Optical MineralogyOptical Mineralogy

WS 2008/2009WS 2008/2009

ExaminationsExaminations

1) Mid-term - December 11 @ normal time 1) Mid-term - December 11 @ normal time (13:30)(13:30)

THEORY TESTTHEORY TEST

2) Finals - February 10 (probably)2) Finals - February 10 (probably)

PRACTICAL TEST (1 THIN SECTION)PRACTICAL TEST (1 THIN SECTION)

Reflection and RefractionReflection and Refraction

1) Reflection:1) Reflection:

Angle of Incidence = Angle of Reflection

(i1 = ref1; i2= ref2)

2) Refraction:2) Refraction:

Angle of Incidence ≠ Angle of Angle of Incidence ≠ Angle of Refraction (light is ‘bent’)Refraction (light is ‘bent’)

(i3 ≠ r3)

Refraction - Snell‘s LawRefraction - Snell‘s Law

Snell‘s Law

v1/v2 = sini/sinr = n2/n1

....as n2=c/v2 and n1=1....

n2 = v1/v2 = sini/sinr

Note: v2 is difficult to measure but sin1 and sin2 are not ….

sini = AB/CB CB = AB/sini sinr = CD/CB CB = CD/sinr

AB/sini = CD/sinr

…but… v1 = AB and v2 = CD

v1/sini = v2/sinr

Polarisation by double refractionPolarisation by double refraction

In most minerals (all except those of the cubic system), non-polarized light is split into 2 polarized rays

The rays have different n n = BIREFRINGENCE These rays are mutually perpendicular Example: calcite rhomb - light is split into an ordinary ray

(o-ray) and an extraordinary ray (e-ray)

The Polarizing MicroscopeThe Polarizing Microscope

Thin SectionsThin Sections

Glass slide Glue (Epoxy resin) Thin rock slice (30 µm = 0,03 mm) Glue (n = 1,54) Glass cover slip (≈ 1 mm)

+ + + + =

30 µmCover slipRock sliceGlass slide

Observations can be made in:

• PLANE POLARISED LIGHT (PPL) - with the analyser OUT• crystal shape/habit• colour/pleochroism• cleavage/fracture• relief, Becke test refractive index estimation

• CROSSED NICOLS (XN) - with the analyser IN• birefringence• extinction angle• twinning and zoning

Orthoscopic MicroscopyOrthoscopic Microscopy

• Using a ruler, measure the field of view for each objective lens….

• This can then be used to measure maximum and minimum grain size and grain size ranges….

Grain size

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are needed to see this picture.

• Thin sections are 2d cuts through 3d crystals• Habits dependent on crystal system, the angle of cut and how perfectly formed the crystals are:

EUHEDRAL

SUBHEDRAL

ANHEDRAL

PPL - PPL - Crystal habit (shape)

Crystal habitsCrystal habits

Acicular Needle-like

Bladed Blade-like

Equant Length & width roughly equal Fibrous Slender prisms

Poikiloblastic With many inclusions

Prismatic Elongate, prism-like

Tabular Tablet-shaped

….etc., etc….

• Colour is caused by selective absorption of certain wavelengths • Colour (body colour) Colour (body colour) must always be observed using PPLPPL• PleochroismPleochroism = direction controlled absorption

• different colours depend on crystallographic orientation measured by rotating the microscope stage

plag

hbl

pla

g

hbl

- Plagioclase is colourless- Hornblende is pleochroic: light green to olive green

PPL - Colour & PleochroismPPL - Colour & Pleochroism

Absorption and ColourAbsorption and Colour Selective absorption of certain wavelengths Absorption colourAbsorption colour The absorption colour is complimentary to the absorbed

wavelengths! An example: a green mineral (e.g. hornblende):

Red/orange and blue/violet wavelengths are absorbed Transparent for green light

Note: Very rarely, colour effects are from interference and diffraction

Pleochroic scheme: BiotitePleochroic scheme: Biotite

Pale brown with length N-S Dark brown with length E-W

WARNING - many microscopes show false pleochroism where colourless minerals show pleochroism in pale pastel colours….

• How many? • e.g., 0, 1, 2

• Angular relationship? • e.g., 90°, 60°

• How well developed? • Weak, moderate, good

• Beware - Fractures can be easy to mistake as cleavage!

PPL - CleavagePPL - Cleavage

• The amount that a mineral stands out• Can be absent, low, moderate, high or very high• Relief is a measure of the relativerelative refractive index (ΔΔnn) between the mineral and the epoxy • Relief can provide an estimate ofof nn

Garnet: n = 1,72-1,89Quartz: n = 1,54-1,55Epoxy: n = 1,54

Quartz: very low relief Garnet: high relief

PPL - ReliefPPL - Relief

ReliefRelief

Relief can be positive or negative. A mineral can have moderate relief but a refractive index lower than the epoxy (e.g. fluorite):

negative reliefpositive relief

epoxy

Garnet Olivine Quartz Albite Sodalite Fluorite

Very high relief is called CHAGRIN where n > 1.75

nmin > nepoxy nmin < nepoxy

nmin = nepoxy

relief (+) no relief relief (-)

Minerals with different refractive indices (n), cause diffraction, refraction and reflection of the light at grain boundaries:

© Jane Selverstone, University of New Mexico, 2003

Why do we see relief?Why do we see relief?

Becke LineBecke Line

As you As you lower the stagelower the stage (i.e. increase the distance between the objective and sample), (i.e. increase the distance between the objective and sample), the the Becke line moves into the mineral of higher reliefBecke line moves into the mineral of higher relief….OR….….OR….

HHHHHH = Beim HHerablassen des Tisches wandert die hhelle Linie in das hhöherbrechende Mineral.

n2n1

dark light = Becke Line= Becke Line

n2 > n1

Estimating the Refractive Index (Estimating the Refractive Index (nn))

Relief abschätzen

nein1,45 < n < 1,65

ja1,45 > n > 1,65

Chagrin abschätzenBecke-Linie

relativ zu Epoxid

nein1,65 < n < 1,75

jan > 1,75

Mineral ID Sheets….Mineral ID Sheets….