Optical Mineralogy - University of Florida

Optical Mineralogy - University of Florida

Optical Mineralogy Technique utilizing interaction of polarized light with minerals Uses a polarizing microscope Oils - Grain mounts Thin sections rocks Primary way to observe minerals

Important: cheap, quick, easy Only way to determine textures Why use microscopes? Visual properties for ID e.g. texture

Color may be variable Cleavage (may not see, often controls shape) Shape (depends on cut of mineral) Only observable with microscope Separate isotropic and anisotropic minerals and many other optical properties Polarizing Microscope Ocular Bertrand lens Analyzer, upper polarizer, nicols

lens Accessory Slot Objective Polarizer, typically oriented N-S Slightly more modern version Trinocula r head Analyzer, upper polarizer, nicols lens Objectives

conoscope Internal light source, polarized Reflected light source Accessor y plate Vernier scale Four common settings for microscopic

observations of thin sections: 1. Plane polarized light, analyzer (upper polarizer, nicols lens) out 2. Plane polarized light, analyzer in (cross nicols) 3. Conoscopic polarized light, bertrand lens in 4. Conoscopic polarized light, bertrand lens in, gypsum plate in accessory slot Setting #1: No upper analyzer Quartz crystals in plane polarized light Setting #2: Upper analyzer inserted

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

Electromagnetic energy derived from excess energy of electrons Energy released as electrons drop from excited state to lower energy shells perceived as light Particle, Wave or both

Particles = photons For mineralogy, consider light a wave Important wave interference phenomenon Light as wave Energy vibrates perpendicular to direction of propagation

Light has both electrical and magnetic energy Two components vibrate perpendicular to each other Electrical component interacts with electrical properties of minerals, e.g. bond strength, electron densities Electric vibration direction Magnetic vibration

direction For mineralogy well only consider the electrical component Fig. 7-2 Properties of light Wavelength Amplitude Velocity Relationship and units of properties

= wavelength, unit = L, color of light A = amplitude, unit = L, intensity of light v = velocity, unit = L/t, property of material f = frequency e.g. how often a wave passes a particular point, unit = 1/t f = v/frequency is constant, v and variable Visable light spectrum

1 nm = 10-9 m f (hertz) 1 100 Full range of electromagnetic radiation (nm) Fig. 6-6

If two light waves vibrate at an angle to each other: Vibrations interfere with each other Interference creates a new wave Direction determined by vector addition Vibration directions of single wave can be split into various components

Each component has different vibration direction Electrical components only Note two waves have the same v and Two light waves A & B interfere to form resultant

wave R One light wave X has a component V at an angle Fig. 7-3 Light composed of many waves

Wave front = connects same point on adjacent waves Wave normal = line perpendicular to wave front Light ray (Ray path) = direction of propagation of light energy, e.g. direction of path of photon Note: wave normal and light ray are not necessarily parallel Wave normal and ray path not always parallel 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 Fig. 7-2c take Wave normal and ray paths may be coincident Propogation of light through

Isotropic material Wave normal and ray paths may not be coincident Propogation of light through Anisotropic material Fig. 7-2d and e Isotropic materials

Anisotropic materials Wave normals and ray paths are parallel Velocity of light is constant regardless of direction in these minerals Wave normals and ray paths are not parallel Velocity of light is variable depending on direction of wave normal and ray path These difference have major consequences

for interaction of light and materials Birefringence demonstration????????? Polarized and Non-polarized Light Non-polarized light

Vibrates in all directions perpendicular to direction of propagation Occurs only in isotropic materials Air, water, glass, etc. Fig. 7-4 Non-Polarized Light Light vibrates in all directions perpendicular to ray path Multiple rays,

vibrate in all directions Highly idealized only 1 wavelength Fig. 7-4 Polarized light Vibrates in only one plane Generation of polarized light:

In anisotropic material, light usually resolves into two rays Two rays vibrate perpendicular to each other The energy of each ray absorbed by different amounts If all of one ray absorbed, light emerges vibrating in only one direction Called Plane Polarized Light Anisotropic medium: light

split into two rays. One fully absorbed Polarized light vibrates in only one plane: Planepolarized light Fig. 74b Polarization also caused by reflection:

Glare Raybans cut the glare Interaction of light and matter Velocity of light depends on material it passes through In vacuum, v = 3.0 x 1017 nm/sec = 3.0 x 108 m/sec

All other materials, v < 3.0 x 1017 nm/sec When light passes from one material to another f = constant If v increases, also must increase If v decreases, decreases Vair > Vmineral

f = v/ Isotropic vs. Anisotropic Isotropic geologic materials Isometric minerals; also glass, liquids and gases Electron density identical in all directions

Think back to crystallographic axes Direction doesnt affect the electrical property of light Light speed doesnt vary with direction Light NOT split into two rays Anisotropic geologic materials:

Minerals in tetragonal, hexagonal, orthorhombic, monoclinic and triclinic systems Interactions between light and electrons differ depending on direction Light split into two rays vibrate perpendicular to each other Light speed depends on direction of ray and thus vibration direction Reflection and Refraction

Light hitting boundary of transparent material Some reflected Some refracted Reflected light

Angle of incidence = angle of reflection Amount controls luster For reflection: Angle of incidence, i = angle of reflection, r Light ray reflective boundary Fig. 7-6a Refracted light

Angle of incidence angle of refraction Angle of refraction depends on specific property, Index of refraction, n n = Vv/Vm Vv = velocity in a vacuum (maximum) Vm = velocity in material Note n is always > 1

Big N means slow v Little n means fast v Angle of refraction given by Snells law Wave normal n=low, fast v sin 1 n2 sin 2 n1

N=big, slow v Snells law works for isotropic and anisotropic material if: are angles between normals to boundary Direction is wave normal, not ray path

Measuring n important diagnostic tool Not completely diagnostic, may vary within minerals More than one mineral may have same n n cant be measured in thin section, but can be estimated P. 306 olivine information

Indices of refraction{ Optical properties } Critical Angle - CA A special case of Snells law Light going from low to high index material (fast to slow, e.g. air to mineral)

Can always be refracted Angle of refraction is smaller than angle of incidence Light going from high to low index material

May not always be refracted Light is refracted toward the high n material At some critical angle of incidence, the light will travel along the interface If angle of incidence is > CA, then total internal reflection CA can be derived from Snells law All internal reflection N = high High index to low index material:

light cannot pass through boundary if angle of incidence > CA Critical angle is when angle of refraction = 90 n = low Fig. 7-7 Dispersion Material not always constant index of refraction

n = f() Normal dispersion, within same material: n higher for short wavelengths (blue) n lower for long wavelengths (red) Fig. 7-8

Because of dispersion, important to determine n for particular wavelength Typically n given for = 486, 589, and 656 nm Common wavelengths for sunlight

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