Extinction angle and Types of Extinction in Minerals.pdf
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Extinction refers to the dimming of light when viewed through a mineral under cross-polarized light in a petrographic microscope. There are four main types of extinction: parallel extinction occurs when the mineral's long axis or cleavage plane is parallel to the vibration direction of light; inclined extinction occurs when these are not parallel in monoclinic and triclinic minerals; symmetrical extinction occurs when the vibration direction bisects two cleavage planes; and some minerals lack distinct cleavage planes and therefore have no measurable extinction angle. The extinction angle is the amount of rotation required for a mineral to become extinct and is a diagnostic characteristic.
Extinction angle and Types of Extinction in Minerals.pdf
- 1. EXTINCTION INTRODUCTION EXTINCTION ANGLE TYPES OF EXTINCTION Preparedby:AasishGiri [email protected]
- 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.