# Optical Mineralogy - PowerPoint PPT Presentation

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Optical Mineralogy. WS 2008/2009. Theory Exam…. Thursday 18th December @ 13:30 90 minutes Answer 3 questions from 5 Total of 30% of the course. Last week…. BIAXIAL INDICATRIX EXTINCTION ANGLES. Biaxial indicatrix - summary. Extinction Angle. I = 153,0°. Extinction angle

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Optical Mineralogy

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## Optical Mineralogy

WS 2008/2009

### Theory Exam….

• Thursday 18th December @ 13:30

• 90 minutes

• Answer 3 questions from 5

• Total of 30% of the course

### Last week….

• BIAXIAL INDICATRIX

• EXTINCTION ANGLES

### Extinction Angle

I = 153,0°

Extinction angle

e = I – II = 29,5°

For MONOCLINIC and TRICLINIC crystals….

Only the MAXIMUM extinction angle is diagnostic of a mineral measure lots of grains

II = 182,5°

### Compensator (Gypsum plate)

Gypsum plate (-plate) = helps in measuring the relative size of n (e.g. difference between fast and slow rays)

• Vibration direction of the higher n ray (slow ray) is NE-SW

• Vibration direction of the lower n ray (fast ray) is NW-SE

• Retardation = 550nm (= 1 order)

• Observed retardation (in diagonal position):

• Additionobs = Mineral + Gyps

• Subtractionobs = Mineral - Gyps

### Compensator (Gypsum plate)

slow // slow

constructive interference

colour increases

fast // slow

destructive interference

colour decreases

N

Example: Minerals with small birefringence (e.g. Quartz, Feldspar)

Mineral = 100 nm (1o Grey) in diagonal position:

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).

1o Grey

2o Blue

With analyser only

With analyser and compensator

### Subtraction

N

Turn the stage through 90°(Mineralstays 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.

1o Grey

1o Orange

With analyser and compensator

With analyser only

### Marking on vibration directions

• 1 – NE-SW diagonal position (extinction +45°), XPL

• Note the interference colour

• 2 – insert the gypsum plate

• Note the interference colour (addition or subtraction)

• 3 – rotate the mineral 90º

• Note the interference colour (addition or subtraction)

• 4 – Mark on the fast and slow rays

• How do these relate to pleochroic scheme?

• Also a helpful way to tell the order of the polarisation colour ….

### Hauptzone + or - = Length fast or length slow?

ng

na

ng

ALWAYS align length of mineral NE-SW

• If n parallel to slow ray (higher n) = addition

• Length slow

• Hauptzone +

• If n perpendicular to slow ray (lower n) = subtraction

• Length fast

• Hauptzone -

### Optical v Hauptzone character

Uniaxial minerals….

Prismatic crystals:

Optical and Hauptzone sign are the same….

Tabular crystals:

Optical and Hauptzone sign are different….

### Some examples….

Prismatic crystal:

Long dimension of mineral is parallel to the slow ray (n) =length slow = Hauptzone +

Optically positive +

Tabular crystal:

Long dimension of mineral is parallel to the slow ray (n) = length slow = Hauptzone +

Biaxial negative -

sillimanite

muscovite

### Exsolution (XN)

Exsolution lamellae albite in K-feldspar (perthite)

Exsolution lamellae of orthopyroxene in augite

### Undulose extinction (XN)

Undulose extinction in quartz, the result of strain

### Zoning (XN)

Reflects compositional differences in solid solution minerals

### Twinning (XN)

simple (K-feldspar)

polysynthetic (plagioclase)

cross-hatched or ‘tartan‘ (microcline)

sector (cordierite)

### Orthoscopic properties - summary

Orthoscopic, PPL

• Crystal shape/form

• Transparent or opaque

• Colour and pleochroism

• Relief and (variable) refractive index

• Cleavage, fracture

Orthoscopic, XN (in the diagonal position)

• Isotropic or anisotropic

• Maximum polarisation colour  birefringence (n)

• Extinction angle  crystal system

• Length fast or slow

• Zoning (normal, oscillatory, etc.)

• Twinning (simple, polysynthetic, sector)