optical mineralogy technique utilizing interaction of polarized light with minerals technique...
TRANSCRIPT
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