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Optical Mineralogy

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?.

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Optical Mineralogy

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  1. 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

  2. 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

  3. Polarizing Microscope Ocular Bertrand lens Analyzer, upper polarizer, nicols lens Accessory Slot Objective Polarizer, typically oriented N-S

  4. Slightly more modern version Trinocular head Reflected light source Analyzer, upper polarizer, nicols lens Accessory plate Objectives Vernier scale conoscope Internal light source, polarized

  5. Four common settings for microscopic observations of thin sections: • Plane polarized light, analyzer (upper polarizer, nicols lens) out • Plane polarized light, analyzer in (cross nicols) • Conoscopic polarized light, bertrand lens in • Conoscopic polarized light, bertrand lens in, gypsum plate in accessory slot

  6. Setting #1: No upper analyzer Setting #2: Upper analyzer inserted Quartz crystals in plane polarized light Same quartz crystals with analyzer inserted (cross polarizers aka crossed nicols)

  7. 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

  8. 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

  9. 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

  10. Fig. 7-2 Electric vibration direction Magnetic vibration direction For mineralogy – we’ll only consider the electrical component

  11. Properties of light Wavelength Amplitude Velocity

  12. Relationship and units of properties • l = 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/l, frequency is constant, v and l variable

  13. l (nm) Fig. 6-6 f (hertz) 1 Å Visable light spectrum 100 Å Full range of electromagnetic radiation 1 nm = 10-9 m

  14. 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

  15. Fig. 7-3 Note – two waves have the same v and l Electrical components only Two light waves A & B interfere to form resultant wave R One light wave X has a component V at an angle 

  16. 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

  17. Wave normal and ray path not always parallel Fig. 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

  18. Fig. 7-2d and e 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

  19. Isotropic materials • Wave normals and ray paths are parallel • Velocity of light is constant regardless of direction in these minerals • Anisotropic materials • 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

  20. Birefringence demonstration?????????

  21. 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

  22. 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

  23. 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”

  24. Anisotropic medium: light split into two rays. One fully absorbed Fig. 7-4b Polarized light vibrates in only one plane: “Plane-polarized light”

  25. Polarization also caused by reflection: • “Glare” • Raybans cut the glare

  26. 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

  27. When light passes from one material to another • f = constant • If v increases, l also must increase • If v decreases, l decreases Vair > Vmineral f = v/l

  28. 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 doesn’t affect the electrical property of light • Light speed doesn’t vary with direction • Light NOT split into two rays

  29. 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

  30. Reflection and Refraction • Light hitting boundary of transparent material • Some reflected • Some refracted • Reflected light • Angle of incidence = angle of reflection • Amount controls luster

  31. Fig. 7-6a For reflection: Angle of incidence, i = angle of reflection, r Light ray “reflective” boundary

  32. 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

  33. Angle of refraction given by Snell’s law Wave normal n=low, fast v N=big, slow v

  34. Snell’s law works for isotropic and anisotropic material if: •  are angles between normals to boundary • Direction is wave normal, not ray path

  35. 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

  36. P. 306 – olivine information } Optical properties Indices of refraction {

  37. Critical Angle - CA • A special case of Snell’s 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

  38. 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 Snell’s law

  39. Fig. 7-7 All internal reflection High index to low index material: light cannot pass through boundary if angle of incidence > CA Critical angle is when angle of refraction = 90º N = high n = low

  40. Dispersion • Material not always constant index of refraction • n = f(l) • Normal dispersion, within same material: • n higher for short wavelengths (blue) • n lower for long wavelengths (red)

  41. Fig. 7-8

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

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