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PPM. -84.0. -92.0. -100.0. 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

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Study of rocks





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


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


Petrographic microscope
Petrographic Microscope

Ocular Lens

Upper Polarizer

Objective Lens



Substage Assembly

Including lower polarizer

Light and blue filter

Thin section
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

Properties of light



Direction of Travel

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.

Plane polarized light ppl
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

Crossed polars
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

Passage of light



Passage of Light

(1) Reflection from an external or internal surface.

Angle of incidence (i) = angle of reflection (r)

2 refraction
(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.





Refractive index
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.

Deer howie and zussman
Deer, Howie and Zussman

Refractive Indices are listed for rock- forming minerals in D.H.Z. as n (isotropic),εω(uniaxial) or αβγ(biaxial).

δ(birefringence) is the maximum difference between values of R.I.

Garnet Group

Opaque mineral
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

Transparent mineral
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


Becke line
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


Microcline with exsolved albite

showing Becke Line between the two minerals


To measure relative refractive index of two touching minerals or mineral epoxy
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.

Relief minerals or mineral/epoxy

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


R.I.= 1.624, 1.666

In quartz

R.I. = 1.544, 1.553


Cleavage minerals or mineral/epoxy

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.


e.g. hornblende ~ 54o/126o

Plagioclase: ~90o

Pyroxene e.g. augite ~ 90o;

Fracture: minerals or mineral/epoxy

Irregular cracks not related to atomic structure e.g. olivine

Olivine in gabbro (PPL)

Metamict texture
Metamict Texture minerals or mineral/epoxy

Intense fracturing cause by radiation

Disruption of crystal lattice can decrease optical properties

The mineral may appear isotropic



Colour in ppl
Colour in PPL minerals or mineral/epoxy

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

Isotropic minerals

a minerals or mineral/epoxy1



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


a1 = a2 = a3

α = β = γ = 90o

Between crossed polars

Garnet minerals or mineral/epoxy

rotated in XPL

Between crossed polars

Isotropic minerals always look black regardless of orientation of crystal or rotation of stage


n minerals or mineral/epoxy



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

Isotropic minerals1

XPL minerals or mineral/epoxy


Isotropic Minerals

Colourin 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

Garnet in metasediment