Optical 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 microscope Why 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 microscope The petrographic microscope Also called a polarizing microscope In 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 your eye amplitude A light travels as waves wavelength light ray waves travel from source to eye light source Frequency of waves sec to pass a given point hz f v v velocity We are dealing with white light in microscopy Violet 400 nm Red 700 nm White ROYGBV can be separated by dispersion in a prism What happens as light moves through the scope propagatio n direction plane of vibration vibration direction light vibrates in all planes that contain the light ray i e all planes perpendicular to the propagation direction 1 Light passes through the lower polarizer west left Unpolarized light Plane polarized light east right Only the component of light vibrating in E W direction can pass through lower polarizer light intensity decreases Though polarized still white light PPL plane polarized 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 observe hbl hbl Plagioclase is colorless Hornblende is pleochroic plag plag Mineral properties Index of refraction R I or n n velocity in air velocity in mineral Light is refracted when it passes from one substance to another refraction is accompanied by a change in velocity n1 n1 n2 n2 n2 n1 n2 n1 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 birefringence Mineral 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 n Olivine has high relief Plag has low relief plag olivine olivine n 1 64 1 88 plag n 1 53 1 57 epoxy n 1 54 What causes 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 surroundings Hi relief nxtl nepoxy Lo relief Hi relief nxtl nepoxy nxtl nepoxy 2 Insert the upper polarizer west left north back east right south front Black extinct Now what happens What reaches your eye XPL crossed nicols crossed polars Why would anyone design a microscope that prevents light from reaching your eye Jane Selverstone University of New Mexico 2003 3 Now insert a thin section of a rock west left Unpolarized light east right Light vibrating E W Light vibrating in many planes and with many wavelengths Light and colors reach eye How does this work Conclusion has to be that minerals somehow reorient the planes in which light is vibrating some light passes through the upper polarizer 4 Note the rotating stage Most mineral grains change color as the stage is rotated these grains go black 4 times in 360 rotationexactly every 90o These minerals are anisotropic Glass and a few minerals stay black in all orientations These minerals are isotropic Some 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 biaxial How light behaves depends on crystal structure Isotropic Isometric All crystallographic axes are equal Uniaxial Biaxial Hexagonal tetragonal All axes c are equal but c is unique Orthorhombic monoclinic triclinic All axes are unequal 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 special directions optic axes the mineral thinks that it is isotropic i e no splitting occurs Uniaxial and biaxial minerals can be further subdivided into optically positive and optically negative depending on orientation of fast and slow rays relative to xtl axes Splitting of light what does it mean For some exceptionally clear minerals where we can see this is hand sample this is double refraction calcite displays this Light is split into 2 rays one traveling at a different speed and this difference is a function of thickness and orientation of the crystal Norden Bombsight patented in 1941 utilized calcite in the lenses to gauge bomb delivery based on speed altitude of plane vs target ALL anisotropic minerals have this property and we can see that in thin sections with polarized light Anisotropic crystals Calcite experiment and double refraction O O ray Ordinary Double images E Obeys Snell s Law and goes Ray 2 rays with