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Cleavage,
parting, fracture,
hardness, tenacity,
and specific gravity reflect how minerals
respond to the application of external forces.
 Cleavage
is the tendency of minerals to break parallel to planes of
weakness due to fewer or weaker chemical bonds. The lower bonding force
usually leads to wider spacing between atoms, because the attractive force is
not great enough to adjacent planes of ions closely together.
Cleavage
is one of the most diagnostic properties of minerals because it is so strongly
dependent on mineral structure. Cleavage is described in terms of its
quality (perfect, good, fair, poor), and the crystallographic direction or shape
of the cleavage fragments produced (e.g.: cubic, octahedral, rhombohedral,
prismatic). Multiple cleavage planes that are parallel to each other count
as one cleavage direction; the calcite crystal shown to the left has three
directions of cleavage.
 Minerals
may lack cleavage entirely, or have one or several directions of cleavage.
The monoclinic micas (muscovite and biotite) have only one perfect
cleavage. The hexagonal carbonate minerals (calcite and dolomite) have
three directions of perfect cleavage that yield rhombohedral fragments.
Galena (shown to the right), an isometric mineral, also has three perfect
directions of cleavage, but because the cleavage directions are at right angles
to each other the resulting fragments are cubes. Orthoclase, microcline,
the plagioclase group minerals, and the pyroxene group minerals all have two
directions of cleavage at right angles to each other. Fluorite
has four directions of cleavage, yielding octahedral fragments. Sphalerite
has six directions of cleavage that are usually very difficult to see,
especially on fine-grained samples.
Parting,
which can be mistaken for cleavage, is the tendency of a mineral
to break along planes of structural weakness caused by twinning.
Unlike cleavage, parting does not occur in all specimens of a particular
mineral, but only in crystals that are twinned. Twinned crystals will part
along the composition plane of the twin.
In
some mineral structures, the strengths of the chemical bonds are approximately
the same in all or some directions. Failure in such a mineral will produce
an irregular broken surface called a fracture.
The patterns formed on the irregular fracture surfaces are highly diagnostic in
some minerals. For example, quartz and glass break to form curved
surfaces, referred to as "conchoidal fractures" (after the curved
shape of the conch shell). Although hornblende and the other amphibole
group minerals have two directions of cleavage at approximately 56o
and 124o to each other, breakage in other directions results in
fracture surfaces that are characteristically jagged, with sharp edges.
Hardness
(H) is the resistance offered by a mineral to
scratching, as determined by comparison with other minerals of know
hardness. A series of ten common minerals ranked in order of increasing
hardness comprises the Mohs scale of hardness (listed below). The
normal testing procedure is to use samples of these minerals to try to scratch
the unknown mineral, crosschecking by trying to scratch them with the unknown.
|
Relative
Hardness |
Absolute
Hardness (cutting
resistance, after Rosiwal) |
| 1 Talc |
0.03 |
| 2 Gypsum |
1.25 |
| 3 Calcite |
4.5 |
| 4 Fluorite |
5.0 |
| 5 Apatite |
6.5 |
| 6 Orthoclase
feldspar |
37 |
| 7 Quartz |
120 |
| 8 Topaz |
175 |
| 9 Corundum |
1,000 |
| 10 Diamond |
140,000 |
Finger
nails (H = 2 or higher depending on diet), copper pennies (H = 3),plate glass (H
= 5.5), and a steel knife or file (H = 6.5) can also be used for comparison,
although the hardnesses of these items are variable.
 Minerals
such as talc, graphite, molybdenite (H = 1), and cinnabar, galena, and stilbite
(H = 2-2.5) are so soft that they will mark paper. In fact, one of the
many commercial uses of graphite is as the "lead" in pencils.
Hardness
and a number of other properties depend on the direction, or atomic
environment, which is determined by the mineral structure. In most minerals,
the differences in hardness with direction are too slight to measure without
special equipment. However, the hardness of a few minerals varies so
greatly with direction that the differences can be measured easily and are
diagnostic of the mineral. For example, the triclinic
aluminosilicate mineral kyanite, which usually occurs as elongated tabular
crystals or in bladed aggregates, has a hardness of 5 parallel to the direction
of crystal elongation and a hardness of 7 at right angles to the direction of
elongation.
There
are a few rules to keep in mind when testing hardness. 1) Always test a
fresh surface of the mineral. Weathered or corroded surfaces will be
softer than normal. 2) Cross check the results of your hardness
tests. For example, in addition to using mineral B to scratch mineral A,
also try to using mineral A to scratch mineral B. Without this cross check
you may not be able to tell which minerals is actually doing the scratching.
