Optical MicroscopySlide 2Minerals and propogation of lightSlide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19‘Splitting’ of light  what does it mean?Anisotropic crystalsSlide 22Slide 23Difference between our 2 raysSlide 25Slide 26Michel-Lévy Color Chart – Plate 4.11Slide 28Slide 29Slide 30Slide 31Slide 32Rotation of crystal?ExtinctionSlide 35Twinning and Extinction AngleVernier scaleSlide 38Slide 39Slide 40Slide 41Appearance of crystals in microscopeSlide 43Optical Microscopy•Study of how light passes through thin sections – rock cut and polished to about 0.3 mm thickness•Use properties of light absorption and propogation through a mineral  affected by atomic arrangement and composition•Learn the properties of light associated with techniques governing the use of a petrographic microscopeWhy use the microscope??•Identify minerals (no guessing!)•Determine rock type•Determine crystallization sequence•Document deformation history•Observe frozen-in reactions•Constrain P-T history•Note weathering/alteration•Fun, powerful, and cheap!Minerals and propogation of light•Opaque minerals – minerals in which light does not go through  always black even in thin sections. Typically these have molecules with higher atomic density (which includes many ore minerals). How light reflects off of these minerals is used to identify them with a reflected light microscope.•Nonopaque minerals – minerals in which light does go through  use these properties to identify them with the petrographic microscopeThe petrographic microscopeAlso called a polarizing microscopeIn order to use the scope, we need to understand a little about the physics of light, and then learn some tools and tricks…What happens as light moves through the scope?light sourceyour eyelight raywaves travel from source to eyewavelength,amplitude, Alight travels as wavesFrequency = # of waves/sec to pass a given point (hz)f = v/ v = velocityWe are dealing with white light in microscopy:We are dealing with white light in microscopy:Violet (400 nm)  Red (700 nm)White = ROYGBV(can be separated by dispersiondispersion in a prism)light vibrates inall planes that containthe light ray(i.e., all planesperpendicular tothe propagationdirectionplane of vibrationvibration directionpropagation directionWhat happens as light moves through the scope?1) Light passes through the lower polarizerwest (left)east (right)Plane polarized lightPPL=plane polarized lightUnpolarized lightOnly the component of light vibrating in E-W direction can pass through lower polarizer – light intensity decreasesThough polarized, still white light!Mineral properties: color & pleochroism• Color is observed only in PPL• Not an inherent property - changes with light type/intensity• Results from selective absorption of certain  of light• Pleochroism results when different  are absorbed differently by different crystallographic directions -rotate stage to observeplaghblplaghbl-Plagioclase is colorless-Hornblende is pleochroicMineral properties: Index of refraction (R.I. or n)Light is refracted when it passes from one substance to another; refraction is accompanied by a change in velocityn1n2n2n1n2>n1n2<n1n =velocity in airvelocity in mineral• n is a function of crystallographic orientation in anisotropic minerals isotropic minerals: characterized by one RI uniaxial minerals: characterized by two RI biaxial minerals: characterized by three RI• n gives rise to 2 easily measured parameters: relief & birefringenceMineral properties: relief• Relief is a measure of the relative difference in n between a mineral grain and its surroundings • Relief is determined visually, in PPL• Relief is used to estimate nolivineplagolivine: n=1.64-1.88plag: n=1.53-1.57epoxy: n=1.54- Olivine has high relief- Plag has low reliefWhat causes relief?nxtl > nepoxynxtl < nepoxynxtl = nepoxyHi relief (+) Lo relief Hi relief (-)Difference in speed of light (n) in different materials causes refraction of light rays, which can lead to focusing or defocusing of grain edges relative to their surroundings2) Insert the upper polarizerupper polarizerwest (left)east (right)Now what happens?What reaches your eye?Why would anyone design a Why would anyone design a microscope that prevents light from microscope that prevents light from reaching your eye???reaching your eye???XPL=crossed nicols (crossed polars)south (front)north (back)Black!! (“extinct”)© Jane Selverstone, University of New Mexico, 20033) Now insert a thin section of a rockwest (left)east (right)Light vibrating E-WHow does this work??Unpolarized lightLight vibrating in many planes and with many wavelengthsLight and colors reach eye!Conclusion has to be that minerals somehow reorient the planes in which light is vibrating; some light passes through the upper polarizer4) Note the rotating stageMost mineral grains change color as the stage is rotated; these grains go black 4 times in 360° rotation-exactly every 90oGlass and a few minerals stay black in all orientationsThese minerals are anisotropicThese minerals are isotropicSome generalizations and vocabulary•All isometric minerals (e.g., garnet) are isotropic – they cannot reorient light. Light does not get rotated or split; propagates with same velocity in all directions–These minerals are always black in crossed polars.•All other minerals are anisotropic – they are all capable of reorienting light (transmit light under cross polars).•All anisotropic minerals contain one or two special directions that do not reorient light.–Minerals with one special direction are called uniaxial–Minerals with two special directions are called biaxialIsotropicUniaxialBiaxialHow light behaves depends on crystal structureIsometric– All crystallographic axes are equalOrthorhombic, monoclinic, triclinic–All axes are unequalHexagonal, tetragonal– All axes  c are equal but c is unique•Isotropic minerals: light does not get rotated or split; propagates with same velocity in all directions•Anisotropic minerals:•Uniaxial - light entering in all but one special direction is resolved into 2 plane polarized components that vibrate perpendicular to one another and travel with different speeds•Biaxial - light entering in all but two special directions is resolved into 2 plane polarized components…–Along the