Class Notes (811,690)
Canada (494,876)
EASC 205 (2)

Kevin's Optical Mineralogy Class Notes

13 Pages
Unlock Document

Earth Sciences
EASC 205
Kevin Cameron

Introduction to Petrology -Petrology: “the study of rocks” -There are two components of petrology: 1. Optical Mineralogy: -relief, cleavage, twinning, extinction, retardation, birefringence, optical character 2. Petrography: -the branch of geology dealing with the mineralogical and textural description of a rock in order to completely name the rock -involves the use of hand sample and thin section techniques -Optical Mineralogy: examines optical properties of minerals using the petrographic microscope -Optical properties of minerals depend on the way that light is transmitted through the crystal -therefore, it is dependent on: crystal structure, crystal symmetry, and chemical composition of the mineral Light -The strength of the chemical bonds dictates the cleavage -cleavage occurs along planes where the bonds are the weakest -Light is explained by two theories: 1. Particle Theory 2. Wave Theory -Light: electromagnetic radiation that has properties of waves -has electric and magnetic components and properties -The electromagnetic spectrum is concerned with: -visible light: light with a wavelength between 380-770 nm -1 mm = 1000 μm = 1000 nm -monochromatic light: light with one single wavelength -white light: light which combines all the wavelengths -“dark”/black light: no wavelengths -Light can be described using the same nomenclature applied to any wave phenomenon -Light is described in terms of velocity, frequency, and wavelength -Light does not consist of a single wave but an infinite number of waves that travel together from a light source -Wave Normal: a line perpendicular to the wave front, representing the direction the wave is moving -Wave fronts: all parallel surfaces connecting equivalent points on adjacent waves -successive wave fronts are 1 wavelength apart Velocity of Light and Refractive Index -Energy of light is related to its frequency and velocity 𝐸 = ℎ𝑓 = ℎ𝑉 𝜆 -If light travels through a substance, velocity decreases 𝑉 = 𝑓𝜆 -When the light passes through a substance, frequency remains constant -Thus, if the velocity is reduced on passage through a substance the wavelength must also decrease -Refractive index, n, of a material is defined as the ratio of the speed of light in a vacuumv V , to the speed of light in a material through which it passes, m 𝑉𝑣 𝜂 = 𝑉𝑚 -Vvis the maximum velocity, therefore n>1 -Vmdepends on the density of a material: V dmcreases with an increasing density -A high Vmimplies a low n -Refractive indices depend on the wavelength of light because different wavelengths are interfered with to different extents by the atoms that make up the material -In general, refractive index varies with wavelength Optically Isotropic Versus Anisotropic Materials -Structure of a material can change the velocity of light as it passes through crystals. Materials can be divided into two classes based on the material and the nature of the chemical bonds holding the material together: 1. Isotropic -materials whose refractive index does not depend on the direction that the light travels -have a single refractive index -minerals that crystallise in the isometric system (and isotropic materials such as glass, gases, most liquids, and amorphous solids) -the wave normal and the direction of propagation of the light rays are perpendicular to the wavefront (parallel to each other) 2. Anisotropic -materials whose refractive index does depend on the direction that the light travels -these materials will have a range of refractive indices between two extremes -There are two categories of anisotropic minerals: a. Uniaxial – minerals characterised by two refractive indices and one optic axes -minerals of the tetragonal and hexagonal crystal systems b. Biaxial – minerals characterised by three refractive indices and two optic axes -minerals of the orthorhombic, monoclinic, and triclinic crystal systems -light rays are not perpendicular to the wave normal Reflection and Refraction of Light -When light strikes an interface between two substances with different refractive indices, two things occur: 1. Reflection: light bounces of the surface -Law of Reflection: θ1=θ2 2. Refraction: bending of light as it travels into a transparent substance with different refractive indices -Snell’s Law: the angle of refraction, r, is dependent on the angle of incidence, i, and the refractive index of the materials on either side of the interface 𝑛𝑖sin 𝑖 = 𝑛 s𝑟n⁡(𝑟) -can be used to calculate how much the light will bend on traveling into another medium -In general, light is refracted towards the normal to the boundary on entering a material with a higher refractive index and is refracted away from the normal on entering a material with a lower refractive index Refraction of Light -Refractometry involves the determination of the refractive index of materials -If the indices of refraction of the mineral and surrounding medium are the same, light passes through the boundary unrefracted -If the refractive index