Geology Reference
In-Depth Information
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Figure 3.14
Representative Specimens from Four Mineral Groups
force is applied, but the fracture
surfaces are commonly uneven or con-
choidal (curved) rather than smooth.
An Austrian geologist, Friedrich Mohs,
devised a relative hardness scale for
10 minerals. He arbitrarily assigned a
hardness value of 10 to diamond, the
hardest mineral known, and lesser values
to the other minerals. Relative hardness
is easily determined by the use of Mohs
hardness scale (Table 3.2). Quartz will
scratch fl uorite but cannot be scratched
by fl uorite, gypsum can be scratched by
a fingernail, and so on. So
hardness
is
defi ned as a mineral's resistance to abra-
sion and is controlled mostly by internal
structure. For example, both graphite
and diamond are composed of carbon,
but the former has a hardness of 1 to 2,
whereas the latter has a hardness of 10.
a
Calcite (CaCO
3
) is the most common
carbonate mineral.
The sulfi de mineral galena (PbS) is the
ore of lead.
b
Specifi c gravity and density are two sepa-
rate concepts, but here we will use them
more or less as synonyms. A mineral's
specifi c gravity
is the ratio of its weight
to the weight of an equal volume of pure
water at 4°C. Thus, a mineral with a specifi c gravity of 3.0 is
three times as heavy as water.
Density
, in contrast, is a mineral's
mass (weight) per unit of volume expressed in grams per cu-
bic centimeter. So the specifi c gravity of galena (Figure 3.14b) is
7.58 and its density is 7.58 g/cm
3
. In most instances, we will re-
fer to a mineral's density, and in some of the following chapters,
we will mention the density of various rocks.
Structure and composition control a mineral's specifi c
gravity and density. Because ferromagnesian silicates contain
iron, magnesium, or both, they tend to be denser than non-
ferromagnesian silicates. In general, the metallic minerals,
such as galena and hematite, are denser than nonmetals. Pure
gold with a density of 19.3 g/cm
3
is about 2.5 times as dense
as lead. Diamond and graphite, both of which are composed
of carbon (C), illustrate how structure controls specifi c grav-
ity or density. The specifi c gravity of diamond is 3.5, whereas
that of graphite varies from 2.09 to 2.33.
c
Gypsum (CaSO
4
·2H
2
O) is a common
sulfate mineral.
Halite (NaCl) is a good example of a
halide mineral.
d
direction, and angles of intersection of cleavage planes.
Biotite, a common ferromagnesian silicate has perfect
cleavage in one direction (
Figure 3.16a). Biotite is a sheet
silicate with the sheets of silica tetrahedra weakly bonded
to one another by iron and magnesium ions.
Feldspars possess two directions of cleavage that inter-
sect at right angles (Figure 3.16b), and the mineral halite has
three directions of cleavage, all of which intersect at right
angles (Figure 3.16c). Calcite also possesses three directions
of cleavage, but none of the intersection angles is a right
angle, so cleavage fragments of calcite are rhombohedrons
(Figure 3.16d). Minerals with four directions of cleavage
include fl uorite and diamond (Figure 3.16e). Ironically, dia-
mond, the hardest mineral, can be cleaved easily. A few min-
erals, such as sphalerite, an ore of zinc, have six directions of
cleavage (Figure 3.16f).
Cleavage is an important diagnostic property of miner-
als, and recognizing it is essential in distinguishing between
some minerals. The pyroxene mineral augite and the amphi-
bole mineral hornblende, for example, look much alike: Both
are dark green to black, have the same hardness, and possess
two directions of cleavage. But the cleavage planes of augite
intersect at about 90 degrees, whereas the cleavage planes of
hornblende intersect at angles of 56 degrees and 124 degrees
(
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Talc has a distinctive soapy feel, graphite writes on paper,
halite tastes salty, and magnetite is magnetic (
Figure 3.18).
Calcite possesses the property of
double refraction
, meaning
that an object when viewed through a transparent piece of
calcite will have a double image. Some sheet silicates are plas-
tic and, when bent into a new shape, will retain that shape;
others are fl exible and, if bent, will return to their original
position when the forces that bent them are removed.
◗
Figure 3.17).
In contrast to cleavage,
fracture
is mineral breakage along
irregular surfaces. Any mineral can be fractured if enough
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