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. Biaxial indicatrix - summary. Extinction Angle. I = 153,0°. Extinction angle

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Optical Mineralogy

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Optical mineralogy

Optical Mineralogy

WS 2008/2009


Theory exam

Theory Exam….

  • Thursday 18th December @ 13:30

  • 90 minutes

  • Answer 3 questions from 5

  • Total of 30% of the course


Last week

Last week….

  • BIAXIAL INDICATRIX

  • EXTINCTION ANGLES


Biaxial indicatrix summary

Biaxial indicatrix - summary


Extinction angle

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

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 plate1

Compensator (Gypsum plate)

slow // slow

constructive interference

colour increases

fast // slow

destructive interference

colour decreases


Addition

Addition

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

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

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

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

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

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 (XN)

Exsolution lamellae albite in K-feldspar (perthite)

Exsolution lamellae of orthopyroxene in augite


Undulose extinction xn

Undulose extinction (XN)

Undulose extinction in quartz, the result of strain


Zoning xn

Zoning (XN)

Reflects compositional differences in solid solution minerals


Zoning

Zoning


Twinning xn

Twinning (XN)

simple (K-feldspar)

polysynthetic (plagioclase)

cross-hatched or ‘tartan‘ (microcline)

sector (cordierite)


Orthoscopic properties summary

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)


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