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Optical Mineralogy

Optical Mineralogy. WS 2007/2008. Last week…. Length fast and length slow Twinning Zoning Exsolution Undulose extinction How the diagrams in Tröger relate to the optical properties of minerals. Optical character. Long dimension of mineral is parallel to the slow ray ( n  , n Z ) =

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Optical Mineralogy

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  1. Optical Mineralogy WS 2007/2008

  2. Last week…. • Length fast and length slow • Twinning • Zoning • Exsolution • Undulose extinction • How the diagrams in Tröger relate to the optical properties of minerals

  3. Optical character Long dimension of mineral is parallel to the slow ray(n , nZ) = LENGTH SLOW Long dimension of mineral is perpendicular to the slow ray (n , nZ) = LENGTH FAST sillimanite zoisite

  4. Conoscopic light nw nw nw nw ne ne ne ne Various slices through the indicatrix  INTERFERENCE FIGURE Divergent light through the condensor  Light cone (±30°) Objective lens What do we see? N-S Analysator Mineral Condensor W Polariser E

  5. Conoscopic ray paths Retardation () is NOT constant! • n dependent on angle  Different interference colours

  6. Uniaxial interference figure O E Fig. 7-14 • Colour rings showing interference colours = ISOCHROMES • Crossing lines that remain in extinction = ISOGYRES • Intersection of isogyres = MELATOPE = the OPTIC AXIS (c) • Sectors between the isogyres = QUADRANTS • Interference colours increase with distance from the melatope (c-axis) II I III IV

  7. Measuring the optical character (+) ng (+) Mineral with gypsum plate: • NE & SW: ne’ || ng Addition • NW & SE: no || ng Subtraction • Isochromes in I. and III. quadrants are higher by one order • Isochromes in II. and IV. quadrants are lower by one order • Isogyres  red-violet (1º red) Close to the isogyre (G≈ 100 nm) • 2° blue (650 nm) in NE & SW (100+550) • 1° yellow (450 nm) in NW & SE (100-550) no ne’ no ne’ Sub Add Add Sub ne’ ne’ no no (+) Mineral: ne' > no ne slow ray

  8. Measuring the optical character (-) ng (-) Mineral with gypsum plate: • NE & SW: ne’ || ng Subtraction • NW & SE: no || ng Addition • Isochromes in I. and III. quadrants are lower by one order • Isochromes in II. and IV. quadrants are higher by one order • Isogyres  red-violet (1º red) Close to the isogyre (G≈ 100 nm) • 1° yellow (450 nm) in NE & SW (100-550) • 2° blue (650 nm) in NW & SE (100+550) no ne’ no ne’ Add Sub Sub Add ne’ ne’ no no (+) Mineral: ne' < no ne fast ray

  9. Uniaxial Optic Axial Figures (OAF) without gypsum plate: same for (+) and (-) (+) with gypsum plate blue in I. quadrant (-) with gypsm plate yellow in I. quadrant

  10. OAF with uncentred melatope (Z) Konoskopische Bilder optisch einachsiger Kristalle bei unterschiedlicher Schnittlage; Isochromaten sind in Grautönen dargestellt. In der oberen Reihe sind Schnittlagen fast senkrecht zur optischen Achse skizziert, in der unteren Reihe deutlich schräger zur optischen Achse, so dass das Isogyrenkreuz außerhalb des Gesichtsfeldes liegt.

  11. How do we get an OAF? • In XN, find a grain that remains in extinction through 360º - centre it • Change to high-powered objective and focus • Make sure grain stays in field of view • Maximise light (open diaphragm, remove sub-stage lens) • Remove left ocular • You should see an interference figure - draw it • Insert gypsum plate and note optic sign

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