of two materials are different, then the light travelling through the boundary is refracted and a grain will appear to “stand out” -Relief: the degree to which a mineral grain appears to stand out from the mounting material -mounting medium refractive index = 1.54 = quartz, therefore, quartz appears invisible -Dispersion of Light: an effect produced because the refractive indices are different for each wavelength of light. -the light at the violet end of the spectrum is more strongly refractive than the light at the red end of the spectrum -The refractive index for longer wavelengths (red) are lower than those for shorter wavelengths (violet) Light Absorption and Colour -The colour of a material is the colour of light that is not absorbed on transmission or reflection Polarisation of Light -Normal white light  vibrates in all directions perpendicular to its path of propagation -Plane Polarised Light (PPL): -light that is constructed to vibrate in only one plane (vibration direction) perpendicular to the direction of propagation -The light ray passes through a filter which has a single, preferred vibration direction -There are 2 common ways that light can be polarised: 1) Reflection off a non-metallic surface (glare) 2) Selective absorption: passing light through a substance that absorbs light vibrating in all directions except one -anisotropic crystals have this property in certain directions (called privileged directions) -Polaroid film: long-chain organic molecules that are aligned in one direction and placed in a plastic sheet -If a beam on non-polarised light encounters a polariser, only light vibrating parallel to the polarisation direction of the polariser will be allowed to pass -If another polariser with its polarisation direction oriented perpendicular to the first polariser is placed in front of the beam of polarised light, then no light will pass through the second polariser Absorption of Light -when light encounters a transparent material some of its energy is dissipated as heat energy -when this absorption of energy occurs selectively for different wavelengths of light, the light that gets transmitted through the material will show only those wavelengths of light that are not absorbed -the transmitted wavelength will then be seen as colour (absorption colour of the material) -Ore minerals/metallic minerals are opaque materials  cannot use transmitted light, needs reflective light -Some minerals have more than one absorption colour because the different wavelengths are absorbed to different extents -dependent on the direction of vibration of the light as it passes through the mineral -Pleochroism: the change in colour of anisotropic minerals when the stage is rotated (observed in PPL) -produced because PPL is split into two rays when passing through an anisotropic mineral -this property is most distinct when the optic axis (usually C-axis) is parallel to the microscope stage -Orientations with the optic axis perpendicular to the microscope stage will not display pleochroism -Isotropic minerals have no optic axis therefore they do not exhibit pleochroism -a mineral must be coloured to show pleochroism Optical Properties Observed in Crossed Polarised Light (XPL): Isotropic vs. Anisotropic -Isotropic: when viewed in XPL they will appear black and stay black on a complete rotation of the stage -Why? Isotropic minerals do not have a preferred direction, meaning the velocity of light is the same in all directions in these minerals. -light passing through the mineral “sees” the same electronic environment in all directions regardless of the direction the light takes through the mineral -isotropic minerals do not affect the polarisation direction of the light which has passed through the lower polariser. Therefore, light which passes through the mineral is absorbed by the upper polariser -Anisotropic: when viewed in XPL they will allow some light to pass and thus will be generally “light” or show colour (unless in specific orientations) -Why? Anisotropic minerals do affect the polarisation of the light passing through them -Therefore, some component of the light is able to pass through the upper polarisers -anisotropic minerals will appear dark, “extinct,” every 90° of rotation of the stage when viewed between crossed polarisers Double Refraction -Minerals which have more than one refractive index have a property which is known as double refraction  only anisotropic minerals (e.g. Calcite) -The amount of double refraction depends on the difference in refractive index e.g.) Calcite: nω=1.658, n ε 1.486  Δn = 1.658-1.486 = 0.172 (Large Value) e.g.) Quartz: Δn=n -ω =ε.553-1.544=0.009  very small value, therefore it will not bend/separate the different light rays very well -a mineral must be transparent to see double refraction in hand sample -double refraction is the result of anisotropic minerals splitting light into 2 rays which travel through the mineral with different velocities (different refractive indices): -Fast ray: has a higher velocity and a lower refractive index -Slow ray: has a lower velocity and a higher refractive index -Each