Optical Mineralogy WS 2008/2009

Examinations 1) Mid-term - December normal time (13:30) THEORY TEST 2) Finals - February 10 (probably) PRACTICAL TEST (1 THIN SECTION)

Reflection and Refraction 1) Reflection: Angle of Incidence = Angle of Reflection (  i1 =  ref1;  i2 =  ref2 ) 2) Refraction: Angle of Incidence ≠ Angle of Refraction (light is ‘bent’) (  i3 ≠  r3 )

Refraction - Snell‘s Law Snell‘s Law v 1 /v 2 = sin  i /sin  r = n 2 /n 1....as n 2 =c/v 2 and n 1 =1.... n 2 = v 1 /v 2 = sin  i /sin  r Note: v 2 is difficult to measure but sin  1 and sin  2 are not …. sin  i = AB/CB  CB = AB/sin  i sin  r = CD/CB  CB = CD/sin  r  AB/sin  i = CD/sin  r …but… v 1 = AB and v 2 = CD  v 1 /sin  i = v 2 /sin  r 

Polarisation 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 Microscope

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

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 habits Acicular Needle-like Bladed Blade-like EquantLength & width roughly equal Fibrous Slender prisms PoikiloblasticWith many inclusions Prismatic Elongate, prism-like Tabular Tablet-shaped ….etc., etc….

Colour is caused by selective absorption of certain wavelengths Colour (body colour) PPL Colour (body colour) must always be observed using PPL Pleochroism Pleochroism = direction controlled absorption different colours depend on crystallographic orientation measured by rotating the microscope stage plag hbl plag hbl - Plagioclase is colourless - Hornblende is pleochroic: light green to olive green PPL - Colour & Pleochroism

Absorption and Colour Absorption colour Selective absorption of certain wavelengths  Absorption 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: 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 - Cleavage

The amount that a mineral stands out Can be absent, low, moderate, high or very high relative Δn Relief is a measure of the relative refractive index (Δn) between the mineral and the epoxy of n Relief can provide an estimate of n Garnet:n = 1,72-1,89 Quartz:n = 1,54-1,55 Epoxy:n = 1,54 Quartz: very low relief Garnet: high relief PPL - Relief

Relief Relief can be positive or negative. A mineral can have moderate relief but a refractive index lower than the epoxy (e.g. fluorite): negative relief positive relief epoxy Garnet Olivine Quartz Albite Sodalite Fluorite Very high relief is called CHAGRIN where n > 1.75

n min > n epoxy n min < n epoxy n min = n epoxy relief (+)no reliefrelief (-) 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?

Becke Line As you lower the stage (i.e. increase the distance between the objective and sample), the Becke line moves into the mineral of higher relief….OR…. As you lower the stage (i.e. increase the distance between the objective and sample), the Becke line moves into the mineral of higher relief….OR…. HHHHh h HHH = Beim Herablassen des Tisches wandert die helle Linie in das höherbrechende Mineral. n2n2 n1n1 = Becke Line dark light = Becke Line n 2 > n 1

Estimating the Refractive Index (n) Relief abschätzen nein 1,45 < n < 1,65 Becke-Linie relativ zu Epoxid ja 1,45 > n > 1,65 Chagrin abschätzen nein 1,65 < n < 1,75 ja n > 1,75

Mineral ID Sheets….