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Unformatted text preview: Optical Mineralogy Technique utilizing interaction of polarized light with minerals Primary way to observe minerals Important: Oils Grain mounts Thin sections rocks cheap, quick, easy Only way to determine textures Light Electromagnetic energy Particle, Wave or both derived from excess energy of electrons Energy released as electrons drop from excited state to lower energy shells perceived as "light" Particles = photons For mineralogy, consider light a wave Important wave interference phenomenon Light as wave Light has both electrical and magnetic energy Vibration perpendicular to direction of propagation 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 we'll only consider the electrical component Fig. 72 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 f = frequency e.g. how often a wave passes a particular point, unit = 1/t f = v/ (nm) f (hertz) Full range of electromagnetic radiation Visable light spectrum 9 Fig. 66 If two light waves vibrate at an angle to each other: Vibration directions of single wave can be split into various components Vibrations interfere with each other Interference creates a new wave Direction determined by vector addition Each component has different vibration direction 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. 73 Light composed of many waves Note: wave normal and light ray are not necessarily parallel 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 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 Fig. 72c Isotropic materials Anisotropic materials Wave normals and ray paths are parallel Velocity of light constant in these minerals Wave normals and ray paths are not parallel Velocity of light variable depending on direction of wave normal and ray path These difference have major consequences for interaction of light and materials 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. 72d and e Polarized Light Unaltered light (not polarized) Vibrates in all directions perpendicular to direction of propagation Multiple rays, vibrate in all directions
Fig. 74 Light may be polarized Generation of polarized light: Vibrates in only one plane Called "Plane Polarized Light" In anisotropic material, ray resolves into two rays Two rays vibrate perpendicular to each other One ray absorbed by material Light emerges vibrating in only one direction Isotropic medium: light split into two rays. One fully absorbed Polarized light vibrates in only one plane: "Plane polarized light" Fig. 7 4b 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 f = v/ Isotropic vs. Anisotropic Isotropic geologic materials Isometric minerals; also glass, liquids and gases Electron density identical in all directions Direction doesn't affect the electrical property of light Light speed doesn't vary with direction Anisotropic geologic materials: Minerals in tetragonal, hexagonal, orthorhombic, monoclinic and triclinic systems Interactions between light and electrons differ depending on direction Light speed depends on direction of ray and thus vibration direction Reflection and Refraction Light hitting boundary of transparent material Reflected light Some reflected Some refracted Angle of incidence = angle of reflection Amount controls luster For reflection: Angle of incidence, i = angle of reflection, r Light ray "reflective" boundary
Fig. 76a Refracted light Angle of incidence angle of refraction Angle of refraction depends on specific property n = Vv/Vm Index of refraction, n Note n is always > 1 Vv = velocity in a vacuum (maximum) Vm = velocity in material Big N means slow v Little n means fast v Angle of refraction given by Snell's law
n=low, fast v sin 1 n2 = sin 2 n1
n=big, slow v Measuring n important diagnostic tool Not completely diagnostic, may vary within minerals More than one mineral may have same n n can't be measured in thin section, but can be estimated P. 306 olivine information Indices of refraction Snell's law works for isotropic and anisotropic material if: are angles between normals to boundary Direction is wave normal, not ray path Which wave is important in anisotropic material Critical Angle CA Light going from low to high index material (fast to slow) 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 bent toward the interface 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 Snell's law High index to low index material: light cannot pass through boundary if angle of incidence > CA All internal reflection Fig. 77 Dispersion Material not always constant index of refraction Normal dispersion, within same material: n = f( ) n higher for short wavelengths (blue) n lower for long wavelengths (red) Fig. 78 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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- Spring '11