1. Study of Rocks 1) Field outcrop observe relationship between rocks
preliminary identification of large minerals
generalized rock composition and type
take samples
2) Microscopic determination
mineralogy
textural relationships
rock composition, type
origin and history
3) Other analytical techniques such as
Electron Microprobe, ICPMS,
Scanning Electron Microscope
X-ray diffraction
Isotopic analysis
Mineral spectroscopy
More detailed understanding of origin and history of rock
PPM
-84.0
-92.0
-100.0
NMR

2. Petrographic Microscope
Ocular Lens
Petrographic Microscope
Upper Polarizer
Objective Lens
Stage
Focus
Substage Assembly
Including lower polarizer
Light and blue filter

3. Thin section
Thin rectangular slice of rock that light can pass through.
One side is polished smooth and then
stuck to a glass slide with epoxy resin
The other side is ground to 0.03 mm thickness, and then polished smooth.
May be covered with a thin glass cover slip
0.03 mm

4. Properties of Light Light travels as an electromagnetic wave
In a solid, liquid or gaseous medium the electromagnetic light waves interact with the electrons of the atom.
Direction of Travel
(wavelength)
(Amplitude)

5. Plane Polarized light (PPL)
In air, light normally vibrates in all possible directions perpendicular to the direction of travel (A)
Plane Polarized Light vibrates in one plane (B)
PPL is produced by substage polarizer which stops all other vibration directions

6. Crossed Polars
A second polarizer can be inserted above the stage, perpendicular to the substage polarizer.
In air or an isotropic medium, it will stop light from first polarizer
Isotropic garnet in XPL
Isotropic garnet in PPL

7. Passage of Light (1) Reflection from an external or internal surface.
Angle of incidence (i) = angle of reflection (r) i r

8. (2) Refraction
The velocity of light depends on the medium through which it passes
Light is an electromagnetic wave which interacts with electrons
The distribution of electrons are different for each material and sometimes for different directions through a material
When light passes from one medium to another there is a difference in velocity
Light rays apparently bend at the contact
Angle of incidence ≠ Angle of Refraction. i r i r

9. Refractive Index
The amount of refraction is related to the difference in velocity of light in each medium.
Refractive index (R.I.) for air is defined as 1
The absolute refractive index for a mineral (n) is the refraction relative to that in air.
depends on the atomic/crystal structure
is different for each mineral
is constant for a mineral
is a diagnostic property of the mineral
between 1.3 and 2.0
There may be one, two or three values of R.I. depending on the atomic structure of the mineral.

10. Deer, Howie and Zussman
Refractive Indices are listed for rock- forming minerals in D.H.Z. as n (isotropic), ε ω (uniaxial) or α β γ (biaxial).
δ (birefringence) is the maximum difference between values of R.I.
Garnet Group

11. Opaque Mineral Sulphides and oxides PPL does not pass through
Minerals looks black in PPL regardless of orientation of mineral or polarizers
Mineral cannot be identified in transmitted light; needs reflected light
Opaque mineral in granite
Rotated 45o in PPL

12. Transparent mineral
PPL passes through the 30μm thickness of the thin section
The electromagnetic light waves interact with the electrons in the minerals and slow down
The higher the density of electrons the slower the light wave travels
CPX in gabbro
PPL

13. Becke Line
A white line of light between two minerals allows the Relative Refractive Index (R.R.I.) to be measured
This is relative to an adjacent medium which can be glass, epoxy, or another mineral
R.I. epoxy: 1.54 to 1.55
The edge of the grain acts like a lens distorting the light
Perthite:
Microcline with exsolved albite
showing Becke Line between the two minerals
(PPL)

14. To measure relative refractive index of two touching minerals or mineral/epoxy
Use PPL (upper polarizer out)
Partly close the substage diaphragm, reducing light by 50-75%
Slightly raise and lower the microscope stage, observing the movement of the Becke Line at boundary of grain.
When decreasing the distance between the ocular and the stage, (raising the stage) the line moves into the material of lower R.I.

15. Relief
Apparent topographic relief of mineral grains caused by differences in R.I.
Positive relief - high R.I.
Negative relief - low R.I.
R.I. epoxy = 1.54 to 1.55
Apatite
R.I.= 1.624, 1.666
In quartz
R.I. = 1.544, 1.553
PPL

16. Cleavage
Parallel cracks in mineral related to crystal structure, often diagnostic of a mineral
In thin sections cleavage is developed during grinding of thin section
Note how many directions of cleavages are present
Measure the angle between cleavages or between cleavage and some mineral feature e.g. edge of grain, extinction.
Amphiboles
e.g. hornblende ~ 54o/126o
Plagioclase: ~90o
Pyroxene e.g. augite ~ 90o;

17. Fracture:
Irregular cracks not related to atomic structure e.g. olivine
Olivine in gabbro (PPL)

18. Metamict Texture Intense fracturing cause by radiation
Disruption of crystal lattice can decrease optical properties
The mineral may appear isotropic
Zircon
Allanite

19. Colour in PPL
Due to absorption of selective wavelengths of light by electrons e.g absorption of red gives a green colour
May be diagnostic of the mineral e.g. green chlorite
Beware: biotite and hornblende may be either brown or green
Brown biotite in granite
Green chlorite in granite
Green/blue hornblende in amphibolite

20. Isotropic Minerals Isometric (cubic) minerals e.g. garnet, halite
Amorphous materials: glass, epoxy resin, air
Atomic structure is the same is all directions
Light travels through the mineral with equal velocity in all directions
Refractive Index: one value (n) regardless of orientation
NaCl a1 a2 a3
a1 = a2 = a3
α = β = γ = 90o

21. Between crossed polars
Isotropic minerals always look black regardless of orientation of crystal or rotation of stage
Garnet
rotated in XPL

22. Indicatrix
An imaginary figure which indicates the vibration directions and size of refractive index
The length of a semi-axis shows the size of R.I. in that direction through the mineral
For isotropic minerals, R.I. (n) and hence the length of the indicatrix semi-axes are the same for all directions through the mineral
Therefore, the indicatrix for isotropic minerals is a sphere with only one value of R.I. (n)
Isotropic Indicatrix n

23. Isotropic Minerals Colour in PPL may be diagnostic
Absorption of light is the same in all directions so the colour will be the same regardless of orientation of crystal and remains constant when stage is rotated
Cleavage: rare but fracture common
Always in extinction between crossed polars
PPL
Garnet in metasediment
XPL