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)