Post on 24-Feb-2016
description
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)