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Optical Mineralogy

Optical Mineralogy

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Optical Mineralogy

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  1. Optical Mineralogy WS 2012/2013

  2. The week before last…. • BIAXIAL INDICATRIX • EXTINCTION ANGLES

  3. Biaxial indicatrix - summary

  4. 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°

  5. Compensator (Gypsum plate) Gypsumplate (-plate) = helps in measuringthe relative sizeofn(e.g. allowsidentificationoffastandslowrays) • 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

  6. Addition 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 (1oRed) Gobs = GMineral + GGyps  Gobs = 650 nm (2o Blue) ? Whentheinterferencecolouris 1ohigher (addition), thenthe NE-SW directionisthehighern - slowray (parallel tonofthegypsumplate). 1o Grey 2o Blue Withanalyseronly Withanalyserandcompensator

  7. Subtraction Turn the stage through 90°(Mineralstays at 100 nm) GMineral = 100 nm (1o Grey) GGips = 550 nm (1oRed) Gobs = |GMineral – GGips|  Gobs = 450 nm (1o Orange) ? Whentheinterferencecolouris 1olower (subtraction), thenthe NE-SW directionisthelowern - fast ray. 1o Grey 1o Orange Withanalyserandcompensator Withanalyseronly

  8. Marking on vibration directions • 1 – Rotate into extinction and draw the grain and its privileged vibration directions • 2 – Rotate 45° until the polarisation colour is brightest • Note the interference colour • 3 – insert the gypsum plate • Note the interference colour (addition or subtraction) • 4 – rotate the mineral 90º • Note the interference colour (addition or subtraction) • 5 – Mark the fast (short line) and slow (long line) rays • How do these relate to pleochroic scheme? • Also a helpful way to tell the order of the polarisation colour ….

  9. Length fast or length slow? ng na ng ALWAYS align length of mineral NE-SW Ifslowray (n)ofcompensatoris parallel totheslowrayofthemineral(highern) (Addition) Lengthslow Ifslowray (n)ofcompensatorisperpendiculartoslowrayofthemineral (lowern) (Subtraction) Length fast = Hauptzone - = Hauptzone +

  10. Hauptzone + or -?

  11. Optical character and Hauptzone Uniaxialminerals…. Prismatic crystal: If HZ + and Optically + If HZ - and Optically - Tabular crystal: If HZ + and Optically - If HZ - and Optically +

  12. Optical character and HZ Long dimension of mineral is parallel to the slow ray(n) = LENGTH SLOW (HZ +) = PRISMATIC CRYSTAL Long dimension of mineral is parallel to the slow ray (n) = LENGTH SLOW (HZ +) = TABULAR CRYSTAL Sillimanite (+) Muscovite (-)

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

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

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

  16. Zoning

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

  18. So why do we see polarisation colours?

  19. Retardation (Gangunterschied) • After time, t, whentheslowrayisabouttoemergefromthemineral: • The slowrayhastravelleddistance d….. • The fast rayhastravelledthedistance d + …..  = retardation Fast wave with vf (lower nf) Slow wave with vs (higher ns) Slow wave: t = d/vs Fast wave: t = d/vf + /vair …and so d/vs = d/vf + /vair  = d(vair/vs - vair/vf)  = d(ns - nf)  = d ∙ Δn d Mineral Polarised light (E_W) Retardation,  = d ∙ Δn (in nm) Polariser (E-W)

  20. Interference • Polariser forces light to vibrate E–W • Light split into two perpendicular rays • Analyser forces rays to vibrate in the N-S plane and interfere. • Destructive interference (extinction):  = k∙ k = 0, 1, 2, 3, … • Constructive interference (maximum intensity):  = (2k+1) ∙ /2 k = 0, 1, 2, 3, …

  21. Retardation,  550 550 550 550 550 550 Wavelength, 400 440 489 550 629 733 13/8 l 11/4 l 11/8 l 1 l 7/8l 3/4 l Nogreen(eliminated) red + violetpurpleinterferencecolour Fig 7-7 Bloss, Optical Crystallography, MSA

  22. Retardation, 800 800 800 800 800 800 800 Wavelength, 400 426 457 550 581 711 800 2l17/8l13/4l11/2l 13/8l1 1/8l1l Noredorviolet (eliminated) greeninterferencecolour Fig 7-7 Bloss, Optical Crystallography, MSA

  23. 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)