INTRODUCTION
In optical mineralogy, mineral extinction refers to the complete loss of visible light transmission through a mineral when it is viewed under a polarizing microscope with crossed polarisers. This occurs when the mineral is oriented at an angle that is perpendicular to the plane of polarization of the transmitted light, which causes the mineral to appear completely dark against a bright background.
Mineral extinction is an
important optical property of minerals that is used in the identification and
characterization of minerals. It can provide information about the
crystallographic orientation and structural features of the mineral, as well as
its optical properties such as birefringence and Pleochroism.
In optical mineralogy, the angle of
mineral extinction is measured relative to the vibration direction of the
polarized light. The angle of extinction can be used to determine the
crystallographic orientation of the mineral within the sample, as well as its
symmetry and optical behavior.
Mineral extinction can occur in
different directions depending on the crystal structure of the mineral, and the
direction of extinction can provide important information about the
crystallographic axes and symmetry of the mineral.
Overall,
mineral extinction is an important property of minerals in optical mineralogy
that provides valuable information about their crystallographic orientation,
structure, and optical properties.
IMPORTANCE
Mineral
extinction is an important concept in optical mineralogy because it is one of
the key optical properties that can be used to identify minerals under a
polarizing microscope. When a thin section of a rock is placed under crossed
polarisers, the minerals in the rock interact with the light in specific ways,
producing a range of colors and patterns.
Mineral
extinction occurs when a mineral is oriented at an angle that is perpendicular
to the plane of polarization, causing it to appear dark or black against a bright
background. This effect is useful in identifying minerals because it is a
characteristic property of each mineral that can be used to distinguish it from
other minerals.
By observing
the angle and direction of mineral extinction under a polarizing microscope,
petrologists can identify minerals in rocks and determine their
crystallographic orientation and structural features. This information is then
used to interpret the geological history of the rocks and the conditions under
which they were formed. In addition to aiding in mineral identification and
interpretation of geological history, mineral extinction can also be used to
determine the composition and texture of rocks, as well as to investigate the
physical and chemical properties of minerals.
Overall,
mineral extinction is an essential concept in optical mineralogy, providing
valuable information about the properties and characteristics of minerals that
are essential for the study of rocks and the Earth's geology.
EXTINCTION ANGLE AND TYPES
In optical
mineralogy, mineral extinction angle refers to the angle between the
crystallographic axis of a mineral and the direction of vibration of the
polarized light used in the microscope. When a mineral is placed in a
polarizing microscope, it will appear bright or dark depending on the angle at
which the crystal is oriented relative to the polarization direction of the
light.
When a mineral
is oriented such that the crystallographic axis is parallel to the vibration
direction of the polarized light, it will appear bright, indicating that it is
transmitting light. However, when the mineral is oriented at an angle that is
perpendicular to the vibration direction of the polarized light, it will appear
dark, indicating that it is not transmitting light. This angle at which the
mineral appears dark is known as the extinction angle.
There are four categories of mineral extinction angles:
- Parallel Extinction
- Inclined Extinction
- Symmetrical Extinction
Fig : Different kinds of extinction angles
1. Parallel Extinction
With parallel extinction the mineral is extinct when the cleavage or length is aligned parallel to one of the cross hairs.
The extinction
angle is 0°.
Either the
slow ray or fast ray vibration direction is parallel to the trace of cleavage
or length of the mineral.
2. Inclined Extinction
With inclined
extinction the mineral is extinct when the cleavage of length is aligned
parallel to one of the cross hairs.
The extinction
angle will be greater than 0°.
Neither
vibration direction is aligned parallel to the trace of the cleavage of the
length of the mineral.
If the slow
ray vibration direction is closest to the length or trace of cleavage, the
mineral is length slow. If the fast ray is closest the mineral is length fast.
3. Symmetrical Extinction
Symmetrical
extinction may be observed in minerals that display either two cleavages or two
distinct crystal faces.
If the
extinction angles EA1 and EA2 measured from the two cleavage or crystal faces
to the same vibration direction, are the same, extinction is symmetrical.
4. No Extinction Angle
Many minerals
lack distinct cleavages or do not display an elongation or crystal faces.
Although they
go extinct once every 90° of stage rotation, there is no cleavage, elongation
or crystal face from which to measure an extinction angle.
In these
situation we say that the mineral has no extinction angle.
5.Other Types of Extinction
In addition to
the before mentioned types of extinction, some minerals may not go totally
extinct at any stage position.
Usually this
is the result of strain in the crystal lattice chemical zoning.
In many deformed
rocks, the mineral grains are bent or distorted so that different parts of the
grain go extinct at different times. If the extinction follows an irregular or
wavy pattern it is called undulose extinction.
Many minerals grow so that they are compositionally zoned. Because extinction angle may be controlled by chemical composition in mono-clinic and tri-clinic minerals, the extinction angle may vary systematically with composition so that the center of the grain may display one extinction angle and the rim another.
CONCLUSION
The concept of
mineral extinction in petrology refers to the point at which a particular
mineral phase ceases to exist in a rock due to a change in its chemical or
physical environment. This can occur during various geological processes such
as metamorphism, alteration, or weathering.
The
consequences of mineral extinction can be significant for the interpretation of
the rock's history and the determination of its origin. For example, the
presence or absence of certain minerals can provide clues about the temperature
and pressure conditions under which the rock formed.
Furthermore,
the extinction of specific minerals can affect the rock's physical and
mechanical properties. For instance, the disappearance of mica from a rock can
result in a reduction in its shear strength.
In conclusion,
the concept of mineral extinction is an important aspect of petrology that
plays a crucial role in understanding the processes that shape the Earth's
crust and the rocks that make it up.
