Optical Mineralogy

WS 2012/2013

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....

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

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).

?

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

?

Hauptzone + or -?

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

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

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

Conoscopic ray paths

Retardation () is NOT constant!

n dependent on angle Different interference colours

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

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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34

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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34

Uniaxial Optic Axial Figures (OAF)

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

(+) with gypsum plateblue in I. quadrant

(-) with gypsum plateyellow 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 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