Geoscience Reference
In-Depth Information
a)
3.0
Forsterite
(Mg
2
SiO
4
)
Fayalite
(Fe
2
SiO
4
)
Mg content (%)
100
80
60
40
20
0
2.0
4.40
4.00
Saturated
Argillaceous rocks
Carbonate rocks
Silicic rocks
Unconsolidated (argillaceous)
Unconsolidated (silicic)
1.0
3.60
Dry
3.20
0.0
2.80
0.0
0.2
0.4
0.6
0.8
1.0
b)
Fractional porosity (
f
)
100
Figure 3.32
Correlation between bulk density and porosity for
various sedimentary rocks. The two trends represent dry (air-
lled)
75
and water-saturated samples.
50
Porosity tends to collapse with depth of burial, owing to
the increased con
25
ning pressure, leading to an equivalent
increase in density. Depth-related porosity/density vari-
ations are only signi
0
cant in sedimentary rocks, with the
actual compaction/density variations varying both geo-
graphically and lithologically. It is normally greatest in
argillaceous rocks and least in carbonates (except chalk).
The change in density/porosity is non-linear, often being
ation decreases markedly at depth owing to the collapse of
the majority of the pore space and the comparative incom-
pressibility of the matrix minerals, but usually remains
signi
cant well beyond depths of interest to miners. Note
from the figure how the bulk density converges toward the
matrix density. If gravity responses are to be modelled (see
Section 3.10.3
)
in terrains containing Phanerozoic sedi-
mentary rocks, it is usually necessary to account for the
depth dependency of density.
0
20
40
60
80
100
Fe content (%)
Figure 3.31
Variations in (a) density and (b) magnetic susceptibility
of olivine due to variations in Mg and Fe content. Based on
Kumazawa and Anderson (
1969
). Based on magnetic data in Bleil
and Petersen (
1982
).
The data in
Fig. 3.32
show the linear relationship
between porosity and bulk density predicted by
Eq.
(3.26)
. The data lie on mixing lines between pure matrix
materials, usually calcite or quartz (all data are from sedi-
mentary materials), and the pore contents (air or water);
and they extend from the density of the matrix (normally
2.65
2.75 g/cm
3
) at zero porosity to the densities of air and
water at 100% porosity. The signi
-
3.8.3
Density and lithology
uence of low-
density pore contents on the overall density is clearly
evident.
If the pore space is not totally saturated, or contains
more than one
fluid phase, the density of the pore contents
is then the average of the constituents weighted according
to their relative abundance. The low density of air means
that saturated and unsaturated rocks may have quite dif-
ferent densities, but fresh and salt water have similar dens-
ities so salinity makes little difference. Ice may occupy pore
space in permafrost areas resulting in a slightly lower
density than if saturated with water.
cant in
Figures 3.29
and
3.30
show, as expected, that crystalline
rock types have densities which lie within the range of
the rock-forming minerals. Density is not diagnostic of
rock type, but a number of relationships are evident in
For unweathered and unaltered rocks, felsic rocks are
normally less dense than intermediate rocks, which are
less dense than ma
c rocks, and ultrama
c rocks nor-
mally have the highest densities. This correlation between
mineralogy and chemistry is primarily due to the relative
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