Geology
250: Mineralogy
Physical Properties of Minerals
The
physical properties of minerals are a direct result of their chemical and
structural characteristics. Because they are readily determined (i.e., by
examination or simple tests), they are important to the rapid identification of
minerals. Other methods for studying minerals involve more elaborate
tests (e.g., optical, x-ray, or SEM), and although they provide important
information, can be time-consuming.
I. Physical Properties
A. Color
1. Color of a mineral is a result of the
interaction of electromagnetic radiation (visible light) with the surface of a
mineral (primarily the electrons).
2. Minor amounts of some elements
(especially the transition elements) can greatly effect the perceived color of
a substance.
3. Color also produced by defects in
crystal structure, structural planes, and by small inclusions.
B. Luster
1. Luster refers to the general
appearance of a mineral surface in reflected light.
2. Metallic versus non-metallic;
minerals with metallic luster are usually opaque and have a colored streak
3. Non-metallic Lusters: Vitreous,
Resinous, Pearly, Greasy, Silky, Adamantine
C. Form (Crystal
habit and aggregates) see Klein.
D. Twins and Intergrowths
Perfectly formed crystals are
uncommon, and many have defects. Twinning is a special type of
defect wherein two parts of a crystal have different crystallographic
orientations that are related by a symmetry operation (usually reflection, rotation,
or a inversion).
Twins are
symmetrically intergrown crystals.
Twin Law - a
center, axis, or plane of twinning
Contact (related
by a twin plane) versus interpenetration twins (twinned crystals share a
common volume).
Lowest
energy configuration of a crystal is perfectly ordered. Internal
energy of a twinned crystal is only slightly higher. That is, during
growth, an ion can occupy a position that has only slightly higher energy,
causing the crystal growth to occur in a new direction. This is a growth
twin.
Transformation
twinning occur as a result of a polymorphic transformation, usually from a
higher-temperature form.
Under
deformation, crystals attain a lower energy configuration by mechanical, or
deformation, twins along a glide axis due to the sliding of atoms within a
lattice.
Multiple
(cyclic) and polysynthetic twins.
Parallel
intergrowths are common because they are a lower energy
configuration than randomly oriented crystals
E. Cleavage, Parting, and Fracture
All
manifestations of the way a minerals responds to an applied stress.
Strain occurs along planes of weakness in the atomic structure.
Cleavage
directions (or forms) are described using Miller Index notation.
Quality
of cleavage is also described (e.g., perfect in micas, to poor in beryl or
apatite.
Parting is used
to describe the breakage planes in minerals that occur along structurally weak
planes, but that are not cleavages, which are shown by all specimens of a
mineral.
Fracture is used
to describe the way a mineral breaks when the strength of the bonds is
approximately the same in all directions.
Conchoidal
Fibrous
or splintery
Hackly
Uneven or
irregular.
F. Magnetism
1. Some iron-bearing minerals are
naturally magnetic (ferromagnetic)
2. Others are weakly magnetic when
subjected to a strong magnetic field and are termed paramagnetic.
G. Hardness
A scale knowns as Mohs hardness was made about 200 years ago by Freidrich Mhos. It is meauredon scale from 1 to 10. On this scale, softer minerals have low numbers and harder minerals have higher numbers.
While the structure of a crystal
depends on the strongest bonds that are present in the crystal structure, the
physical properties depend on the weakest bonds. For example, silicate
minerals all have Si4+ in tetrahedral coordination, and the
strength of the Si--O bond in all silicates is approximately the same, their hardnesses
range from 1 - 7.
Hardness
is the resistance of a mineral surface to scratching.
Furthermore,
the strength of ionic bonds between divalent ions is stronger than the bonds
between monovalent ions. Similarly, ions with larger radii tend to form
weaker bonds because electrostatic attraction falls off in proportion to the
square of the distance. (eg. Al2O3
[corundum=9] and Fe2O3 [hematite=6]).
| Talc | 1 |
| Gypsum | 2 |
| Calcite | 3 |
| Fluorite | 4 |
| Apatite | 5 |
| Orthoclase | 6 |
| Quartz | 7 |
| Topaz | 8 |
| Corundum | 9 |
| Diamond | 10 |
Hardness of other common Objcects
| Fingernail | 2.5 |
| Copper penny | 3 |
| Glass | 5.5 |
To figure out the hardness of a mineral, try to scratch it with an object of known hardness. For instance, if a fingernail can't scratch an unknown minera, we know it has a hardness of more than 2.5. If the unknown mineral can't scratch glass, we know it has a hardness of less than 5.5
H. Streak
1. Fine-grained powder of a substance a
more reliable indicator of color.
2. Streak is determined using a
streak plate (unglazed porcelain tile).
3. More diagnostic for minerals with
metallic luster.
I. Effervescence - gaseous reaction
produced by carbonate when dissolved in HCl; some require heating to start
reaction.
J. Specific Gravity
Specific
gravity is the ratio between the weight of a substance and the weight of an
equal volume of water. E.g., 10 cc of platinum weighs 19 times that
of 10 cc of H2O.
Specific
gravity of a mineral depends on the elements that are present, and their
packing density.
K. Tenacity - Resistance to breaking,
crushing, bending, and tearing.
Minerals
with metallic bonds yield to stress by deforming plastically (malleable).
This is possible because the cations are surrounded by a 'cloud' of mobile
electrons. Thus they can slide past another without being repulsed
electrostatically.
A sectile
mineral can by cut into shavings, and a ductile mineral can be drawn
into a wire.
A flexible
mineral can be bent, but does not regain its original shape the way an elastic
mineral does.
Brittle minerals
break and powder easily.
L. Luminescence
1. Emission of light by a mineral that is
not related to visible light. Similar to color, luminescence is caused by
the excitation of electrons. Most common in the transition metals.
Shorter wavelengths (UV) have greater energy, and excited electrons give off
heat plus light (visible) when they fall back to initial state.
2. Fluorescence - luminescence due
to exposure to ultraviolet light
3. Phosphorescence - luminescence
continues after exciting rays are cut off
Also:
Thermoluminescence and Triboluminescence
M. Radioactivity
1. Some elements are naturally unstable
and spontaneously decay by the release of energy.
2. Can be easily measured using a Geiger
counter or scintillation counter. Also by using photographic film.
N. Piezoelectricity, Pyroelectricity
1. The electrical polarity that develops
along a crystal's polar axis in proportion to a mechanical strain along this
axis.
2. Same as piezoelectricity, but related
to change in temperature.