1.6 Isotropic and anisotropic minerals

1.6 Isotropic and anisotropic minerals - Microscopy...

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Unformatted text preview: Microscopy Microscopy Ocular Bertrand lens Analyzer, upper polarizer, nicols lens Accessory Slot Objective Polarizer, typically oriented N­S Additional parts 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? Why use microscopes? Visual properties for ID – e.g. texture Color – may be variable Cleavage (may not see, often controls shape) Shape (depends on cut of mineral) Only observable with microscope Separate isotropic and anisotropic minerals Isotropic vs Anisotropic 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 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 Not Extinct Quartz crystals in plane polarized light Extinct Same quartz crystals with analyzer inserted (cross polarizers aka crossed nicols) One grain Individual crystals Feldspar Feldspar partially extinct, Since it not completely extinct, it is not isotropic Anisotropic Minerals Anisotropic Minerals Variable values of n within mineral Has property of double refraction Light entering material usually split into two rays Vibration directions perpendicular to each other Sometimes light not split into two rays – acts like isotropic mineral Two rays vibrate at 90º to each other 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 Optic axis Special direction where rays not split into two rays Wave normal and ray path coincide Hexagonal and tetragonal have one optic axis Uniaxial Orthorhombic, monoclinic, and triclinic have two optic axes Biaxial Interference Phenomena 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 Interference Colors Not true colors Viewed in crossed nicols (upper polarizer inserted) Intermediate interference colors Low interference colors Colored minerals Colored minerals Viewed in plane polarized light Biotite, a pleochroic mineral, natural color Cross nicols Muscovite showing interference colors Interference with monochromatic 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 Distance for slow ray d = thickness (distance) Typically 30 µm Distance for fast ray Note: here you need to imagine the two rays follow the same path even though they are refracted Fig. 7­14 Fig. 7­14 Retardation and Birefringence Retardation and Birefringence Derive definition of retardation Retardation controlled by two things: Thickness of mineral, d Difference in speed of fast and slow ray – (ns – nf) – must be positive number Units have to be length, typically reported as nm Birefringence Birefringence Birefringence is the difference between ns and nf δ = (ns – nf) ∆ = retardation Distance for slow ray d = thickness (distance) Typically 30 µm Distance for fast ray Note: here you need to imagine the two rays follow the same path even though they are refracted Remember: ∆ = d(ns­ nf) (ns­ nf) = δ δ = birefringence Fig. 7­14 Fig. 7­14 Interference Colors Interference Colors Not true colors Viewed in crossed nicols (upper polarizer inserted) Intermediate interference colors δ = intermediate values Low interference colors δ = low values Colored minerals Colored minerals Viewed in plane polarized light Biotite, a pleochroic mineral, natural color Cross nicols Muscovite showing interference colors Origin of interference colors 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 λ Privileged direction of analyzer All light blocked = extinct Resolved vibration direction Fig. 7­15a Fig. 7­15a If retardation is half integer of wavelength Components resolve into vibration direction 90º to original Light passes through analyzer Original polarized direction Resolved vibration directions Privileged direction of analyzer ∆ = 1/2 λ All light passes Fig. 7­15b Fig. 7­15b Fig. 7­4 bloss Fig. 7­4 bloss 1½ λ 1λ 2λ 2λ A more realistic depiction Note – this is still monochromatic light Interference with polychromatic Interference with polychromatic light Polychromatic light All wavelengths Some λ = integer value of ∆ Most λ ≠ integer value of ∆ Interference colors depend on what wavelength resolved at analyzer Depending on magnitude of birefringence: If few wavelengths passes through analyzer, see only one color Sometimes multiple λ pass through analyzer, see white For standard thin section (d=30µm): Quartz: ∆ = 250 nm 1st order white Kyanite: ∆ = 500 nm; 1st order red 750/500 = 1.5 750 λ passes through Calcite: ∆ = 2500 nm; 4th order white; cream Red 2500/625= 4 Red Green 2500/500 = 5 Indigo 2500/416.6 = 6 500/500 = 1 Blue is blocked Visible λ All pass through 416.6, 500, 625 λ passes through ­ white Fig. 7­17 Fig. 7­17 Color chart Color chart Shows range of interference, depends on retardation Plot of d (thickness of thin section) versus ∆ (retardation) Diagonal lines are birefringence, δ = ns ­ nf Color chart Color chart 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 Color chart Primary use is to determine retardation Simply read the retardation off the bottom of the chart Retardation controlled by mineral thickness and birefringence; ∆ = dδ By observing color, can determine amount of ∆ By knowing thickness, can determine value of δ Determining thickness of thin Determining thickness of thin section Use quartz (or other easily identifiable, common mineral) Maximum δ is 0.009 From back of book: ns = 1.553; nf = 1.544 Actual birefringence depends on orientation of grain Maximum birefringence when c axis is parallel to stage Birefringence = 0 when c axis is perpendicular to stage Intermediate birefringence for intermediate orientation Procedure Procedure Find quartz with highest birefringence (correct cut of mineral) 2. Find where the retardation (given by color), intersects lines for birefringence 3. Calculate it from formula for birefringence: δ = ∆ /d Or read off thickness from chart 1. Fig.7­18 Fig.7­18 Typical slide thickness is 30 µm (0.03 mm) Quartz will be first order white to yellow Thin sections may not be perfect Variable thicknesses Thin on edges Thick sections – 70 µm Used for inclusions Freeze/thaw of fluid inclusions Determining birefringence Determining birefringence Maximum δ is a useful diagnostic value Easily determined in thin section with known thickness Distribution of birefringence: Some with zero δ Some with maximum δ Most with intermediate δ Procedure for determining 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 δ = ∆ /d Or find maximum birefringence from chart 1. Fig. 7­18b Fig. 7­18b Extinction Extinction Many grains in a thin section go dark (extinct) every 90º 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 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 Accessory Plates Primary functions: Determine optic sign Determine sign of elongation Construction: Usually gypsum ­ full wave plate, ∆ = 550 nm Common mica ­ ½ wave plate, ∆ = 150 nm Retardation is known Orientation of principle vibration directions is known, set at 45º 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 Fig. 7­20 Procedure to determine fast and 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 45º 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 Fig. 7­21 ...
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