Optical Mineralogy

WS 2012/2013

Theory exam!

….possibilities in the last week of semester:

Mo 4th February, 09:00-10:30

Do 7th February, 09:00-11:00Fr 8th February, 08:00-19:00

….alternatives in the penultimate week of the semester:

Mo 28th January, 08:00-10:00Mo 28th January, 10:00-12:00Do 31st January, 08:00-10:00Do 31st January, 10:00-12:00Fr 1st February, 08:00-19:00

Last week - Uniaxial interference figures

without gypsum plate:same for (+) and (-)

(+) with gypsum plateblue in I. quadrant

(-) with gypsum plateyellow in I. quadrant

Biaxial Interference Figures

Biaxial negative (acute bisectrix looking down X) Condensor forms a cone of light through sample at O OX OS OA

decreasing n n

OA OT OUincreasing n n

OX OQ OPincreasing n n

black

black

Biaxial Interference Figures

The result is an interference figure with ‘figure-of-8’ isochromes….

Fig 10-15 Bloss, Optical Crystallography, MSA

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.

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 2V

Measuring 2V

15o

60o

30o

45o

15o

30o

90o

5o

60o

Maximum separation of isogyres Curvature of isogyres

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 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 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 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 - 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)