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Optical Mineralogy WS 2012/2013. Theory exam! ….possibilities in the last week of semester: Mo 4th February, 09:00-10:30 Do 7th February, 09:00-11:00.

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Presentation on theme: "Optical Mineralogy WS 2012/2013. Theory exam! ….possibilities in the last week of semester: Mo 4th February, 09:00-10:30 Do 7th February, 09:00-11:00."— Presentation transcript:

1 Optical Mineralogy WS 2012/2013

2 Theory exam! ….possibilities in the last week of semester: Mo 4th February, 09:00-10:30 Do 7th February, 09:00-11:00 Fr 8th February, 08:00-19:00 ….alternatives in the penultimate week of the semester: Mo 28th January, 08:00-10:00 Mo 28th January, 10:00-12:00 Do 31st January, 08:00-10:00 Do 31st January, 10:00-12:00 Fr 1st February, 08:00-19:00

3 Last week - Uniaxial interference figures without gypsum plate: same for (+) and (-) (+) with gypsum plate blue in I. quadrant (-) with gypsum plate yellow in I. quadrant

4 Biaxial Interference Figures l Biaxial negative l Biaxial negative (acute bisectrix looking down X) O l Condensor forms a cone of light through sample at O l OX  OS  OA decreasing  n   n  l OA  OT  OU increasing  n   n  l OX  OQ  OP increasing  n   n  black

5 Biaxial Interference Figures The result is an interference figure with ‘figure-of-8’ isochromes…. Fig Bloss, Optical Crystallography, MSA

6 Biaxial Interference Figures Centered B xa Figure Fig Bloss, Optical Crystallography, MSA ISOGYRES ISOCHROMES

7 Biaxial Interference Figures Same figure rotated 45 o Optic axes are now E-W Clearly isogyres must rotate Fig 10-16B Bloss, Optical Crystallography, MSA

8 Biaxial Interference Figure Upper row: Cut perpendicular to acute bisectrix (2V approx. 30°); Middle row: Cut close to an Optic Axis; Lower row: Cuts nearly perpendicular to the obtuse bisectrix.

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10 Determining the optical sign (+ or -) In A-D, sections are perpendicular to the acute bisectrix. In E and F, they are perpendicular to one of the optic axes.

11 Biaxial Optic Sign B(+) B(+) Z = B xa thus  closer to  than 

12 Biaxial Optic Sign (+) Without gypsum plate: Polarisation colours increase away from melatopes With gypsum plate: Colours increase by 1º in NE quadrant = +ve

13 Biaxial Optic Sign B(-) B(-) X = B xa thus  closer to  than 

14 Measuring 2V

15 15 o 60 o 30 o 45 o 15 o 30 o 90 o 5o5o 60 o Maximum separation of isogyresCurvature of isogyres

16 How do we get an OAF? 1. In XPL, find a grain that remains in extinction through 360º - centre it 2. Change to high-powered objective and focus 3. Make sure grain stays in field of view 4. Maximise light (open diaphragm, remove sub-stage lens) 5. Remove left ocular and adjust condensor settings 6. You should see an interference figure - draw it 7. Rotate isogyre so it is bent towards NE quadrant 8. Insert gypsum plate and note optic sign

17 How do we get a BISECTRIX interference figure? 1. In XPL, find a grain that shows low polarisation colour (1°) …. a bit of a guess …. 2. Change to high-powered objective and focus 3. Make sure grain stays in field of view 4. Maximise light (open diaphragm, remove sub-stage lens) 5. Remove left ocular and adjust condensor settings 6. You should see an interference figure - draw it 7. Rotate as shown 8. Insert gypsum plate and note optic sign

18 Conoscopic observations - summary Find an isotropic section (remains black) l Optical character FNo interference figure  cubic or amorphous FUniaxial interference figure  hexagonal, trigonal, tetragonal FBiaxial interference figure  orthorhombic, monoclinic, triclinic l Using the gypsum plate FUniaxial positive or negative FBiaxial positive, negative or neutral l Estimate the 2V angle (curvature or separation of isogyres)


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