GEOLOGY 333 LAB 5. Light Mechanics

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1 GEOLOGY 333 LAB 5 OPTICAL MICROSCOPY & MINERALS IN THIN SECTION Light Mechanics Light Waves: Visible light travels in waves, which have measurable wavelengths, frequencies, and velocities Wavelength ( ): the horizontal distance of 1 wave (peak to peak or trough to trough) Frequency ( ): the number of wavelengths per unit time Velocity (v): the speed of the wave = * 1

2 Light Mechanics Light Waves: Velocity (v): the speed of the wave = * o Velocity of light in a vacuum (c) = 3 * 10 8 m/s o Velocity decreases when traveling through air or any other material o When velocity changes, changes, while stays the same Light Mechanics Light Waves: Light color depends on the wavelength: (When all s s within the visible spectrum are at equal intensities, they combine to form white light) 2

3 Light Mechanics Interference: Amplitude (A): the height of the wave Phase: the position of one wave with respect to another In-phase Constructive interference Partially outof-phase Out-of-phase Partially destructive interference Completely destructive interference Light Mechanics Polarized Light: Unpolarized light: waves vibrate at 90 to direction of movement in all planes (normal light) Polarized light: waves vibrate in a single plane only 3

4 Light Mechanics Polarized Light: Two polarized lenses with polarization planes at 90 block light completely. Petrographic microscopes have two such lenses. However light passes through most minerals, it is refracted (waves shift propagation direction), so some light still passes through both polarizers. Petrographic Microscope 4

5 Petrographic Microscope Microscope Rules: 1. Never touch the objectives! Only rotate the big ring when you want to change magnification. 2. Be gentle with the thin sections. They break 4. If you don t know what easily. it does, don t touch it!! 3. Avoid large changes in focusing. It s possible to crack thin sections with the objective lens. 5. These microscopes are very expensive, so use with great care. Optical Microscopy Optical microscopy is the use of a petrographic microscope to study minerals and rocks in thin section Plane Polarized Light (PPL) - No upper polarizing lens 1) Pleochroism and Color 2) Opaque minerals 3) Cleavage 4) Crystal Shape 5) Refraction 6) Relief Cross Polarized Light (XPL) - Insert upper polarizing lens 1) Isotropic 2) Anisotropic a) Double Refraction b) Birefringence 3) Twinning 5

6 Plane Polarized Light Pleochrosim: A change in color when the sample is rotated on the stage. Plane Polarized Light Opaque minerals: No light passes through the minerals, so they look black in plane polarized light (PPL), even when the stage is rotated (ex: hematite, magnetite, and pyrite) Magnetite 6

7 Plane Polarized Light Cleavage: Will look like a set of parallel straight lines. May see up to two cleavage planes. see up to two cleavage planes. Amphibole: 2 cleavage planes at 120 and 60 Plane Polarized Light Cleavage: Will look like a set of parallel straight lines. May see up to two cleavage planes. Pyroxene: 2 cleavage planes at ~90 7

8 Plane Polarized Light Fracture: Will look jagged with irregular lines. Olivine Plane Polarized Light Crystal Shape: Are there well-developed crystal faces? Nepheline with euhedral (well-developed) crystal faces 8

9 Plane Polarized Light Refraction: Remember! When light passes through most minerals, the waves shift propagation direction (i.e. they re bent). Index of refraction (n): n = v vacuum /v crystal n is always > 1 Examples: n air = n epoxy = n minerals = Light travels more slowly through minerals with high n than with minerals with low n. Snell s Law: The angle at which light is refracted after passing from one substance to another. This is dependent on the refractive indices of both substances. Plane Polarized Light Relief: The contrast between a mineral and it s surroundings. Garnet: High Relief M&A p.18 Quartz: Low Relief Feldspar: Low Relief Corundum: High Relief 9

10 Cross-Polarized Light Isotropic: Mineral has the same refractive index (n) in all directions and will appear black in XPL M&A p.65 Garnet Cross-Polarized Light Anisotropic: Mineral has different refractive indices (n) in different e directions and will appear as different e colors o in XPL. Most minerals are anisotropic. M&A p.25 Olivine 10

11 Cross-Polarized Light Double Refraction: All anisotropic minerals cause transmitted light to be double refracted, or split into two rays that travel at different velocities. Two rays vibrate perpendicular to each other. During one full rotation of stage, anisotropic mineral grain will become dark (extinct) 4 times at 90 intervals (when polarizing lenses block all light transmitted through mineral). Cross-Polarized Light Double Refraction: Travel through the crystal structure of the mineral also changes the wavelength (color) When rays come together after passing through the mineral, they interfere constructively with each other resulting in an INTERFERENCE color. In-phase Constructive interference 11

12 Cross-Polarized Light Birefringence (Interference Colors): Each anisotropic mineral has certain birefringence (amount of separation of two rays) resulting in a characteristic interference color in XPL. Low Bi. M&A p.23 High Bi. Cross-Polarized Light Twinning: A mineral is split into two parts with different orientations. o M&A p.57 Plagioclase with polysynthetic twinning. Two sets of zebra stripes. Each set goes extinct at alternating 90 rotations of the stage. 12

13 Biotite Brown pleochroism Medium relief 1 cleavage plane Medium birefringence M&A p.43 Muscovite Colorless in PPL Medium relief 1 cleavage plane High birefringence M&A p.45 13

14 Quartz Low relief No cleavage Low birefringence M&A p.49 Olivine High relief Irregular Fracture High birefringence M&A p.33 14

15 Amphibole Green/brown pleochroism High relief 2 cleavage planes at 120 and 60 Medium birefringence M&A p.41 Pyroxene Medium relief 2 cleavage planes at 90 Medium birefringence M&A p.35 15

16 Plagioclase Low relief Polysynthetic twinning Low birefringence M&A p Calcite High relief 2 cleavage planes at 120 and 60 Very high birefringence M&A p.63 16