review of optical mineralogy geol 5310 advanced igneous and metamorphic petrology 9/9/09

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  • Review of Optical Mineralogy

    GEOL 5310Advanced Igneous and Metamorphic Petrology


  • Nature of LightVisible light is a form of electromagnetic radiation, which can be characterized as pulses or waves of electrical energyTravels in straight lines with a transverse wave motion

    Unpolarized lightPolarized light

  • Attributes of LightWavelength () - distance between wave peaks; measured in angstroms (); defines color of visible lightAmplitude (A) - height of light waves; corresponds to the intensity/brightness of lightFrequency () - number of light waves passing a fixed point per second; measured in cycles/secondVelocity (v = ); speed of light in a vacuum = 31018 /sec = ce.g. for orange light in a vacuum, = 6000, = 51014 /sec

    Light slows down as it passes through denser substances. Because the frequency of light never changes as it passes through different substances, a decrease in light velocity reflects a proportional decrease in its wavelength.

  • Electromagnetic SpectrumFrom Bloss, 1961

  • Reflection and Refraction of Light When light passes from a low density medium (e.g. air) into a higher density non-opaque medium (e.g. a mineral), part will be reflected and part will be pass through, but be bent and slowed refracted. Angle of reflection (r) equals the incident angle (i)Angle of refraction (r) will differ from the incident angle depending on the change in velocity between the two substances

  • Refractive Index and Snells LawIndex of Refraction n nsubstance = c / vsubstance >1light velocity in air c, so nair ~ 1Snells Law- predicts the angle of refraction at the interface of two substances with different refractive indicies: ni sin i = nr sin r r = sin-1 (ni/nr x sin i)

  • Successive Refraction

  • Refraction, Relief, and the Becke LineRelief is the degree to which a phase stands out from its surroundings and is an expression of the contrast in index of refraction dark outline

  • Becke Line Test From Bloss (1961)

  • DispersionBecause n is related to light velocity, which is related to wavelength ((v = ), different wavelengths of light will have different refraction indicies within a particular substance Illuminating a mineral with white light may thus lead to color dispersion

  • Polarization of LightLight emanating from a point source vibrates in all directions normal to the propagation direction

    Light can be polarized (made to vibrate in one plane) by selective absorption (OR) or by reflectance (OL)

  • AnisotropyIndicies of refraction can vary in all minerals (except those in the isometric system) depending on the orientation of light ray. Such minerals are said to be anisotropic. Isometric minerals, glass, liquids and gasses have a single refraction index value regardless of the orientation of light rays. Such substances are said to be isotropic.

  • Optical Indicatrices A 3-d map of the indices of refraction for various vibration directions of light rays Orientation of the indicatrix within a mineral is symmetrical with the crystallographic axisIsotropic

    IsometricAnisotropic Uniaxial


  • Isotropic IndicatrixA sphere whose radius corresponds to the characteristic refraction index- nDiagram shows change in n for different wavelengths of light in same mineral4861Blue5893Yellow6563 Redn=c/v =c/

  • Optical Recognition of Isotropic MineralsFrom Bloss (1961)Total Extinction under X-polarsSlowing of ray = shortening of wavelength, but no change in polarity

  • Anistropic MineralsAll randomly oriented anisotropic minerals cause double refraction (splitting) of light resulting in mutually perpendicular-polarized light rays.

    One ray has a higher n (slow ray, or the ordinary ray) than the other ray (the fast ray, or extraordinary ray)

    Fast raySlow ray

  • Birefringence (), Retardation(), and Interference Colors = nslow ray nfast ray = d*

  • Uniaxial IndicatrixOptic Axis = C axis in tetragonal and hexagonal crystals

  • Sections of Uniaxial Indicatrices = - = 0 (circular section) = - (random section)= - (principal section)maximum birefringenceTotal extinction in x-polar light

  • Re-Polarization of Light through a Non-circular Section of the Uniaxial Indicatrix

  • Extinction of Uniaxial Minerals

  • Conoscopic Interference Figures of Uniaxial MineralsOrthoscopic ConoscopicIsochromes zones of equal retardationIsogyres represent the areas where the and vibration directions are oriented N-S, E-W

  • Uniaxial Optic Axis (OA) FigureCircular section parallel to stage = 0

  • Off-centered OA FigureRandom section parallel to stage, < 0, max

  • Very Off-centered OA FigureRandom section parallel to stage, 0, < max

  • Flash FigurePrincipal section parallel to stage, = max

  • Determining the Optic Sign of Uniaxial MineralsConnect the quadrants that go down in color (to yellow), compare with slow direction of gypsum plate for sign+

  • Biaxial IndicatrixPrincipal vibration axesgreatest nlowest nintermediate n<
  • Circular Sections and Optic AxesCircular SectionCircular SectionOptic AxesOptic Plane

  • 2V and the Optic Sign+-Trace of CircularSections

  • Random Section through the Biaxial IndicatrixVibration plane parallel to stageDouble refractionrays

  • Variable Birefringence within a Biaxial Mineral=0=max

  • Biaxial Optic Axis FiguresLook for a mineral with the lowest interference colors, i.e. ~0

  • Acute Bisectrix Figures (Bxa)Melatope (emergence of optic axes)

  • Determining the Optic Sign of Biaxial MineralsD+D-UUDDDD is fast ray is intermediate is slow ray-+XXUUUU

  • Estimating 2V by Curvature of Isogyre

  • Estimating 2V by Separation of Isogyres

  • Extinction AngleSymmetricalParallelInclined

  • Sign of ElongationslowrayExample Length slowInterference colors increaseSlowing down the slow ray



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