|
|
|
|
Physical Properties of Crystals @ Jewel Info 4 U
By:Ritika
Summary: The physical
properties of crystals like
hardness, cleavage, optical
properties, heat and electrical
conductivity differ from crystal to
crystal. Crystallographers deem it
necessary to learn more about these
properties so that they can
determine what the crystals can be
used for.
Crystallography is the scientific
study of crystals where the
arrangement of atoms and molecules
in solid matter are determined. With
the developments made in this field
of science, crystallographers have
found that crystals have numerous
physical properties. It is important
to know that all crystals do not
possess the same properties and
hence crystals have been classified
into different classes and groups.
Crystals have hence been divided
into 32 different classes to make
studying about them easier. Some of
the important physical properties of
crystals are discussed below.
 Color
: The color in crystals appears
as some wavelengths of light are
absorbed by the solid matter. Some
metals like chrome and iron and some
others have color as they are able
to absorb these particular
wavelengths of light. Color is not a
very good property to use for
identification of crystals as many
different kinds of matter are of the
same or similar color and can hence
cause confusion.
Refraction of light: There is
a change in the refraction of light
through materials. The velocity at
which light passes through a given
material is inversely related to its
index of refraction. For a vacuum, n
= 1.0. It is found that in most of
the minerals the range of the index
of refraction is between 1.4 and
3.2.
Birefringence: This property
is when there are two different
refractive indices of light that are
noticed, which cross-polarized when
it enters matter. It is noticed that
if one ray enters matter, there are
two rays that emerge, these are
called the ordinary and
extraordinary rays. Should the
material be rotated, the ordinary
ray will remain still and the
extraordinary ray makes a circle
around the ordinary ray. This effect
is easily seen in Calcite and Sodium
Nitrate. The extraordinary rays
which exit always display
polarization at right angles.
Dispersion: This is the
property when the refracted indices
of light spread through a material
in such a manner that it causes a
variation in the wavelength from red
to violet. When the dispersion of
light in the material is larger,
there is a greater amount of white
light separated when the rays are
exiting from the particular
material. A good example is Diamond,
where the dispersion of light is
extremely great, hence one can see
so many colors in this crystal.
Color Change: This property
is of two types:
One type of color change is
Pleochroism in which the material
seems to possess many different
colors when it is looked at from
different directions. When two
colors are exhibited the effect is
know as Dichroism. When three colors
are exhibited the effect is called
Trichorism. The mineral Iolite
(cordierite) changes colors from
dark blue to colorless, hence it is
an example of dichroism.
The other type of change in color is
noticed by the existence of
artificial light, natural light,
fluorescent light or even
incandescent light. This type of
change in color is noticed in nickel
sulfate making it look like
Alexandrite, which is a priceless
gemstone.
Polarization of light: This
property can be seen in some
materials. This property is found
naturally in tourmaline. As it is
believed to be placed between what
is called a "polarization sandwich,"
sodium chlorate allows different
colors through it, since there are
changes in the angles between the
polarizers.
Cleavage: This property
refers to the ability of a crystal
to break along certain specific
planes with a lot more ease when
compared with the other directions.
Naturally crystals can break either
perfectly or imperfectly. There is a
marked difference between these two
forms of cleavage. A perfect
cleavage is one that will always
break on the cleavage plane, which
is found in fluorite, calcite and
diamond. This property of the
diamond makes cutting the gem very
challenging as the planes the person
is cutting along may not really be
the cleavage planes. On the other
hand imperfect cleavage is one in
which it can break along any plane.
Quartz and beryl are good examples.
When quartz breaks, some of the
broken pieces look very similar to
glass.
Piezoelectric Effect: This is
that physical property found in a
compressed crystal, which causes a
flow of charge and a drop in voltage
across the opposite poles in the
crystal. These materials are easily
used in communication equipment.
This effect is easily seen in both
Rochelle salt as well as natural
quartz, which are known to be able
to supply voltage when mechanical
force is applied. Materials like
germanium, silicon, galena and
silicon carbide are used as semi
conductors as they carry current
unequally in different directions.
Of the 32 classes of crystals 20 are
piezoelectric. Crystals of the
piezoelectric classes lack a center
of symmetry. When an electric field
is applied to any material it
develops a dielectric polarization.
Materials that naturally have a
charge separation are called polar
materials. The structure of the
crystal determines whether it is
polar or not. Of the 32 classes only
10 have polar crystals. Polar
crystals are all pyroelectric and
hence these ten classes are
generally referred to as the
pyroelectric classes.
Some crystal structures display
ferroelectric behavior.
Ferromagnetism, which is the similar
to ferroelectric behavior, is a
property where due to the electric
field being absent, polarization is
not displayed by the ferroelectric
crystal. In the presence of an
electric field the ferroelectric
crystal displays permanent
polarization. With the application
of a large counter charge this
polarization can be reversed just
like it is reversed in a
ferromagnetic. It is important to
note here that though this effect is
called ferroelectric, there is no
presence of the ferrous metal and
this effect is produced by the
structure of the crystal.
The physical properties of crystals
have been spelt out in detail in the
above paragraphs. Crystallographers
have made the study of crystals
simple and convenient based on these
properties.
Related Links :
|
 |
Belly Button Piercing
The Egyptians, the Indians and many other cultures pierced this humble button to show the significance of the person sporting a pierced navel - Royalty, Warriors, exceptional Beauty. |
|
|
|
|
|
|
|
