Optical Mineralogy WS 2007/2008. Last week…. Length fast and length slow Twinning Zoning Exsolution Undulose extinction How the diagrams in Tröger relate.

Optical MineralogyOptical Mineralogy

WS 2007/2008WS 2007/2008

Last week….Last week….

Length fast and length slow

Twinning

Zoning

Exsolution

Undulose extinction

How the diagrams in Tröger relate to the optical properties of minerals

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Optical characterOptical character

Long dimension of mineral is parallel to the slow ray (n , nZ) =

LENGTH SLOW

Long dimension of mineral is perpendicular to the slow ray (n ,

nZ) =

LENGTH FAST

sillimanite

zoisite

Conoscopic lightConoscopic light

Objectivelens

Mineral

Condensor

Divergent light through the condensor

Light cone (±30°)Light cone (±30°)

WPolariser

N-S Analysator What do we see?What do we see?

n

n

n

n

nn

nn

Various slices through the indicatrixVarious slices through the indicatrix

INTERFERENCE FIGUREINTERFERENCE FIGUREE

Conoscopic ray pathsConoscopic ray paths

Retardation () is NOT constant! n dependent on angle Different interference Different interference

colourscolours

Uniaxial interference figureUniaxial interference figure

Fig. 7-14Fig. 7-14

O E

Colour rings showing interference colours = ISOCHROMESISOCHROMES

Crossing lines that remain in extinction = ISOGYRESISOGYRES

Intersection of isogyres = MELATOPEMELATOPE = the OPTIC AXIS (c)

Sectors between the isogyres = QUADRANTSQUADRANTS

Interference colours increase with distance from the melatope (c-axis)II

IIIIIIIIII

IVIV

Measuring the optical character (+)Measuring the optical character (+)

(+) Mineral with gypsum plate:

NE & SW: ne’ || n Addition

NW & SE: no || n 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 isogyre ( ≈ 100 nm) 2° blue (650 nm) in NE & SW (100+550) 1° yellow (450 nm) in NW & SE (100-

550)

(+) (+) Mineral:ne' > no

nnee slow ray slow ray

ne’

no

AddAddAddAddSubSub

SubSub

nn

no

ne’

ne’

ne’

no

no

Measuring the optical character (-)Measuring the optical character (-)

(+) (+) Mineral:ne' < no

nnee fast ray fast ray

ne’

no

SubSubSubSubAddAdd

AddAdd

nn

no

ne’

ne’

ne’

no

no

(-) Mineral with gypsum plate:

NE & SW: ne’ || n Subtraction

NW & SE: no || n 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 ( ≈ 100 nm) 1° yellow (450 nm) in NE & SW (100-

550) 2° blue (650 nm) in NW & SE (100+550)

Uniaxial Optic Axial Figures (OAF)Uniaxial Optic Axial Figures (OAF)

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

(+) with gypsum plateblue in I. quadrant

(-) with gypsm plateyellow in I. quadrant

OAF with uncentred melatope (Z)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?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,

remove sub-stage lens)5. Remove left ocular6. You should see an interference figure -

draw it7. Insert gypsum plate and note optic sign