Optical MineralogyOptical Mineralogy
WS 2007/2008WS 2007/2008
Last week - Uniaxial interference figuresLast week - Uniaxial interference figures
without gypsum plate:same for (+) and (-)
(+) with gypsum plateblue in I. quadrant
(-) with gypsm plateyellow in I. quadrant
Biaxial Interference FiguresBiaxial Interference Figures
Biaxial negativeBiaxial negative (acute bisectrix looking down X) Condensor forms a cone of light through sample at OO OX OX OS OS OA OA
decreasing n n
OA OA OT OT OU OUincreasing n n
OX OX OQ OQ OP OPincreasing n n
black
black
Biaxial Interference FiguresBiaxial Interference Figures
The result is an interference figure with ‘figure-of-8’ isochromes….
Fig 10-15 Bloss, Optical Crystallography, MSA
Biaxial Interference FigureBiaxial 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.
Determining the optical sign (+ or -)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.
Measuring 2VMeasuring 2V
15o
60o
30o
45o
15o
30o
90o
5o
60o
Maximum separation of isogyres Curvature of isogyres
How do we get an OAF?How do we get an OAF?
1. In XN, find a grain that remains in extinction through 360º - centre it
2. Change to high-powered objective and focus3. Make sure grain stays in field of view4. Maximise light (open diaphragm, remove sub-
stage lens)5. Remove left ocular and adjust condensor settings6. You should see an interference figure - draw it7. Rotate isogyre so it is bent towards NE quadrant8. Insert gypsum plate and note optic sign
How do we get a BISECTRIX interference How do we get a BISECTRIX interference figure?figure?
1. In XN, find a grain that shows low polarisation colour (1°) …. a bit of a guess ….
2. Change to high-powered objective and focus3. Make sure grain stays in field of view4. Maximise light (open diaphragm, remove sub-
stage lens)5. Remove left ocular and adjust condensor settings6. You should see an interference figure - draw it7. Rotate as shown8. Insert gypsum plate and note optic sign
Conoscopic observations - summaryConoscopic observations - summary
Find an isotropic section (remains black)
Optical character No interference figure cubic or amorphous Uniaxial interference figure hexagonal, trigonal, tetragonal Biaxial interference figure orthorhombic, monoclinic, triclinic
Using the gypsum plate Uniaxial positive or negative Biaxial positive, negative or neutral
Estimate the 2V angle (curvature or separation of isogyres)
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