Optical Mineralogy WS 2012/2013

Exam week…. l Final week of semester (4–8 February) l 3 hours at your normal Übung time l 1 hour theory, 2 hours practical l Simple pass or fail....

Last week…. l Length fast (Hauptzone -), length slow (Hauptzone +) l Twinning l Zoning l Exsolution l Undulose extinction l How the diagrams in Tröger relate to the optical properties of minerals

Addition Example: Minerals with small birefringence (e.g. Quartz, Feldspar)  Mineral = 100 nm (1 o Grey) in diagonal position: With analyser only With analyser and compensator 1 o Grey2 o Blue  Mineral = 100 nm (1 o Grey)  Gips = 550 nm (1 o Red)  obs =  Mineral +  Gyps   obs = 650 nm (2 o Blue) When the interference colour is 1 o higher (addition), then the NE- SW direction is the higher n - slow ray (parallel to n  of the gypsum plate). ?

Subtraction Turn the stage through 90° (  Mineral stays at 100 nm )  Mineral = 100 nm (1 o Grey)  Gips = 550 nm (1 o Red)  obs = |  Mineral –  Gips |   obs = 450 nm (1 o Orange) When the interference colour is 1 o lower (subtraction), then the NE- SW direction is the lower n - fast ray. With analyser only With analyser and compensator 1 o Grey1 o Orange ?

Hauptzone + or -?

Optical character Long dimension of mineral is parallel to the slow ray ( n , n Z ) = LENGTH SLOW = HAUPTZONE + Long dimension is perpendicular to the slow ray ( n , n Z ) = LENGTH FAST = HAUPTZONE - sillimanite zoisite

Optical character and Hauptzone Prismatic crystal: If HZ + and Optically + If HZ - and Optically - Tabular crystal: If HZ + and Optically - If HZ - and Optically + Uniaxial minerals….

Conoscopic light – looking down an optic axis Objective lens Mineral Condensor Divergent light through the condensor  Light cone (±30°) W Polariser N-S Analysator What do we see? Various slices through the indicatrix  INTERFERENCE FIGURE E

Conoscopic ray paths Retardation (  ) is NOT constant!  Different interference colours   n dependent on angle  Different interference colours

Uniaxial interference figure Fig O E ISOCHROMES l Colour rings showing interference colours = ISOCHROMES ISOGYRES l Crossing lines that remain in extinction = ISOGYRES MELATOPE l Intersection of isogyres = MELATOPE = the OPTIC AXIS (c) QUADRANTS l Sectors between the isogyres = QUADRANTS l Interference colours increase with distance from the melatope (c-axis) l o-ray tangential, e-ray radial I III II IV

Measuring the optical character (+) (+) Mineral with gypsum plate : NE & SW: n e’ || n   Addition NW & SE: n o || n   Subtraction FIsochromes in I. and III. quadrants are higher by one order FIsochromes in II. and IV. quadrants are lower by one order FIsogyres  red-violet (1º red) Close to the melatope (  ≈ 100 nm) l 2° blue (650 nm) in NE & SW ( ) l 1° yellow (450 nm) in NW & SE ( ) (+) (+) Mineral: n e' > n o n e slow ray n e’ nono Add Add Sub Sub nnnn nono nono nono

Measuring the optical character (-) (+) (+) Mineral: n e' < n o n e fast ray n e’ nono Sub Sub Add Add nnnn nono nono nono (-) Mineral with gypsum plate : NE & SW: n e’ || n   Subtraction NW & SE: n o || n   Addition FIsochromes in I. and III. quadrants are lower by one order FIsochromes in II. and IV. quadrants are higher by one order FIsogyres  red-violet (1º red) Close to the isogyre (  ≈ 100 nm) l 1° yellow (450 nm) in NE & SW ( ) l 2° blue (650 nm) in NW & SE ( )

Uniaxial Optic Axial Figures (OAF) without gypsum plate: same for (+) and (-) (+) with gypsum plate blue in I. quadrant (-) with gypsum plate yellow in I. quadrant

OAF with uncentred melatope (Z) Konoskopische Bilder optisch einachsiger Kristalle bei unterschiedlicher Schnittlage; Isochromaten sind in Grautönen dargestellt. In der oberen Reihe sind Schnittlagen fast senkrecht zur optischen Achse skizziert, in der unteren Reihe deutlich schräger zur optischen Achse, so dass das Isogyrenkreuz außerhalb des Gesichtsfeldes liegt.

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, insert sub- stage lens) 5. Remove left ocular 6. You should see an interference figure - draw it 7. Insert gypsum plate and note optic sign