Optical mineralogy examines the interaction of minerals with visible light.

The differences in interaction can be used to

(a) identify minerals

(b) derive sometimes the conditions in which they have formed

The interaction between each mineral and visible light depends on the internal crystal structure of the mineral = optical crystallography

Optical Mineralogy

Electromagnetic spectrum & visible portion

Violet (400 nm) Red (700 nm)

White = ROYGBV(can be separated by dispersion)

Wave Theory

Wavelength = Distance between peaks

Frequency = no. of waves/sec to pass a given point (hertz)

frequency = v/ , v = velocity

Electromagnetic waves (visible light, X-rays, etc.)

f

Pertinent Units

Wavelength: nm (=mμ) = 10-9m;

old units, Å = 10-8m

Frequency: hz (cycles/sec)

Energy units (at the atomic scale) are also sometimes used. Energy in eV = ћν, where ћ is Planck,s constant. 1Å is equivalent to 1.24x10-24eV.

Polarization

Non-polarized (“usual”) light:

Each photon vibrates as a wave form in a single plane. Vibration refers to the rise and fall of the sine wave.

Light beam is composed of numerous photons, each vibrating in a different plane.

Vibration occurs in all directions ~ perpendicular to propagation direction

Plane of interface = surface of crystalPlane of interface = surface of crystal

Plane of incidence = T-N-U-V-M-S-O Plane of incidence = T-N-U-V-M-S-O

Plane of incidence contains incident, reflected and refracted ray Plane of incidence contains incident, reflected and refracted ray paths as well angles i, l and rpaths as well angles i, l and r

Polarization

Polarization

Note symbolization used:

Reflected ray polarized perpendicular to the plane of incidence.

Refracted ray polarized in plane of incidence.

reflected and refracted raysboth become polarized

incoming ray is non-polarized

Interference

A: Particles in phase if displaced from rest position by same amount in same direction– a1 - a2 - a3 are all in phase– b1 - b2 - b3 are also all in phase (but not with a1…)

• particles perfectly out of phase: equal-but-opposite displacement• b1 and c1 are not perfectly out of phase

Fig 7-1 Bloss, Optical Crystallography, MSA

Interference

path difference () = distance between any 2 points on a wave form

• usually expressed as x between any 2 points in phase = i (i=any integer) between any 2 points perfectly out of phase = ((2i+1)/2)

Fig 7-1 Bloss, Optical Crystallography, MSA

PolarizationOur microscopes have two polarizers:– polarizer (below stage) is E-W– analyzer (above stage) is N-S

OcularOcular

Bertrand Amici lensBertrand Amici lens

AnalyzerAnalyzer

ObjectiveObjective

Microscope stageMicroscope stageCondenser lensCondenser lens

PolarizerPolarizer

Light source with Light source with blue filterblue filter

Iris diaphragmIris diaphragm

RefractionIncident ray and reflected ray:

1) of incidence i = of reflection r

2) coplanar in the plane of incidence

Refracted ray:1) Slower in water or glass

2) r (refraction) i (incidence)

Depends on v

IncidentIncident

ReflectedReflectedairair

RefractedRefracted

water

iirr

rr

For a substance x:nx = vvacuum/vx

nair = 1.0003 light is slower in water, glass, crystalsIs nwater greater or less than 1?Larger n associated with slower v !

Snell’s Law (1621): ni sin i = nr sin r

for two known media (air/water), sin i/sin r = nr / ni = const.

So can predict angle change (or use it to determine nr)

Index of refractionIndex of refraction

Refractive index of balsam: 1.537Refractive index of balsam: 1.537

Relief of minerals in thin sections:Relief of minerals in thin sections:

Minerals in thin sections:Minerals in thin sections:n >>> 1.537 high relief (positive relief)n >>> 1.537 high relief (positive relief)n = 1.5-1.6, low reliefn = 1.5-1.6, low reliefn<<<1.537 high relief (negative relief)n<<<1.537 high relief (negative relief)

First observation parameter of minerals in thin sections!!!: First observation parameter of minerals in thin sections!!!: reliefrelief

ReliefRelief

Second observation parameter of minerals in thin sections!!!: Second observation parameter of minerals in thin sections!!!: refractive index smaller or larger than 1.537 refractive index smaller or larger than 1.537 or index of Mineral A is larger or smaller than index of Mineral Bor index of Mineral A is larger or smaller than index of Mineral B

MicroscopeMicroscoperaisedraisedupwardsupwards

Becke line methodPositive relief Negative relief

The Optical Indicatrix

When analyzer inserted = crossed-nicols or XPL shorthand (vs PPL) no light passes

extinct, even when the stage is rotated

Fig. 6-6

East

West

North South

P P

PP

A

AFig 6-6 Bloss, Optical Crystallography, MSA