Optical Mineralogy

1. Optical Mineralogy WS 2008/2009

2. Examinations 1) Mid-term - December 11 @ normal time (13:30) THEORY TEST 2) Finals - February 10 (probably) PRACTICAL TEST (1 THIN SECTION)

3. 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)

4. Refraction - Snell‘s Law sini = AB/CB  CB = AB/sini sinr = CD/CB  CB = CD/sinr  AB/sini = CD/sinr …but… v1 = AB and v2 = CD  v1/sini = v2/sinr  Snell‘s Law v1/v2 = sini/sinr = n2/n1 ....as n2=c/v2 and n1=1.... n2 = v1/v2 = sini/sinr Note: v2 is difficult to measure but sin1 and sin2 are not ….

5. 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)

6. The Polarizing Microscope

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

8. Orthoscopic Microscopy 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

9. Grain size • 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….

10. PPL - Crystal habit (shape) • 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

11. Crystal 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….

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

13. Absorption and 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

14. 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….

15. PPL - Cleavage • 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!

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

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

18. Why do we see relief? relief (+) no relief relief (-) nmin > nepoxy nmin = nepoxy nmin < nepoxy Minerals with different refractive indices (n), cause diffraction, refraction and reflection of the light at grain boundaries: © Jane Selverstone, University of New Mexico, 2003

19. Becke Line dark light = 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…. • HHH = Beim Herablassen des Tisches wandert die helle Linie in das höherbrechende Mineral. n1 n2 n2 > n1

20. Estimating the Refractive Index (n)

21. Mineral ID Sheets….