Optical MineralogyOptical Mineralogy Technique utilizing interaction of Technique utilizing interaction of

polarized lightpolarized light with minerals with minerals Uses a Uses a polarizing microscopepolarizing microscope Oils - Grain mountsOils - Grain mounts Thin sections – rocksThin sections – rocks

Primary way to observe mineralsPrimary way to observe minerals Important:Important:

cheap, quick, easycheap, quick, easy Only way to determine texturesOnly way to determine textures

Why use microscopes?Why use microscopes?

Visual properties for ID – e.g. textureVisual properties for ID – e.g. texture Color – may be variableColor – may be variable Cleavage (may not see, often controls shape)Cleavage (may not see, often controls shape) Shape (depends on cut of mineral)Shape (depends on cut of mineral)

Only observable with microscopeOnly observable with microscope Separate isotropic and anisotropic Separate isotropic and anisotropic

minerals and many other optical minerals and many other optical propertiesproperties

Polarizing MicroscopePolarizing Microscope

Polarizer, typically oriented N-S

Objective

Accessory Slot

Analyzer, upper polarizer, nicols lens

Bertrand lens

Ocular

Slightly more modern Slightly more modern versionversion

conoscope

Internal light source, polarized

Trinocular head

Reflected light source

Vernier scale

Analyzer, upper polarizer, nicols lens

Accessory plate

Objectives

Four common settings for microscopic Four common settings for microscopic observations of thin sections:observations of thin sections:1.1. Plane polarized light, analyzer (upper Plane polarized light, analyzer (upper

polarizer, nicols lens) outpolarizer, nicols lens) out

2.2. Plane polarized light, analyzer in (cross Plane polarized light, analyzer in (cross nicols)nicols)

3.3. Conoscopic polarized light, bertrand lens inConoscopic polarized light, bertrand lens in

4.4. Conoscopic polarized light, bertrand lens Conoscopic polarized light, bertrand lens in, gypsum plate in accessory slotin, gypsum plate in accessory slot

Quartz crystals in plane polarized light

Same quartz crystals with analyzer inserted (cross polarizers aka crossed nicols)

Setting #1: No upper analyzer Setting #2: Upper analyzer inserted

Setting # 3: Conoscopic polarized light, bertrand lens in, highest magnification

Setting #4: Conoscopic polarized light, bertrand lens in, gypsum plate in accessory slot, highest magnification

Characteristics of lightCharacteristics of light Electromagnetic energyElectromagnetic energy

derived from excess energy of electronsderived from excess energy of electrons Energy released as electrons drop from Energy released as electrons drop from

excited state to lower energy shells – excited state to lower energy shells – perceived as “light”perceived as “light”

Particle, Wave or bothParticle, Wave or both Particles = photonsParticles = photons For mineralogy, consider light a waveFor mineralogy, consider light a wave Important wave interference phenomenonImportant wave interference phenomenon

Light as waveLight as wave Energy vibrates perpendicular to direction Energy vibrates perpendicular to direction

of propagationof propagation Light has both electrical and magnetic Light has both electrical and magnetic

energyenergy Two components vibrate perpendicular to Two components vibrate perpendicular to

each othereach other Electrical component interacts with Electrical component interacts with

electrical properties of minerals, e.g. bond electrical properties of minerals, e.g. bond strength, electron densitiesstrength, electron densities

Fig. 7-2Fig. 7-2

Electric vibration direction

Magnetic vibration direction

For mineralogy – we’ll only consider the electrical component

Properties of lightProperties of lightWavelength

Velocity

Amplitude

Relationship and units of propertiesRelationship and units of properties = = wavelengthwavelength, unit = L, color of light, unit = L, color of light A = A = amplitudeamplitude, unit = L, intensity of light, unit = L, intensity of light v = v = velocityvelocity, unit = L/t, property of , unit = L/t, property of

materialmaterial f = f = frequencyfrequency – e.g. how often a wave – e.g. how often a wave

passes a particular point, unit = 1/tpasses a particular point, unit = 1/t f = v/f = v/frequency is constant, v and frequency is constant, v and

variablevariable

Fig. 6-6Fig. 6-6

Visable light spectrum

Full ra

nge o

f ele

ctrom

agn

etic ra

dia

tion

(nm) f (hertz)

