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Chapter 10 - A

Chapter 10 - A. Identification of minerals with the petrographic microscope. Content. Sample preparation Microscope alignment Determination of the refractive index Use of interference colors Conoscopic observation of interference figures. Microscopy. Transmitted light microscopy

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Chapter 10 - A

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  1. Chapter 10 - A Identification of minerals with the petrographic microscope

  2. Content • Sample preparation • Microscope alignment • Determination of the refractive index • Use of interference colors • Conoscopic observation of interference figures

  3. Microscopy • Transmitted light microscopy • Transparent crystals • Light transmits through mineral grains • Common rock-forming minerals • Reflected light microscopy • Opaque crystals • Light reflects from highly polished surface • Usually ore minerals  This course: transmitted light microscopy

  4. Sample preparation:Transmitted light microscopy • Grain mount: • Finely ground fragments; immersed in oil and scattered on glass plate; covered by thin sheet of glass • Thin section: • Cut slab from rock sample – area of interest • Bottom - polished and cemented onto glass slide • Top - ground to desired thickness; covered with balsam and thin cover glass • Rock-forming minerals now transparent

  5. Why use a cover glass?

  6. Microscope alignment • Important in order to: • have light going through the center of all lenses, of the stage, the condenser • get two polarizers filtering light at vibration directions perpendicular to each other • Oculars – one or both adjusted for each eye; cross-hair in focus • Stage – center exactly in the optic axis; object not to move during stage rotation • Condenser – when switched on light beam should be centered around cross-hair • Polarizer – one set at 0º and one at 90º

  7. Other settings • Brightness of light – comfortable for your eyes – very bright will give headaches and burns out filaments • Iris – determines the diameter of the light beam coming from the source – different setting for different magnifications • Condenser lens – use to get high resolution at high magnification • Focusing – to avoid collision: first bring sample close to objective lens (not against) and increase distance until sample in focus

  8. Determination of the refractive index • Grain mount • Edges of crystal – act as small prisms which concentrate light as a ring of light – the Becke line • When increasing the distance from sample to objective (defocusing), the Becke line is always refracted in the direction of a medium of higher RI • In practice: • Change liquids until two adjacent liquids defines the range for the index of the mineral

  9. Determination of refractive Index

  10. Birefringence (δ) • When a ray of light is split into two separate polarized rays – each with a single vibration direction perpendicular to that of the other ray • True maximum birefringence value (δ) of mineral • Isotropic: δ = n – n = 0 • Uniaxial: δ = nε – nω • Biaxial: δ = nγ – nα • Under the microscope: • Observed under crossed polarized light as: • Interference colors • Only in anisotropic minerals

  11. Birefringence/double refraction • Doubly refracted waves are polarized but separate, vibrating in different planes – no interaction • Need interference – study interference colours and properties • To get interference – a second polarizer inserted – the analyzer: • Crossed polarizer/upper polarizer/crossed nichols • Used to analyze the interference effects of light in minerals

  12. Interference colours First order colors Second order colors Third order colors

  13. Birefringence • A characteristic that all anisotropic minerals have, intensity differs • High birefringent minerals – third/fourth order interference colours • Med birefringent minerals – second order interference colours • Low birefringent minerals - first order interference colours • For specific mineral birefringence depends on orientation: • Maximum birefringence - orientation of grain shows highest possible interference colour for the specific mineral • Minimum or no birefringence – orientation of grain shows lowest or no interference colour for specific mineral • Intermediate birefringence – orientation of grains shows interference colours intermediate between minimum and maximum

  14. Interference colours Determine order of colour and so value for birefringence – interference color chart

  15. Use of interference colorsTrue birefringence • In sample: crystals in random orientations  each grain different interference colors, each with corresponding birefringence • Minimum birefringence • Circular section (perpendicular to optical axis) give lowest order or no interference colours – refractive indices on both axes equal or almost equal • Also referred to as the isotropic section • True birefringence • Longest elliptical section (parallel to optical axis) give highest order colors • Refractive index on major axis = largest; on minor axis = smallest • THUS: to determine the true birefringence of mineral – choose grain with highest interference colors and read of the value of birefringence from the color chart

  16. Use of interference colors:Accessory plates (compensators) • Accessory plate is a crystal with known birefringence and orientation • Determine unknown mineral optical orientation by comparing with known crystal plate orientation • Crystal orientation in plate parallel with mineral orientation • Plate colors interfere constructively with colors of mineral • Addition – Positive (Red plate + color of mineral = blue) • Crystal orientation in plate perpendicular with mineral orientation • Plate colors interfere destructively with colors of mineral • Subtraction – Negative (Red plate - color of mineral = yellow)

  17. POSITIVE NEGATIVE Use of interference colors:Accessory plates (compensators)

  18. Use of interference colors:Extinction • As an anisotropic crystal is rotated a full turn under crossed polarized light, it goes into extinction 4 times • I.e. – at every 90° rotation the mineral goes dark • This happens every time the two perpendicular vibrating directions falls parallel with the two polarizer directions

  19. Use of interference colors:Extinction angle • When optical axis vertical (circular section) – mineral dark during rotation • When inclined – mineral go dark once every 90º • Angle of extinction can be measured for elongated minerals or minerals with strong cleavage • Parallel extinction • Inclined extinction • Symmetrical extinction • No extinction angle

  20. Observation of interference figures using convergent light – conoscopic view • Insert condenser lens • Gives convergent light • Enters sample at 50º - 90º angles • See image of light source • Interference effects at different angles

  21. Conoscopic observation of interference figures • Isotropic • No image

  22. Conoscopic observation of interference figures • Uniaxial • Perpendicular to optical axis

  23. Conoscopic observation of interference figures • Uniaxial • At an angle to the optical axis

  24. Conoscopic observation of interference figures • Uniaxial • Parallel to the optical axis

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