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OPTICAL PROPERTIES OF ROCK FORMING MINERAL BY KULDIP KUMAR (47/13) MANISH KUMAR (78/13) RAHUL KUMAR(80/13)

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Presentation on theme: "OPTICAL PROPERTIES OF ROCK FORMING MINERAL BY KULDIP KUMAR (47/13) MANISH KUMAR (78/13) RAHUL KUMAR(80/13)"— Presentation transcript:

1 OPTICAL PROPERTIES OF ROCK FORMING MINERAL BY KULDIP KUMAR (47/13) MANISH KUMAR (78/13) RAHUL KUMAR(80/13)

2 Rock forming minerals  A rock forming mineral is a mineral that is common in all different types of rocks. Minerals that form first under the given composition of magma /lava.

3 OPTICAL PROPERTIES The properties of a minerals which are related to the behaviour of light while being transmitted through or reflected from it are grouped under optical properties. They are of great significance in the determinative mineralogy and highly useful in identifying individual minerals occurring in very small, physically inconspicuous grain size, as for examples in rocks.

4 The Petrographic Microscope Also called a polarizing microscope

5 Why use the microscope?? Identify minerals (no guessing!) Determine rock type Determine crystallization sequence Document deformation history Observe frozen-in reactions Note weathering/alteration Fun, powerful, and cheap!

6 1) Light passes through the lower polarizer west (left) east (right) Plane polarized light PPL=plane polarized light Unpolarized light Only the component of light vibrating in E-W direction can pass through lower polarizer – light intensity decreases

7 2) Insert the upper polarizer west (left) east (right) Now what happens? What reaches your eye? Why would anyone design a microscope that prevents light from reaching your eye??? XPL=crossed nicols (crossed polars) south (front) north (back) Black!!

8 3) Now insert a thin section of a rock west (left) east (right) Light vibrating E-W Light vibrating in many planes and with many wavelengths How does this work?? Unpolarized light Light and colors reach eye!

9 Conclusion has to be that minerals somehow reorient the planes in which light is vibrating; some light passes through the upper polarizer But, note that some minerals are better magicians than others (i.e., some grains stay dark and thus can’t be reorienting light) Minerals act as magicians!!

10 Quartz (SiO 2 ) Optical properties of Quartz :- In Plane Polarized Light a. Colour : Colourless through various colours to black b. Shape : Trigonal and Hexagonal c. Cleavage : {0110} Indistinct d. Pleochroism : None Under Crossed Nicols a. Twinning : Common in {0001} b. Interference colour : low order c. Extinction angle : < 25 d. Uniaxial(+)

11 Quartz crystals in plane polarized light Same quartz crystals with analyzer inserted (cross polarizers aka crossed nicols) Setting #1: No upper analyzer Setting #2: Upper analyzer inserted

12 Plagioclase (NaAlSi 3 O 8 -CaAl 2 Si 2 O 8 ) Optical properties of Plagioclase :- In Plane Polarized Light a. Colour : Colourless in thin section b. Shape : Triclinic c. Cleavage : (001)&(110) d. Pleochroism : None Under Crossed Nicols a. Twinning : Common in {001} b. Interference colour : low order c. Extinction angle : < 25 d. Biaxial(+) (-)

13 Pyroxene (RSiO 3 ), R= Na,Ca,Al Optical properties of Pyroxene :- In Plane Polarized Light a. Colour : Pale green b. Shape : Prism c. Cleavage : {011} Indistinct d. Pleochroism : Commonly Strong Under Crossed Nicols a. Twinning : Common in {0001} b. Interference colour : moderate order c. Extinction angle : > 25 d. Uniaxial(+)

14 Muscovite KAl 2 (AlSi 3 O 10 )(OH) 2 Optical properties of Muscovite :- In Plane Polarized Light a. Colour : White, grey & silvery b. Shape : Monoclinic c. Cleavage : Perfect on {001} d. Pleochroism : Weak when colored Under Crossed Nicols a. Twinning : Common on [310],less common on {001} b. Biaxial(-) c. Interference colour : moderate- high order d. Extinction Angle : 0

15 BIOTITE K(Mg,Fe) 3,(AlSi3O 10 )(OH) 2 Optical properties of Biotite :- In Plane Polarized Light a. Colour : Dark brown, blackish, yellow, white b. Shape : Monoclinic c. Cleavage : Highly perfect and basal d. Pleochroism : Strong Under Crossed Nicols a. Twinning : Common on [310],less common on {001} b. Biaxial(-) c. Extinction angle : 0 d. Interference colour : moderate high order

16 THANK YOU


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