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Optical Mineralogy WS 2012/2013. THEORY OF OPTICS – first 6 or 7 weeks of lectures and practicals MINERAL CHEMISTRY – lectures January onwards IDENTIFICATION.

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Presentation on theme: "Optical Mineralogy WS 2012/2013. THEORY OF OPTICS – first 6 or 7 weeks of lectures and practicals MINERAL CHEMISTRY – lectures January onwards IDENTIFICATION."— Presentation transcript:

1 Optical Mineralogy WS 2012/2013

2 THEORY OF OPTICS – first 6 or 7 weeks of lectures and practicals MINERAL CHEMISTRY – lectures January onwards IDENTIFICATION OF ROCK FORMING MINERALS – pracs January onwards EXAM – Week beginning February 4th Course structure

3 Downloads (pdfs) for lectures and practicals Mineral identification sheets Links to other resources Lots more.... Website…. There is lots of EXCELLENT freely available material (English and German) on the internet….

4 l Mineral identification l Rock identification l Microstructural/textural investigation ….this leads to (for example)…. l Crystallisation sequence (igneous petrology) l Deformation history (structural geology) l Infer reactions & P–T (metamorphism) l Diagenetic processes (sedimentary petrology) l Alteration processes (e.g., weathering) l.... and it is CHEAP.... Why use a polarizing microscope??

5 Garnet Olivine Clinopyroxene Orthopyroxene Hornblende Glaucophane Biotite Muscovite Chlorite Epidote Minerals you will learn – 20(ish) Spinel Cordierite Andalusite Sillimanite Kyanite Staurolite Calcite Plagioclase feldpar Alkali feldspar Quartz

6 What is the nature of light? l Particles or quanta – photons (Newton) ? …or… l Electromagnetic Waves (Huygens) ? ….it’s both…. Particle-Wave duality (e.g. Einstein, de Broglie) For the purpose of mineral optics, light is best explained as a WAVE….

7 Electromagnetic Radiation Transverse waves that are mutually perpendicular E = Electrical field M = Magnetic field C = Propagation direction For crystal optics we only need to consider the electrical field E M

8 The Electromagnetic Spectrum

9 Visible light: 750 nm (red)  400 nm (violet) The Visible Spectrum DISPERSION through a prism shows us that white light consists of a mixture of all colours of the visible spectrum....

10 Colour

11 Absorption and Colour Absorption colour l Selective absorption of certain wavelengths  Absorption colour l The absorption colour is complimentary to the absorbed wavelengths! l An example: a green mineral (e.g. hornblende): FRed/orange and blue/violet wavelengths are absorbed FTransparent for green light Note: Very rarely, colour effects are from interference and diffraction

12 Reflection and Refraction

13 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 )

14 Velocity of light and refractive index Index of Refraction or Refractive Index (n): The ratio of the velocity of light in a vacuum/air (c) to the velocity of light through a material (v) is called the Index of Refraction or Refractive Index (n): n m = c/v n is inversely proportional to v (n air = c/c = 1) Light is slowed down when it enters a denser material, so n m > 1. This slowing down reduces the wavelength of light (the energy and frequency stay constant): Velocity of light: v =  Light energy: E = h  = hv/ h = Planck‘s constant;  = frequency (remains constant); = wavelength; c ≈ km/s; v = velocity of light in a material (v 1)

15 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 

16 0% 10% 20% 30% 40% n RReflectance Reflectance, R: I 0 = Intensity of incident (source) light I R = Proportion of reflected light n = Refractive IndexLustre Glassy:n = 1,3-1,9  R = 1,7-10% Adamantine:n = 1,9-2,6  R = 10-20% Sub-metallic:n = 2,6-3  R = 20-25% Metallic:n > 3  R > 25%

17 Total Internal Reflection and the Critical Angle Snell’s Law: n 2. sin  int = n 1. sin  ext n 2 sin  crit = n 1 sin(90°) sin(90°) = 1, n 1 = 1 n 2 sin  crit = 1 sin = 1/n 2 sin  crit = 1/n 2 If = T.I.R. If  >  crit = T.I.R. Example: Diamond n = 2,42   crit = 24°

18 Dispersion

19 Dispersion The refractive index of a material depends on the frequency and/or the wavelength of the light. - i.e. red light, with a longer wavelength, is refracted less than blue light: n = c/v  n = c/  For most minerals, dispersion is small and not a problem. However, with other minerals it can be a problem as it affects the other optical properties (e.g. sphene, zircon, some amphiboles)….

20 Polarization Plane polarized lightPPL) Plane polarized light (PPL)  only vibrates in a single plane. Light can be polarized by: F Reflection F Double refraction F Selective absorption F Dispersion Normal light is NOT polarized and vibrates in all possible directions!

21 Polarization by reflection l Reflected polarized light is parallel to the surface l Maximum polarization occurs when the angle between the directions of the reflected and refracted beams is 90°: BREWSTER‘S ANGLE B BREWSTER‘S ANGLE  B B tan  B = n

22 Polarization by selective absorption Dichroism Dichroism = preferred absorption of particular oscillation directions: Tourmaline - a naturally occurring mineral Polaroid - a type of synthetic plastic sheet used to polarize light

23 Polarization by double refraction l In most minerals (all except those of the cubic system), non- polarized light is split into 2 polarized rays l The rays have different n   n = BIREFRINGENCE l These rays are mutually perpendicular l Example: calcite rhomb - light is split into an ordinary ray (o-ray) and an extraordinary ray (e-ray)

24 The Polarizing Microscope

25 Objective and Ocular lenses l Magnification = Objective × Ocular: Fe.g. 40 × 10 = 400x l Numerical aperture, A: A = n ∙ sin  l Maximum resolution, d: Objective Ocular

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

27

28 The Polarizing Microscope

29 Orthoscopic (parallel light) 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 POLARISED LIGHT (XPL) - with the analyser IN birefringence extinction angle twinning and zoning Orthoscopic Microscopy

30 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…. PPL - PPL - Grain size

31 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 = idiomorphic  SUBHEDRAL = hypidiomorphic  ANHEDRAL = xenomorphic PPL - PPL - Crystal habit (shape)

32  EUHEDRAL = idiomorphic  SUBHEDRAL = hypidiomorphic  ANHEDRAL = xenomorphic PPL - PPL - Crystal habit (shape)

33 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….

34 Colour is caused by selective absorption of certain wavelengths Colour (absorption colour) PPL Colour (absorption 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

35 Absorption and Colour Absorption colour l Selective absorption of certain wavelengths  Absorption colour l The absorption colour is complimentary to the absorbed wavelengths! l An example: a green mineral (e.g. hornblende): FRed/orange and blue/violet wavelengths are absorbed FTransparent for green light Note: Very rarely, colour effects are from interference and diffraction

36 Pleochroic scheme: Biotite Pale brown N-S Dark brown E-W

37 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

38 The amount that a mineral stands out in the section 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 (glue) 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

39 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

40 Relief

41 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?

42 Becke Line l 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…. l HHHHh h l HHH = Beim Herablassen des Tisches wandert die helle Linie in das höherbrechende Mineral. +

43 Becke Line

44 The Polarizing Microscope


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