,Fe
2+
)
2
SiO
4
Olivine
0--3
Oxides
2
∼
Sphene CaTiSiO
5
Allanite (Ce,Ca,Y)(Al,Fe)
3
(SiO
4
)
3
(OH)
Apatite
Ca
5
(PO
4
,CO
3
)
3
(F,OH,Cl)
Magnetite
FeFe
2
O
4
Ilmenite
FeTiO
2
Alternatively, there may be layers or blobs of
eclogite in the mantle, representing crust that
was previously near the Earth's surface. Such fer-
tile blobs are an alternative to the plume model
for anomalous volcanism.
Feldspar (K-feldspar, plagioclase) is the most
abundant mineral in the crust, followed by
quartz and hydrous minerals (such as the micas
and amphiboles) (Table 8.3). The minerals of the
crust and some of their physical properties are
given in Table 8.4. A crust composed of these
minerals will have an average density of about
2.7 g/cm
3
. There is enough difference in the
velocities and
V
p
/
V
s
ratios of the more abundant
minerals that seismic velocities provide a good
mineralogical discriminant. One uncertainty is
the amount of serpentinized ultramafic rocks
in the lower crust since serpentinization decrea-
ses the velocity of olivine to crustal values. In
regions of over-thickened crust, the lower por-
tions can have abundant garnet and therefore
high density and high seismic velocity, compa-
rable to mantle values. The Moho in these cases
Table 8.4
Average crustal abundance, den-
sity and seismic velocities of major crustal
minerals
Volume
V
p
V
s
ρ
(g/cm
3
)
Mineral
%
(km/s)
(km/s)
Quartz
12
2.65
6.05
4.09
K-feldspar
12
2.57
5.88
3.05
Plagioclase
39
2.64
6.30
3.44
Micas
5
2.8
5.6
2.9
Amphiboles
5
3.2
7.0
3.8
Pyroxene
11
3.3
7.8
4.6
Olivine
3
3.3
8.4
4.9
can actually be due to the gabbro--eclogite phase
change, rather than to a chemical change. In
these cases,
crustal thickness
is not a good proxy
for mantle temperature or the amount of melt
that has been extracted from the mantle.
The floor of the ocean, under the sediments,
is veneered by a layer of tholeiitic basalt that
