Geol 333 Lab #5 (Optical Microscopy, Minerals in Thin Section) - Student Notes 1. Light Mechanics

A. Wave (Fig. 3a in handout)

• Wavelength = λ = horizontal distance of one full wave cycle (peak to peak or trough to trough) • Frequency = ν (nu) = number of wavelengths per unit time • Velocity of wave = v = λ * ν = speed of wave

* Velocity of light in vacuum (c = 3 x 108 m/s) * Velocity is less when traveling through air or crystals * If v changes, λ changes (color changes) but ν is constant.

• Color depends on wavelength (Fig. 2 in handout)

• All λ’s at equal intensities in visible spectrum combine to form white light

• Interference * Amplitude (A) = height of wave * Phase = position of one wave with respect to another wave, constructive (2 waves in phase, Fig. 5a in

handout) and destructive interference (2 waves out of phase, Fig. 5b and c in handout)

B. Polarized Light

• Unpolarized Light = light waves vibrate at 90° to movement direction but in all planes (normal light, Fig. 3b in handout)

• (Plane) Polarized Light = light waves vibrate in single plane only, e.g., light reflected off shiny surface or light through sunglasses (Fig. 3c in handout)

• Cross Polarization = 2 polarizers (e.g., sunglasses) oriented at 90°, no light waves pass through; BUT most minerals change vibration direction of light and allow some light to pass through

2. Parts of Petrographic (Polarizing) Microscope (Fig. 7 in handout) 3. Petrographic Microscope Etiquette

• VERY expensive instruments ($10,000 each) • Always change objectives with big ring, NOT the objectives themselves • Try not to change any knobs or switches if you don’t know what they do • Switch on bottom is the dimmer for the light • Be gentle in handling thin sections; they break easily • Avoid large changes in focusing, which is done by raising or lowering microscope stage; it IS possible

to collide thin section into objective lens, especially with high power magnification 4. Optical Microscopy - Use of petrographic microscope to study minerals and rocks

A. Plane Polarized Light (PPL) = no upper polarizing lens

i. Opaque Minerals - no light passes through opaque minerals, which look black, e.g., oxides

(hematite and magnetite) and sulfides (pyrite); difficult to study using petrographic microscope

ii. Color and Pleochroism = some minerals, e.g., biotite, absorb different frequencies of light depending on light’s direction of polarization (property of pleochroism). To determine if mineral is pleochroic, rotate stage + look for color changes in mineral. (Photos 7 and 8 on page 15 in M & A)

4. Optical Microscopy Plane Polarized Light (PPL) - continued

iii. Cleavage = parallel lines running through mineral (Photos 9 and 10 on page 17 in M & A show

pyroxene with 2 cleavage directions @ 90° and amphibole with 2 cleavage directions not @ 90°) - Olivine and Quartz = 0 (olivine has fracture - irregular lines) - Biotite = 1 - Feldspar = 2 @ 90° - Calcite = 3 @120° (In thin section we see only 2 sets of cleavage. Why?)

iv. Crystal Shape - Are there well-developed crystal faces or not? Crystal needs room to grow. There are

many different crystal shapes, e.g., rectangular, hexagonal, diamond, needle-like.

v. Refraction = bending of light (pencil in water appears bent) - Index of refraction = n = vvacuum / vcrystal - So n > 1 ALWAYS - n of air = 1.00029 - n of epoxy = 1.54 or 1.55 - n of most minerals = 1.5 - 2 • Light travels more slowly through minerals with high n than those with low n. • Snell’s Law = angle at which light is refracted after passing from one substance to another,

depends on refractive indices of both substances.

vi. Relief = contrast between mineral and its surroundings (usually epoxy used as glue) - High Relief = edges, cleavage planes, and fractures in mineral grain stand out, e.g., mafic

silicates (olivine, pyroxene, amphibole, biotite) and calcite; minerals with high relief have high n - Low Relief = features in mineral grain do NOT stand out, e.g., quartz and feldspar; minerals with

low relief have low refractive index, similar to epoxy, where n = ~1.55 - Photos 11 and 12 on page 18 in M & A (Garnet, kyanite, corundum = high relief; biotite =

medium relief; Quartz = low relief)

B. Cross Polarized Light (XPL) = insert upper polarizing lens

i. Isotropic = same n in all directions; black in XPL; Cubic symmetry minerals (e.g., garnet) and amorphous solids (obsidian = volcanic glass) are isotropic (Photos 64 and 65 of garnet on page 65 in M & A)

ii. Anisotropic = Minerals with different refractive indices in different directions; most minerals

are anisotropic.

Double Refraction = All anisotropic minerals cause transmitted light to be doubly refracted or split into two rays that travel at different velocities, e.g., optical (clear) calcite. 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.

- Travel through crystal structure of mineral also changes wavelength (color) of light

- When rays come together after passing through mineral, they interfere constructively with each

other resulting in INTERFERENCE (Photo 19 on page 25 in M & A and many others!).

4. Optical Microscopy Cross Polarized Light (XPL) - continued

Birefringence (Interference Colors) = each anistropic mineral has certain birefringence (amount of separation of two rays), resulting in characteristic interference color in crossed polarized light; Birefringence chart (Figure 18 on page 23 in M & A) shows range of colors for different minerals; 3 orders of colors (first order/white, gray and vivid colors = low birefringence, third order/pastel and “washed out” colors = high birefringence); assumes mineral thickness = 0.03 mm Birefringence of Quartz from chart? (See Photo 110 on page 115 in M & A) Birefringence of Calcite (not on chart)? (See Photo 63 on page 63 in M & A) Birefringence of Mg-Olivine from chart? (See Photo 27 on page 33 in M & A)

iii. Twinning = Single crystal split into 2 or more parts with different orientations; polysynthetic

twinning (plagioclase feldspar) = regular alternations of twins, “zebra” stripes in XPL (Photos 55 and 56 on page 57 in M & A)

5. Optical Microscopy - Pure Minerals

Olivine - Photos 26 - PPL and 27 - XPL on page 33 in M & A Pyroxene - Photos 29 - PPL and 30 - PPL on page 35 in M & A Amphibole - Photos 34 - PPL, 35 - PPL, 36 - XPL on page 40 + 41 in M & A Biotite - Photos 37 - PPL, 38 - PPL, 39 - XPL Photos 37 - 39 on page 42 + 43 in M & A Muscovite - Photos 40 and 41 on page 45 in M & A Quartz - Photos 45 - PPL, 46 - XPL, 47 - PPL, 48 - XPL on page 49 in M & A Plagioclase feldspar - Photos 54 - 57 - all are XPL on page 56 + 57 in M & A Calcite - Photos 61 - XPL, 62 - XPL, 63 - XPL on page 62 + 63 in M & A

Class Web site has links to these optical features of minerals with many excellent photos of minerals in thin section for later review outside of Lab. <http://classes.geology.illinois.edu/14SprgClass/geo333/333-Lab_Optical_Microscopy.html> Reading Assignment for Next Week Igneous rocks: MacKenzie and Adams p. 67 - 82; Klein + Philpotts p. 164 - 166, 170 - 174

Next week’s Lab will begin with a quiz, which will be based entirely on assigned reading!