Light in Minerals II Behavior of Light in Minerals • Absorption (light is absorbed by materials) – Color (absorption is a function of wavelength) – Pleochroism (absorption is a function of direction) • Refraction (light travels slowly in some materials) – Dispersion (velocity is a function of wavelength) – Birefringence (velocity is a function of direction) Refraction and Reflection Dispersion • When light strikes a polished surface of a material it is split into two rays. • One is reflected and the other refracted. Pleochroism: Behavior of Light in Minerals Tourmaline in • Absorption (light is absorbed by materials) Plane-Polarized Light – Color (absorption is a function of wavelength) – Pleochroism (absorption is a function of direction) • Refraction (light travels slowly in some materials) – Dispersion (velocity is a function of wavelength) – Birefringence (velocity is a function of direction) 1 Birefringence: Birefringence The difference in the indices Birefringence: The difference in the indices Light in Minerals • Polarization: Vibration directions • The Petrographic Microscope • Measuring Index of Refraction • Pleochroism • Birefringence: Optical anisotropy • The Indicatrix: • Uniaxial Minerals • Biaxial Minerals Scope Petrographic Oculars Microscope Bertrand Lens Analyzer Accessory Plate Objectives Stage Polarizer Focus 2 I grain > I oil Becke Line Method • As you increase the working distance, the white line moves to the medium of higher index. • If dispersion curves cross, liquid has higher slope and yellow moves in and blue moves out. Igrain > Ioil I grain < I oil Dispersion • The index of refraction (velocity) I grain = I oil varies with wavelength. • Blue is bent more than red. • Oil has higher dispersion than mineral 3 Optical Anisotropy Pleochroism • Pleochroism: – Different absorption for light vibrating in • Pleochroism is different directions. different – Mineral grains change color on rotation absorption in plane-polarized light. spectra for light vibrating in • Birefringence: different – Different velocities (index or refraction) directions for light vibrating in different directions. • Pleochroism is – Mineral grains change color on rotation observed non- between crossed polarizers cubic Fe-bearing minerals Pleochroism: Birefringence: Tourmaline in The difference in the indices Plane-Polarized Light Birefringence Birefringence • Birefringence is the color of a grain in crossed polars. • Birefringence is the difference between the operative indices of refraction of a grain times the thickness of the grain. • The color observed is controlled by the orientation of the grain and the thickness of the grain. 4 The Optical Indicatrix The Optical Indicatrix • For Hexagonal, Tetragonal and • The vibration direction of light Trigonal crystals the indicatrix is an determines the operative index of ellipsoid of rotation. There is one refraction in anisotropic media. circular section, and these crystals • The index of refraction is represented are said to be uniaxial. by an ellipsoid called the indicatrix. • For Orthorhombic, Monoclinic, and • The elliptical section perpendicular to Triclinic crystals the indicatrix is a the ray (propagation direction) general ellipsoid. There are two determines the operative indices of circular sections. These crystals are refraction. said to be biaxial. Orthoscope The Uniaxial Indicatrix • Normal imaging setup • Parallel light from below • Condenser out • Bertrand out Conoscope Conoscope • Convergent light • Find a grain near its optic axis • [Condenser in] • Gives a figure on back of objective • High power lens objective • Figure gives optic class and optic • Analyzer in sign • Bertrand in or ocular out • Optic class = isotropic, uniaxial, or biaxial • Optic sign = positive or negative 5 Uniaxial Uniaxial Sign Optic Axis Figure Uniaxial Sign The Biaxial Indicatrix Positive + Negative - Biaxial Biaxial Sign Figures Positive + Negative - 6 Estimate 2V Thin Section of Gabbro from Optic Axis Figure What to do when you look Low Power, at a rock in thin section Plane-Polarized Light • Count the number of minerals • Low Power, present. – Plane-Polarized Light • For Each Mineral, Note: – Crossed polars – Grain size • High-Power – Opacity – Plane-Polarized light – Color/Pleochroism • Crossed polars – Relief and Approximate Indices of Refraction (work in from the edge) – Grain Habit or Shape • Conoscope (check out a few grains of each mineral) Low Power, High Power - Orthoscope Crossed Polarizers • Check for oriented inclusions. • Birefringent or Isotropic ? • Look at accessory or other fine- • Maximum birefringence grained phases. • Extinction (parallel, inclined, symmetric, or asymmetric) High Power - Conoscope • Twinning • Check optic class/sign of unknown minerals 7 Textures Reflected Light • Phenocrysts: large crystals that grow in volcanic rock prior to eruption. • Use reflected light to examine • Cumulates: early-formed crystals that settle opaques in gabbros. – Color and contrast • Exsolution: crystals that grow inside other – Bireflectance crystals (sub solidus growth). – Inclusion textures • Twinning: orientation accidents in crystal growth or sub-solidus ordering. • Zoning: concentric changes in composition as crystals grow. • Reaction rims: chemical reactions at crystal rims. Sandstone Texture (cumulate) Zoned Feldspar Phenocryst Twinned K-Feldspar Phenocryst 8 Twinned Plagioclase Twinned Feldspar Feldspar (growth twins) Exsolved Pyroxene Twinned Exsolved Pyroxene Exsolved Pyroxene Zoned Pyroxene with cumulate olivine 9 Deformation Twinning in Deformation Twinning Calcite Kyanite Wadsleyite lamellae in olivine Reaction Rim on Coesite Optics Terms • Dispersion • Polarizer • Pleochroism • Analyzer • Index of refraction • Indicatrix • Anisotropy • Becke line • Birefringence • Refraction • Uniaxial • Diffraction • Biaxial • Conoscope • Orthoscope 10.