1 nm = 10-9 m

1 Å

100 Å

If two light waves vibrate at an angle If two light waves vibrate at an angle to each other:to each other: Vibrations interfere with each otherVibrations interfere with each other Interference creates a new waveInterference creates a new wave Direction determined by vector additionDirection determined by vector addition

Vibration directions of single wave Vibration directions of single wave can be split into various componentscan be split into various components Each component has different vibration Each component has different vibration

directiondirection

Fig. 7-3Fig. 7-3

Two light waves A & B interfere to form resultant wave R

One light wave X has a component V at an angle

Note – two waves have the same v and

Electrical components only

Light composed of many wavesLight composed of many waves Wave front Wave front = connects same point on = connects same point on

adjacent wavesadjacent waves Wave normalWave normal = line perpendicular to = line perpendicular to

wave frontwave front Light ray (Ray path)Light ray (Ray path) = direction of = direction of

propagation of light energy, e.g. propagation of light energy, e.g. direction of path of photondirection of path of photon

Note: wave normal and light ray are Note: wave normal and light ray are not necessarily parallelnot necessarily parallel

Fig. 7-2cFig. 7-2c

Wave front connects common points of multiple waves

It is the direction the wave moves

Ray path is direction of movement of energy, e.g., path a photon would take

Wave normal and ray path not always parallel

Fig. 7-2d Fig. 7-2d and eand e

Wave normal and ray paths may be coincident

Propogation of light through Isotropic material

Wave normal and ray paths may not be coincidentPropogation of light through Anisotropic material

IsotropicIsotropic materials materials Wave normals and ray paths are parallelWave normals and ray paths are parallel Velocity of light is constant regardless of Velocity of light is constant regardless of

direction in these mineralsdirection in these minerals AnisotropicAnisotropic materials materials

Wave normals and ray paths are not parallelWave normals and ray paths are not parallel Velocity of light is variable depending on Velocity of light is variable depending on

direction of wave normal and ray pathdirection of wave normal and ray path These difference have major consequences These difference have major consequences

for interaction of light and materialsfor interaction of light and materials

Birefringence Birefringence demonstration?????????demonstration?????????

Polarized and Non-polarized Polarized and Non-polarized LightLight

Non-polarized lightNon-polarized light Vibrates in all directions perpendicular to direction of Vibrates in all directions perpendicular to direction of

propagationpropagation Occurs only in isotropic materialsOccurs only in isotropic materials

Air, water, glass, etc.Air, water, glass, etc.

Fig. 7-4

Non-Polarized LightNon-Polarized Light Light vibrates in all directions Light vibrates in all directions

perpendicular to ray pathperpendicular to ray path

Fig. 7-4

Multiple rays, vibrate in all directions

Highly idealized – only 1 wavelength

Polarized lightPolarized light Vibrates in only one planeVibrates in only one plane Generation of polarized light:Generation of polarized light:

In anisotropic material, light In anisotropic material, light usuallyusually resolves into two resolves into two raysrays

Two rays vibrate perpendicular to each otherTwo rays vibrate perpendicular to each other The energy of each ray absorbed by different amountsThe energy of each ray absorbed by different amounts If all of one ray absorbed, light emerges vibrating in If all of one ray absorbed, light emerges vibrating in

only one directiononly one direction Called Called “Plane Polarized Light”“Plane Polarized Light”

Fig. 7-Fig. 7-4b4b

Polarized light vibrates in only one plane: “Plane-polarized light”

Anisotropic medium: light split into two rays. One fully absorbed

Polarization also caused by Polarization also caused by reflection:reflection: ““Glare”Glare” Raybans cut the glareRaybans cut the glare

Interaction of light and Interaction of light and mattermatter

Velocity of light depends on material Velocity of light depends on material it passes throughit passes through In vacuum, v = 3.0 x 10In vacuum, v = 3.0 x 1017 17 nm/sec = 3.0 x nm/sec = 3.0 x

10108 8 m/secm/sec All other materials, v < 3.0 x 10All other materials, v < 3.0 x 101717

nm/secnm/sec

When light passes from one material When light passes from one material to anotherto another f = constantf = constant If v increases, If v increases, also must increase also must increase If v decreases, If v decreases, decreases decreases

f = v/

Vair > Vmineral

Isotropic vs. AnisotropicIsotropic vs. Anisotropic

Isotropic geologic materialsIsotropic geologic materials Isometric mineralsIsometric minerals; also glass, liquids and ; also glass, liquids and

gasesgases Electron density identical in all directionsElectron density identical in all directions

