Optical Mineralogy

1. Optical Mineralogy WS 2008/2009

2. Theory Exam…. • Thursday 18th December @ 13:30 • 90 minutes • Answer 3 questions from 5 • Total of 30% of the course

3. Last week…. • BIAXIAL INDICATRIX • EXTINCTION ANGLES

4. Biaxial indicatrix - summary

5. 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°

6. 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

7. Compensator (Gypsum plate) slow // slow constructive interference colour increases fast // slow destructive interference colour decreases

8. 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

9. 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

10. 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 ….

11. 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 -

12. Optical v Hauptzone character Uniaxial minerals…. Prismatic crystals: Optical and Hauptzone sign are the same…. Tabular crystals: Optical and Hauptzone sign are different….

13. 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

14. Exsolution (XN) Exsolution lamellae albite in K-feldspar (perthite) Exsolution lamellae of orthopyroxene in augite

15. Undulose extinction (XN) Undulose extinction in quartz, the result of strain

16. Zoning (XN) Reflects compositional differences in solid solution minerals

17. Zoning

18. Twinning (XN) simple (K-feldspar) polysynthetic (plagioclase) cross-hatched or ‘tartan‘ (microcline) sector (cordierite)

19. 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)