ray vibrates perpendicular to the other -every anisotropic mineral has either one or two directions along which the light is not split into two rays -called the optic axis -Uniaxial minerals have one optic axis (hexagonal and tetragonal crystal systems) -Biaxial minerals have two optic axes (orthorhombic, monoclinic, and triclinic crystal systems) -Isotropic minerals have no optic axis (isometric crystal system) Birefringence -A quantitative measure of double refraction is birefringence -the numerical difference between the maximum and minimum refractive index of a mineral Birefringence=δ=n max-nmin slown fastunitless) -The maximum birefringence is a useful diagnostic property of minerals -The birefringence chart: -Colour intensity decreases as you move to the right -The first pink band is at 530 nm which is the first boundary between colour orders -the numerical value of birefringence depends on the path followed by the light through the mineral -If the path of light is parallel to an optic axis: No birefringence (optic axis perpendicular to microscope stage) -appears black, behaves like an isotropic mineral -If the path of light is perpendicular to an optic axis: Maximum birefringence (optic axis parallel to microscope stage) -any other orientation: an intermediate birefringence -The maximum birefringence is a useful diagnostic property of minerals ****SEE HAND OUT “JANUARY 20, 2014 #1”**** Retardation -the fast and slow rays have different velocities and thus emerge from the crystal at different times -Retardation: the amount that the slow ray lags behind the fast ray as it exits the crystal -related to the difference in refractive indices of the 2 rays and the thickness (d) of the mineral Retardation=Δ=d(n slownfastδ (in nm) ****SEE HANDOUT “JANUARY 20, 2014 #2**** -If the light waves are retarded they are slowed down -Thickness, d, can be determined graphically or numerically -Birefringence, δ, can be determined graphically (using d and Δ) or numerically Interference Colour -Double refraction occurs when the light is split into two components each having a different velocity -because of their differing velocities, the two light waves become out of phase as they travel through the crystal -when they emerge from the mineral, the two rays interfere with each other -When observed with the upper polariser inserted in the light path, the mineral displays interference colours -As with birefringence, interference colour depends on the orientation of the crystal -for a given mineral in a thin section of standard thickness, only the maximum colour is of diagnostic value and defines the birefringence -The birefringence of minerals that exhibit solid solution (for example, olivine) varies with composition Note that very Mg-rich olivine has a lower birefringence than Fe-rich olivine Extinction -the interference colour of each mineral grain in thin section changes in intensity as the stage is rotated -the intensity falls to zero at every 90° of rotation. Why? Extinction occurs when the vibration directions of light passing through the mineral are parallel to the vibrations directions of the lower and upper polarizer -Thus, no light passes through the upper polarizer and the mineral appears “black” -Maximum birefringence colour always occurs at 45° to extinction -Extinction: the position in which a particular grain is black -4 positions on a complete rotation of the stage (unless optic axes is vertical/perpendicular to the stage) -Extinction position is measured relative to some well-defined direction in a crystal (cleavage, crystal shape)  Extinction angle -There are 4 types of extinction: 1) Parallel: the mineral is extinct when the cleavage or length is aligned with one of the crosshairs (N-S or E-W) -For example, Biotite 2) Inclined: the mineral is extinct when the cleavage or length is at some angle to the crosshairs -measured using the graduated scale on the microscope stage -usually recorded as an angle less that 45° 3) Symmetrical: minerals which display two cleavage directions or two distinct crystal faces -if the extinction angle measured from each is the same -For example, amphibole (Hornblende) 4. Undulatory: Different parts of a single mineral grain go extinct at different points of the stage rotation -the extinction in a grain follows an irregular or wavy pattern -may be caused by deformation -grain may be strained or bent -may be caused by chemical zonation (for example, plagioclase) -a mineral grain may not exhibit an extinction angle if it does not display cleavage, crystal faces, or an elongated shape -For example, quartz grains, olivine -said to have no extinction angle Use of the Accessory Plate -the accessory plate allow for the determination of the fast (low n) and slow (high n) rays which exit the mineral -the plates consist of pieces of gypsum, mica, or quartz mounted in a holder and oriented so that the vibration directions of the minerals are parallel to the long and short axes of the holder -Gypsum plate: first order red, 1 λ plate, p
More Less

Related notes for EASC 205

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.