1. OPTICAL PROPERTIES OF ROCK FORMING MINERALBY
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)
Unpolarized light
Plane polarized light
Only the component of light vibrating in E-W direction can pass through lower polarizer – light intensity decreases
PPL=plane polarized light

7. 2) Insert the upper polarizer
south (front)
north (back)
Black!!
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)

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

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

10. Quartz (SiO2) Optical properties of Quartz :-
In Plane Polarized Light
Colour : Colourless through various colours to black
Shape : Trigonal and Hexagonal
Cleavage : {0110} Indistinct
Pleochroism : None
Under Crossed Nicols
Twinning : Common in {0001}
Interference colour : low order
Extinction angle : < 25
Uniaxial(+)

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

12. Plagioclase (NaAlSi3O8-CaAl2Si2O8) Optical properties of Plagioclase :-
In Plane Polarized Light
Colour : Colourless in thin section
Shape : Triclinic
Cleavage : (001)&(110)
Pleochroism : None
Under Crossed Nicols
Twinning : Common in {001}
Interference colour : low order
Extinction angle : < 25
Biaxial(+) (-)

13. Pyroxene (RSiO3) , R= Na,Ca,Al Optical properties of Pyroxene :-
In Plane Polarized Light
Colour : Pale green
Shape : Prism
Cleavage : {011} Indistinct
Pleochroism : Commonly Strong
Under Crossed Nicols
Twinning : Common in {0001}
Interference colour : moderate order
Extinction angle : > 25
Uniaxial(+)

14. Muscovite KAl2(AlSi3O10)(OH)2 Optical properties of Muscovite :-
In Plane Polarized Light
Colour : White , grey & silvery
Shape : Monoclinic
Cleavage : Perfect on {001}
Pleochroism : Weak when colored
Under Crossed Nicols
Twinning : Common on [310],less common on {001}
Biaxial(-)
Interference colour : moderate- high order
Extinction Angle : 0

15. BIOTITE K(Mg,Fe)3,(AlSi3O10)(OH)2 Optical properties of Biotite :-
In Plane Polarized Light
Colour : Dark brown , blackish , yellow , white
Shape : Monoclinic
Cleavage : Highly perfect and basal
Pleochroism : Strong
Under Crossed Nicols
Twinning : Common on [310],less common on {001}
Biaxial(-)
Extinction angle : 0
Interference colour : moderate high order

16. THANK YOU