Study of Rocks1) Field outcrop

observe relationship between rockspreliminary identification of large minerals generalized rock composition and typetake samples

2) Microscopic determinationmineralogytextural relationshipsrock composition, type origin and history

3) Other analytical techniques such as Electron Microprobe, ICPMS,Scanning Electron MicroscopeX-ray diffractionIsotopic analysisMineral spectroscopy

More detailed understanding of origin and history of rock PPM -84.0

-92.0 -100.0

NMR

Petrographic Microscope

Ocular Lens

Objective Lens

Stage

Substage AssemblyIncluding lower polarizer

Light and blue filter

Upper Polarizer

Focus

Thin section

Thin rectangular slice of rock that light can pass through.One side is polished smooth and thenstuck to a glass slide with epoxy resinThe 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

Light travels as an electromagnetic waveIn a solid, liquid or gaseous medium the

electromagnetic light waves interact with the electrons of the atom.

Direction of Travel

(wavelength)

(Amplitude)

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 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 PPL

Isotropic garnet in XPL

Passage of Light

(1) Reflection from an external or internal surface.

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

i r

(2) Refraction The velocity of light depends on the medium through which it passes

Light is an electromagnetic wave which interacts with electronsThe 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

Refractive IndexThe amount of refraction is related to the difference in

velocity of light in each medium.

Refractive index (R.I.) for air is defined as 1The 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 mineralis constant for a mineralis a diagnostic property of the mineralbetween 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 ZussmanRefractive 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

Opaque MineralSulphides and oxidesPPL does not pass throughMinerals looks black in PPL regardless of orientation of

mineral or polarizersMineral cannot be identified in transmitted light; needs

reflected light

Opaque mineral in graniteRotated 45o in PPL

Transparent mineralPPL passes through the 30μm thickness of the thin sectionThe electromagnetic light waves interact with the electrons

in the minerals and slow downThe higher the density of electrons the slower the light

wave travels

CPX in gabbroPPL

Becke LineA white line of light between two minerals allows the Relative

Refractive Index (R.R.I.) to be measuredThis is relative to an adjacent medium which can be glass,

epoxy, or another mineralR.I. epoxy: 1.54 to 1.55

Perthite:Microcline with exsolved albiteshowing Becke Line between the two minerals(PPL)

The edge of the grain acts like a lens distorting the light

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.

ReliefApparent 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

Cleavage Parallel cracks in mineral related to crystal

structure, often diagnostic of a mineralIn thin sections cleavage is developed

during grinding of thin sectionNote how many directions of cleavages

are presentMeasure the angle between cleavages or

between cleavage and some mineral feature e.g. edge of grain, extinction.

Amphiboles e.g. hornblende ~ 54o/126o

Pyroxene e.g. augite ~ 90o;Plagioclase: ~90o

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

Olivine in gabbro (PPL)

Metamict TextureIntense fracturing cause by radiationDisruption of crystal lattice can decrease optical propertiesThe mineral may appear isotropic

Allanite

Zircon

Colour in PPL Due to absorption of selective wavelengths of light by electrons e.g

absorption of red gives a green colourMay be diagnostic of the mineral e.g. green chloriteBeware: biotite and hornblende may be either brown or green

Green chlorite

in granite

Brown biotite

in granite

Green/blue hornblende

in amphibolite

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

a3a1 = a2 = a3

α = β = γ = 90o

NaCl

Between crossed polars

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

Garnetrotated in XPL

IndicatrixAn 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)

n

n

Isotropic Indicatrix

Isotropic MineralsColour 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 commonAlways in extinction between crossed polars

PPL

XPL

Garnet in metasediment