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Optical Mineralogy

Optical Mineralogy. WS 2012/2013. Exam week…. Final week of semester (4–8 February ) 3 hours at your normal Übung time 1 hour theory , 2 hours practical Simple pass or fail . Last week…. Length fast (Hauptzone -), length slow (Haup tzone +) Twinning Zoning Exsolution

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Optical Mineralogy

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  1. Optical Mineralogy WS 2012/2013

  2. Exam week…. • Final weekofsemester (4–8 February) • 3 hoursatyour normal Übung time • 1 hourtheory, 2 hourspractical • Simple pass orfail....

  3. Last week…. • Length fast (Hauptzone -), lengthslow (Hauptzone +) • Twinning • Zoning • Exsolution • Unduloseextinction • Howthediagrams in Tröger relatetotheopticalpropertiesofminerals

  4. Addition Example: Minerals with small birefringence (e.g. Quartz, Feldspar) Mineral = 100 nm (1o Grey) in diagonal position: GMineral = 100 nm (1o Grey) GGips = 550 nm (1oRed) Gobs = GMineral + GGyps  Gobs = 650 nm (2o Blue) ? Whentheinterferencecolouris 1ohigher (addition), thenthe NE-SW directionisthehighern - slowray (parallel tonofthegypsumplate). 1o Grey 2o Blue Withanalyseronly Withanalyserandcompensator

  5. Subtraction Turn the stage through 90°(Mineralstays at 100 nm) GMineral = 100 nm (1o Grey) GGips = 550 nm (1oRed) Gobs = |GMineral – GGips|  Gobs = 450 nm (1o Orange) ? Whentheinterferencecolouris 1olower (subtraction), thenthe NE-SW directionisthelowern - fast ray. 1o Grey 1o Orange Withanalyserandcompensator Withanalyseronly

  6. Hauptzone + or -?

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

  8. Optical character and Hauptzone Uniaxialminerals…. Prismatic crystal: If HZ + and Optically + If HZ - and Optically - Tabular crystal: If HZ + and Optically - If HZ - and Optically +

  9. Conoscopiclight – looking down an optic axis Divergent light throughthecondensor  Light cone (±30°) Objective lens What do wesee? N-S Analysator Mineral Condensor Variousslicesthroughtheindicatrix  INTERFERENCE FIGURE W Polariser E

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

  11. 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) • o-ray tangential, e-ray radial II I III IV

  12. Measuring the optical character (+) ng (+) Mineral withgypsumplate: • NE & SW: ne’ || ng Addition • NW & SE: no || ng Subtraction • Isochromes in I. and III. quadrantsarehigherbyoneorder • Isochromes in II. and IV. quadrantsarelowerbyoneorder • Isogyresred-violet (1º red) Close tothemelatope (G≈ 100 nm) • 2° blue (650 nm) in NE & SW (100+550) • 1° yellow (450 nm) in NW & SE (100-550) 2 1 no ne’ no ne’ Sub Add Add Sub ne’ ne’ no no 4 3 (+) Mineral: ne' > no neslowray

  13. Measuring the optical character (-) ng (-) Mineral withgypsumplate: • NE & SW: ne’ || ng Subtraction • NW & SE: no || ng Addition • Isochromes in I. and III. quadrantsarelowerbyoneorder • Isochromes in II. and IV. quadrantsarehigherbyoneorder • Isogyresred-violet (1º red) • Close totheisogyre (G≈ 100 nm) • 1° yellow (450 nm) in NE & SW (100-550) • 2° blue (650 nm) in NW & SE (100+550) 2 1 no ne’ no ne’ Add Sub Sub Add ne’ ne’ no no 4 3 (+) Mineral: ne' < no ne fast ray

  14. Uniaxial Optic Axial Figures (OAF) withoutgypsumplate: same for (+) and (-) (+) withgypsumplate blue in I. quadrant (-) withgypsumplate yellow in I. quadrant

  15. OAF with uncentredmelatope (Z) KonoskopischeBilderoptischeinachsigerKristallebeiunterschiedlicherSchnittlage; Isochromatensind in Grautönendargestellt. In der oberenReihesindSchnittlagen fast senkrechtzuroptischenAchseskizziert, in der unterenReihedeutlichschrägerzuroptischenAchse, so dass das Isogyrenkreuzaußerhalb des Gesichtsfeldesliegt.

  16. 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, insert 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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