optical mineralogy ws 2012/2013. exam week…. l final week of semester (4–8 february) l 3 hours...
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Optical Mineralogy
WS 2012/2013
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Exam week….
Final week of semester (4–8 February) 3 hours at your normal Übung time 1 hour theory, 2 hours practical Simple pass or fail....
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Last week….
Length fast (Hauptzone -), length slow (Hauptzone +)
Twinning
Zoning
Exsolution
Undulose extinction
How the diagrams in Tröger relate to the optical properties of minerals
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Addition
Example: Minerals with small birefringence (e.g. Quartz, Feldspar)
Mineral = 100 nm (1o Grey) in diagonal position:
With analyser only
With analyser and compensator
1o Grey 2o Blue
GMineral = 100 nm (1o Grey)
GGips = 550 nm (1o Red)
Gobs = GMineral + GGyps
Gobs = 650 nm (2o Blue)
When the interference colour is 1o higher (addition), then the NE-SW direction is the higher
n - slow ray (parallel to n of the gypsum plate).
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Subtraction
Turn the stage through 90° (Mineral stays at 100 nm)
GMineral = 100 nm (1o Grey)
GGips = 550 nm (1o Red)
Gobs = |GMineral – GGips|
Gobs = 450 nm (1o Orange)
When the interference colour is 1o lower (subtraction), then the NE-
SW direction is the lower n - fast ray.
With analyser only
With analyser and compensator
1o Grey 1o Orange
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Hauptzone + or -?
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Optical character
Long dimension of mineral is parallel
to the slow ray (n , nZ) =
LENGTH SLOW = HAUPTZONE +
Long dimension is perpendicular to the slow ray (n , nZ) =
LENGTH FAST = HAUPTZONE -
sillimanite
zoisite
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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….
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Conoscopic light – looking down an optic axis
Objectivelens
Mineral
Condensor
Divergent light through the condensor
Light cone (±30°)
WPolariser
N-S Analysator What do we see?
Various slices through the indicatrix
INTERFERENCE FIGUREE
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Conoscopic ray paths
Retardation () is NOT constant!
n dependent on angle Different interference colours
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Uniaxial interference figure
Fig. 7-14
O E
Colour rings showing interference colours = ISOCHROMES
Crossing lines that remain in extinction = ISOGYRES
Intersection of isogyres = MELATOPE = the OPTIC AXIS (c)
Sectors between the isogyres = QUADRANTS Interference colours increase with distance from
the melatope (c-axis) o-ray tangential, e-ray radial
IIIIII
IV
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Measuring the optical character (+)
(+) Mineral with gypsum plate:
NE & SW: ne’ || ng Addition
NW & SE: no || ng Subtraction
Isochromes in I. and III. quadrants are higher
by one order
Isochromes in II. and IV. quadrants are lower
by one order
Isogyres red-violet (1º red)
Close to the melatope (G ≈ 100 nm) 2° blue (650 nm) in NE & SW (100+550) 1° yellow (450 nm) in NW & SE (100-550)
(+) Mineral:ne' > no
ne slow ray
ne’
no
AddAddSub
Sub
n g
no
ne’
ne’
ne’
no
no
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Measuring the optical character (-)
(+) Mineral:ne' < no
ne fast ray
ne’
no
SubSubAdd
Add
n g
no
ne’
ne’
ne’
no
no
(-) Mineral with gypsum plate:
NE & SW: ne’ || ng Subtraction
NW & SE: no || ng Addition
Isochromes in I. and III. quadrants are lower
by one order
Isochromes in II. and IV. quadrants are
higher by one order
Isogyres red-violet (1º red)
Close to the isogyre (G ≈ 100 nm) 1° yellow (450 nm) in NE & SW (100-550) 2° blue (650 nm) in NW & SE (100+550)
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Uniaxial Optic Axial Figures (OAF)
without gypsum plate:same for (+) and (-)
(+) with gypsum plateblue in I. quadrant
(-) with gypsum plateyellow in I. quadrant
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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.
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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 view4. Maximise light (open diaphragm, insert sub-
stage lens)5. Remove left ocular6. You should see an interference figure - draw
it7. Insert gypsum plate and note optic sign