1.6 Isotropic and anisotropic minerals

1.6 Isotropic and - Microscopy Ocular Bertrand lens Analyzer upper polarizer nicols lens Accessory Slot Objective Polarizer typically oriented NS

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Unformatted text preview: Microscopy Ocular Bertrand lens Analyzer, upper polarizer, nicols lens Accessory Slot Objective Polarizer, typically oriented NS Additional parts conoscope Four common settings for microscopic observations of thin sections: 1. Plane polarized light, analyzer (upper polarizer, nicols lens) out 2. Plane polarized light, analyzer in (cross nicols) 3. Conoscopic polarized light, bertrand lens in 4. Conoscopic polarized light, betrand lens in, gypsum plate in accessory slot Why use microscopes? Visual properties for ID e.g. texture Only observable with microscope Isotropic vs Anisotropic Color may be variable Cleavage (may not see, often controls shape) Shape (depends on cut of mineral) Isotropic vs Anisotropic Many optical methods to distinguish isotropic and anisotropic minerals Distinction of two types important Know if isometric system or not If anisotropic, we'll see there are ways to identify individual systems. Isotropic Minerals Easily identified Always extinct with upper polarizer inserted Rotate stage and remains extinct Vibration direction not changed by material All light blocked by upper polarizer One grain Individual crystals Feldspar partially extinct, Since it not completely extinct, it is not isotropic Anisotropic Minerals Variable values of n within mineral Has property of double refraction Light entering material usually split into two rays Sometimes light not split into two rays acts like isotropic mineral Two rays vibrate at 90 to each other Vibration directions perpendicular to each other Double Refraction experiment For each of the two light rays: Value of n is determined by vibration direction In one direction, the value of n is larger than the other Direction with large n is slow ray Direction with small n is fast ray Different values of n mean different angles of refraction "double refraction" Optic axis Special direction where rays not split into two rays Wave normal and ray path coincide Hexagonal and tetragonal have one optic axis Orthorhombic, monoclinic, and triclinic have two optic axes Uniaxial Biaxial Interference Phenomena For most cuts of anisotropic minerals, light not blocked by analyzer Specific color is interference color Caused by two rays resolving to one when they leave the mineral Interference Colors Viewed in crossed nicols (upper polarizer inserted) Intermediate interference colors Low interference colors Colored minerals Viewed in plane polarized light Cross nicols Biotite, a pleochroic mineral, natural color Muscovite showing interference colors Interference with monochromatic light Monochromatic = one wavelength Light split into fast and slow ray Fast ray travels farther than slow ray in same time Difference in the distances called retardation, Retardation remains same after two rays leave mineral (air is isotropic) = retardation Note: here you need to imagine the two rays follow the same path even though they are refracted Distance for slow ray d = thickness (distance) Typically 30 m Distance for fast ray Fig. 714 Retardation and Birefringence Derive definition of retardation Retardation controlled by two things: Thickness of mineral, d Difference in speed of fast and slow ray Units have to be length, typically reported as nm (ns nf) must be positive number Birefringence Birefringence is the difference between ns and nf = (ns nf) Origin of interference colors Still talking about monochromatic light If retardation is an integer number of wavelengths: Components resolve into vibration direction same as original direction All light is blocked by analyzer Original polarized direction = 1 Privileged direction of analyzer All light blocked = extinct Fig. 715a If retardation is half integer of wavelength Components resolve into vibration direction 90 to original Light passes through analyzer Original polarized direction Privileged direction of analyzer = 1/2 All light passes Fig. 715b Fig. 74 bloss A more realistic depiction Note this is still monochromatic light 1 1 2 2 Interference with polychromatic light Polychromatic light Interference colors depend on what wavelength resolved at analyzer All wavelengths Some = integer value of Most integer value of Depending on magnitude of birefringence: If few wavelengths passes through analyzer, see only one color Sometimes multiple pass through analyzer, see white For standard thin section (d=30m): Visiable Quartz: = 250 nm 1st order white 750/500 = 1.5 500/500 = 1 Kyanite: = 500 nm; 1st order red 2500/416.6 = 6 2500/500 = 5 2500/625= 6 Red Calcite: = 2500 nm; 4th order white; cream Fig. 717 Color chart Shows range of interference, depends on retardation Divided into orders Orders are in multiples of 550 nm Successively higher orders are increasingly washed out Above 4th order, color becomes creamy white Color chart Primary use is to determine retardation Retardation controlled by mineral thickness and birefringence; = d By observing color, can determine amount of By knowing thickness, can determine value of Simply read the retardation off the bottom of the chart Determining thickness of thin section Use quartz (or other easily identifiable, common mineral) Maximum is 0.009 Actual birefringence depends on orientation of grain From back of book: ns = 1.553; nf = 1.544 Maximum birefringence when c axis is parallel to stage Birefringence = 0 when c axis is perpendicular to stage Intermediate birefringence for intermediate orientation Procedure Find quartz with highest birefringence 2. Find where the retardation (given by color), intersects lines for birefringence 3. Calculate it from formula for birefringence: = /d Or read off thickness from chart 1. Fig.718 Typical slide thickness is 30 m (0.03 mm) Quartz will be first order white to yellow Thin sections may not be perfect Thick sections 70 m Variable thicknesses Thin on edges Used for inclusions Freeze/thaw of fluid inclusions Determining birefringence Maximum is a useful diagnostic value Easily determined in thin section with known thickness Distribution of birefringence: Some with zero Some with maximum Most with intermediate Procedure for determining birefringence Find grain with highest interference colors 2. Find retardation on the basis of the color (bottom of chart) 3. Calculate the birefringence using equation = /d Or find maximum birefringence from chart 1. Fig. 718b Extinction Many grains in a thin section go dark (extinct) every 90 of rotation Cause for extinction is orientation of vibration directions Occurs when principle vibration directions are parallel to vibration directions of upper and lower polarizers Light retains original polarized direction Light blocked by analyzer Extinct Birefringent Fig 719 Importance of extinction Allows determination of principle vibration directions When extinct, the orientation of the principle vibration directions are NS and EW Accessory Plates Primary functions: Determine optic sign Determine sign of elongation Construction: Usually gypsum full wave plate, = 550 nm Common mica wave plate, = 150 nm Retardation is known Orientation of principle vibration directions is known, set at 45 to polarizer and analyzer Fast ray is length of holder, slow ray is perpendicular to holder Interference of accessory plate either adds or subtracts from retardation of mineral With slow ray of mineral parallel slow ray of accessory plate retardation increases With slow ray of mineral parallel fast ray of accessory plate retardation decreases Net result: Accessory plate tells you orientation of fast and slow direction in mineral Important for many optical observations mineral total mineral total Fig. 720 Procedure to determine fast and slow 1. 2. 3. 4. 5. Rotate grain to extinction either fast or slow ray parallel to polarized light direction Rotate stage 45 Note interference color Insert accessory plate Observe if color increases or decreases (right or left on chart) Interference plate will also determine order of interference color Rotate grain with gypsum plate inserted Color will alternately go up or down one order Fig. 721 ...
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This note was uploaded on 07/06/2011 for the course GLY 5245 taught by Professor Staff during the Spring '11 term at University of Florida.

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