Think back to crystallographic axesThink back to crystallographic axes Direction doesn’t affect the electrical Direction doesn’t affect the electrical

property of lightproperty of light Light speed doesn’t vary with directionLight speed doesn’t vary with direction Light NOT split into two raysLight NOT split into two rays

Anisotropic geologic materials:Anisotropic geologic materials: Minerals in Minerals in tetragonal, hexagonal, tetragonal, hexagonal,

orthorhombic, monoclinic and triclinic orthorhombic, monoclinic and triclinic systemssystems

Interactions between light and electrons differ Interactions between light and electrons differ depending on directiondepending on direction

Light split into two rays – vibrate Light split into two rays – vibrate perpendicular to each otherperpendicular to each other

Light speed depends on direction of ray and Light speed depends on direction of ray and thus thus vibration directionvibration direction

Reflection and RefractionReflection and Refraction

Light hitting boundary of transparent Light hitting boundary of transparent materialmaterial Some reflectedSome reflected Some refractedSome refracted

Reflected lightReflected light Angle of incidence = angle of reflectionAngle of incidence = angle of reflection Amount controls lusterAmount controls luster

Fig. 7-6aFig. 7-6a

For reflection:Angle of incidence, i = angle of reflection, r

“reflective” boundary

Light ray

Refracted lightRefracted light Angle of incidence ≠ angle of Angle of incidence ≠ angle of

refractionrefraction Angle of refraction depends on Angle of refraction depends on

specific property, specific property, Index of refraction, Index of refraction, nn

n = Vn = Vvv/V/Vmm VVvv = velocity in a vacuum (maximum) = velocity in a vacuum (maximum) VVmm = velocity in material = velocity in material

Note – n is always > 1Note – n is always > 1 Big N means slow vBig N means slow v Little n means fast vLittle n means fast v

Angle of refraction given by Snell’s Angle of refraction given by Snell’s lawlaw

1

2

2

1

sin

sin

n

n

n=low, fast v

N=big, slow v

Wave normal

Snell’s law works for isotropic and Snell’s law works for isotropic and anisotropic material if:anisotropic material if: are angles between normals to are angles between normals to

boundaryboundary Direction is wave normal, not ray pathDirection is wave normal, not ray path

Measuring n important diagnostic Measuring n important diagnostic tooltool Not completely diagnostic, may vary Not completely diagnostic, may vary

within mineralswithin minerals More than one mineral may have same More than one mineral may have same

nn n can’t be measured in thin section, but n can’t be measured in thin section, but

can be estimatedcan be estimated

P. 306 – olivine informationP. 306 – olivine information

Indices of refraction{ }

Optical properties

Critical Angle - CACritical Angle - CA

A special case of Snell’s lawA special case of Snell’s law Light going from low to high index Light going from low to high index

material (fast to slow, e.g. air to material (fast to slow, e.g. air to mineral)mineral) Can always be refractedCan always be refracted Angle of refraction is smaller than angle Angle of refraction is smaller than angle

of incidenceof incidence

Light going from high to low index Light going from high to low index materialmaterial May not always be refractedMay not always be refracted Light is refracted toward the high n Light is refracted toward the high n

materialmaterial At some At some critical angle of incidencecritical angle of incidence, the , the

light will travel along the interfacelight will travel along the interface If angle of incidence is > CA, then total If angle of incidence is > CA, then total

internal reflectioninternal reflection CA can be derived from Snell’s lawCA can be derived from Snell’s law

Fig. 7-7Fig. 7-7

High index to low index material: light cannot pass through boundary if angle of incidence > CA

Critical angle is when angle of refraction = 90º

All internal reflectionAll internal reflection

N = high

n = low

DispersionDispersion

Material not always constant index of Material not always constant index of refractionrefraction n = f(n = f())

Normal dispersion, within same Normal dispersion, within same material:material: n higher for short wavelengths (blue)n higher for short wavelengths (blue) n lower for long wavelengths (red)n lower for long wavelengths (red)

Fig. 7-8Fig. 7-8

Because of dispersion, important to Because of dispersion, important to determine n for particular determine n for particular wavelengthwavelength Typically n given for Typically n given for = 486, 589, and = 486, 589, and

656 nm656 nm Common wavelengths for sunlightCommon wavelengths for sunlight