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SF State GEOL 426 - Optical Mineralogy in a Nutshell

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PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Color and pleochroismSlide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Anisotropic crystalsSlide 25Slide 26Slide 27Slide 28Color chartSlide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Optical Mineralogy in a NutshellUse of the petrographic microscope in three easy lessonsPart ISlides borrowed/adapted from Jane Selverstone (University of New Mexico) and John Winter (Whitman College)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!The 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 wavesMicroscope light is white light,i.e. it’s made up of lots of different wavelengths;Each wavelength of light corresponds to a different colorCan prove this with a prism, which separates white light into itsconstituent wavelengths/colorsWhat happens as light moves through the scope?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?Polarized LightUnpolarized Light1) Light passes through the lower polarizerwest (left)east (right)Plane polarized light“PPL”Unpolarized lightOnly the component of light vibrating in E-W direction can pass through lower polarizer – light intensity decreases2) Insert the upper polarizerwest (left)east (right)Now what happens?What reaches your eye?Why would anyone design a microscope that prevents light from reaching your eye?XPL(crossed nicols or crossed polars)south (front)north (back)Black!“extinct”3) Now insert a thin section of a rockwest (left)east (right)Light vibrating E-WLight vibrating in many planes and with many wavelengthsHow does this work?Unpolarized lightLight 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 polarizerBut, note that some minerals are better magicians than others (i.e., some grains stay dark and thus can’t be reorienting light)Minerals actMinerals actas magicians!as magicians!olivineolivineplagplagPPLXPL• 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 pleochroic in olive greensColor and pleochroismColor and pleochroismBiotiteNow do questions 1 & 2Mineral 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 ngarnet: n = 1.72-1.89quartz: n = 1.54-1.55epoxy: n = 1.54Garnet has high reliefQuartz has low reliefMineral 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 velocityn1n1n2n2n2>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 surroundingsNow do question 3CleavageMost easily observed in PPL (upper polarizer out), but visible in XN as well• No cleavages: quartz, olivine• 1 good cleavage: micas• 2 good cleavages: pyroxenes, amphibolesCleavage2 cleavages intersectingat ~90° pyroxene60°120°2 cleavages intersectingat 60°/120°: amphiboleCleavagerandom fractures, no cleavage:olivineNow do question 4Crystal habit or formblockyacicularbladedprismaticanhedral/irregularelongateroundedfibroustabulareuhedralHabit or formblockyacicularbladedprismaticanhedral/irregularelongateroundedfibroustabulareuhedralNow do question 5Mineral properties: interference colors/birefringence• Colors one observes when polars are crossed (XPL) • Color can be quantified numerically:  = nhigh - nlowBirefringence/interference colorsObservation: frequency of light remains unchanged during splitting, regardless of materialF= V/ if light speed changes,  must also change is related to color; if  changes, color also changesmineral grainplane polarized lightfast ray (low n)slow ray(high n)lower polarizer=retardationdViolet (400 nm)  Red (700 nm)O EDouble imagesDouble imagesRay Ray  2 rays with 2 rays with different propagation different propagation and vibration directionsand vibration directionsEach is polarized ( Each is polarized (  each other)each other)Fig 6-7 Bloss, Optical Crystallography, MSAAnisotropic crystalsCalcite experiment and double refractionAnisotropic crystalsCalcite experiment and double refractionO EO-ray (Ordinary) O-ray (Ordinary) Obeys Snell's Law and goes Obeys Snell's Law and goes straightstraightVibrates Vibrates  plane containing plane containing ray and c-axis (ray and c-axis (“optic axis”“optic axis”))E-ray (Extraordinary)E-ray (Extraordinary)DeflectedDeflectedVibrates Vibrates inin plane containing plane containing ray and c-axisray and c-axisFig 6-7 Bloss, Optical Crystallography,


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SF State GEOL 426 - Optical Mineralogy in a Nutshell

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