Optical Mineralogy WS 2007/2008

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

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

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

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

Biaxial Interference Figures Same figure rotated 45 o Optic axes are now E-W Clearly isogyres must swingDemonstration Fig 10-16B 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.

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

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

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

Measuring 2V 15 o 60 o 30 o 45 o 15 o 30 o 90 o 5o5o 60 o Maximum separation of isogyresCurvature of isogyres

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 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

How do we get a BISECTRIX interference 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 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

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)