Student of Central Department of Geology Tribhuvan University contact : giriaasish1@gmail.com

1. EXTINCTION
INTRODUCTION
EXTINCTION ANGLE
TYPES OF EXTINCTION
Preparedby:AasishGiri
giriaasish1@gmail.com

2. INTRODUCTION : EXTINCTION
• When cross polarized light dims as viewed through a thin section of
a mineral with a petrographic microscope, that phenomena is
called “extinction.”
• Isotropic minerals and substances (like glass) are completely extinct under
cross polars.
• Anisotropic minerals specifically will show one extinction for each 90 degrees
of stage rotation.

3. EXTINCTION ANGLE
• The angle between the length or a prominent cleavage in a mineral and a vibration
direction is a diagnostic property called the extinction angle. Or the angle through
which a crystal is revolved from a definite line (such as that of the
crystallographic axis) to the plane of maximum extinction.
• To determine the extinction angle Rotate the stage of the microscope until the length or
cleavage of the mineral is aligned with the north-south cross hair.
• The extinction angle is the measure between the cleavage direction or habit of
a mineral and the extinction.

4. TYPES OF EXTINCTION
Four major types of extinction are:
1. Parallel Extinction
2. Inclined Extinction
3. Symmetrical Extinction
4. No Extinction Angle

6. 1. PARALLEL EXTINCTION
• Extinction is said to be parallel if the long direction of the crystal or a prominent
cleavage plane is oriented N-S or E-W when the mineral goes extinct.
• 90 degrees would be considered zero degrees, and is known as parallel extinction.
• Only occurs in minerals where the crystallographic axes are at 90 degrees to one
another (Hexagonal, Tetragonal, and Orthorhombic).
• Either the slow ray or fast ray vibration direction is parallel to the trace of cleavage or
length of the mineral.

7. 2. INCLINED EXTINCTION
• It occurs when the elongate direction of the mineral or the direction of
cleavage is NOT E-W or N-S when the mineral goes extinct.
• This type of extinction occurs in minerals where all of the crystallographic
axes are not perpendicular (Monoclinic and Triclinic).
• 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.

8. 3. SYMMETRICAL EXTINCTION
• The phenomenon which occurs when the vibration direction of the
light bisects the angles between two sets of cleavages (as seen in
basal sections of pyroxenes and amphiboles).
• Symmetrical extinction may be observed in minerals that display either two
cleavages or two distinct crystal faces.

9. 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 situations we say that the mineral has no extinction angle.

10. REFRENCES
• https://www.google.com/amp/s/slideplayer.com/amp/4693021/
• https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-
releases/symmetrical-
extinction#:~:text=symmetrical%20extinction%20In%20optical%20mineralogy,sections%20of%20pyroxe
nes%20and%20amphiboles).
• https://hilo.hawaii.edu/~kenhon/mineralogy/lab-syllabus/opt-elong/L-extinction-elongation.htm
• https://en.m.wikipedia.org/wiki/Extinction_(optical_mineralogy)#:~:text=Extinction%20is%20a%20term
%20used,mineral%20in%20a%20petrographic%20microscope.&text=The%20extinction%20angle%20is%
20the,a%20mineral%20and%20the%20extinction
• Gribble, C.D. and Hall, A.J. 1985. A Practical Introduction to Optical Mineralogy. George Allen &
Unwin (Publishers) Ltd., 249 pp.