 Tenacity
is resistance to breaking, crushing, bending, or tearing, and is
described by words such as "brittle", "malleable",
"ductile", "sectile", "flexible", and
"elastic". Most minerals break easily, and are therefore brittle.
Malleable minerals can be hammered out into thin
sheets. Ductile minerals can sustain
considerable deformation, especially stretching, without breaking. The
native metals, gold, silver, and copper (shown to the right), are both ductile
and malleable. Sectile minerals, such as
selenite gypsum, can be cut into thin shavings with a knife. Flexible
minerals, such as chlorite, bend without breaking, but will not return to their
original shape after the pressure is released. Elastic
minerals , such as the micas, will return to their original shape after the
bending pressure is released.
Specific
Gravity is a measure
of the density of a mineral, equivalent to the ratio of the weight of a given
volume of the mineral to the weight of an equal volume of water, measured at 4oC.
The specific gravity of a mineral depends on the weights of the atoms of which
it is composed (its composition), and how closely
they are packed together (its structure).
Minerals composed of elements of high atomic weight will be denser than minerals
composed of elements with low atomic weights. Minerals whose atoms are
held together with strong chemical bonds will have closely packed structures,
and will thus be denser than minerals with comparable compositions but more open
structures.
An
example or two will help to illustrate these principles. The three
aluminosilicate minerals kyanite, sillimanite and andalusite are polymorphs that
have the same chemical composition (Al2SiO5) but different
mineral structures. Kyanite has the most closely packed structure and is
the most dense (3.55 grams/cm3). Andalusite has the least
closely packed structure and it is the lightest (3.16 grams/cm3)
of the three polymorphs. An examination of the specific gravities and
structures of the polymorphs diamond and graphite (C), rutile, brookite,
and anatase (TiO2), calcite and aragonite (CaCO3), quartz,
tridymite, stishovite, cristobalite (SiO2) all show that polymorphs
with closely packed structures are denser than minerals with the same
composition but less closely packed structures.
Diamond
and graphite are a particularly good examples of this. Both are composed
almost entirely of carbon, but their physical properties are very
different. Each carbon atom in a diamond is linked to four neighboring
atoms by strong covalent bonds, resulting in a continuous, closely packed,
three-dimensional lattice. This is reflected in its high specific gravity
(3.51) and hardness (H =10). In graphite, the carbon atoms are linked in
sheets, with much weaker bonds between adjacent sheets. The covalent bonds
between adjacent carbon atoms within a single sheet are strong, and hold the
atoms close together, but the weak van der Waals bonds between sheets cause much
more separation between carbon atoms in adjacent sheets. The easy breaking
of the van der Waal bonds between sheets is reflected in its lower specific
gravity (2.23) and hardness (H = 1).
 Two
groups of isostructural minerals (minerals with the the same structural
arrangement of atoms) illustrate how chemical composition affects specific
gravity. Cerussite (shown to the left) and the other carbonate minerals
have approximately the same structure, in that the cations
in each of these minerals are located in approximately the same place with
respect to the carbonate radicals.
However, the exact distances between the cations varies, due to differences in
size and mass. The same is true of the sulfate minerals. Each of the four
cations (Ca2+, Sr2+, Ba2+, Pb2+) has
a different number of protons, neutrons and electrons, which is reflected in the
atomic weight of the elements. This difference is also reflected in their ionic
radii (a measure of the size of an ion), which affects the strength
of the bond with the carbonate (CO3)-2 and sulfate (SO4)-2
radicals. As shown in the table below, there is a positive correlation
between the atomic weight of the cation and the specific gravity of both the
carbonate and sulfate minerals.
|
Cation |
Atomic
Weight |
Carbonate |
Specific
Gravity |
Sulfate |
Specific
Gravity |
| Calcium (Ca) |
40.08 |
Aragonite |
2.95 |
Anhydrite |
2.89 |
| Strontium (Sr) |
87.62 |
Strontianite |
3.70 |
Celestite |
3.95 |
| Barium (Ba) |
137.34 |
Witherite |
4.30 |
Barite |
4.50 |
| Lead (Pb) |
207.2 |
Cerussite |
6.55 |
Anglesite |
6.20 |
|
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Although the specific gravity
of a mineral can be calculated precisely by measuring the volume and weight of
any mineral specimen, most geologists and rockhounds learn to the density of
minerals (typically referred to as the heft of a
mineral). It is fairly easy to learn to estimate heft by practicing with a
set of minerals of known density.
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