min 2030 final exam: Part 2 - Lec 1 - optical microscope

light waves

electromagnetic radiation that have both electrical and magnetic properties

frequency

V/λ

brightness of the light

is proportional to the amplitude (A)

distance for 1 wave cycle=

wavelength (λ), in nm

wave front

Surface over which the phase of the wave is constant

wave normal

Line perpendicular to the wave front, and parallel to the propagation direction

plane wave front

Plane (2D) surface where phases of parallel wave rays are aligned.

plane polarized light (PPL)

light passed through a polarizing filter which constrains the light to oscillate in a single plane

cross polarized light (XPL)

two polarizers in which the polarizing directions are perpendicular

if nothing changes the orientation of light between the two crossed polarizers, what do you see transmitted on the other side?

nothing

the velocity of light depends on…

the nature of the material it travels through and the wavelength of light

• velocity decreases when changing from a less to more dense material

when velocity of light decreases…

wavelength must also decrease

optically isotrophic

Material in which the velocity of light is the same in all directions. • Isotropic materials: Isometric minerals, glass (i.e., volcanic glass, opal)

Optically Anisotropic

The velocity of light is different in different directions. Anisotropic materials: Non-isometric minerals.

for reflected light

the angle of incidence and of reflection are identical. Mineral luster depends on these reflections, and are affected by the mineral’s refractive index.

a bent (refracted) light path occurs…

when the incident light angle is not 90 degrees

refractive index (RI)

• based on the difference between the speed of light in a vacuum vs. in the material

• n= V_v / V_m = velocity of light in vacuum / velocity of light in material

• denser materials have higher RI (slow light velocity)

angle of refraction

is proportional ton the difference in RI between the two materials

• snells law

critical angle (CA)

he minimum angle of incidence inside the mineral at which the refracted light angle in the lower-index medium reaches 90°, grazing along the interface. Beyond this angle, light cannot escape the mineral and is totally internally reflected.

• important when cutting gemstones (so they the most sparkle)

petrographic microscope

•Relatively inexpensive

Provides a very large amount of information

•Mineral ID in rock samples

•Estimating composition of some solid solution series

•Mineral abundances, morphology (2D), size range, abundance

•Rock textures / micro-structure

•Genetic information!

polished thick section

A thicker type of polished section (100+ micrometers) typically prepared for sulphide ore samples

• Used for reflected light polarized light microscopy.

grain mount slide

• individual grains immersed in oil and covered with glass

• useful to study asbestiform minerals, zeolites

round epoxy grain/ sample mount

• sample mounted in epoxy puck and polished

• used for small individual grains such as zircons for U-Pb dating, or minerals for EPMA work

• used for sulphide-bearing samples for examination by reflected light microscopy, and EPMA work

anisotropic materials

• different light velocity in different directions- change when stage is rotated in XPL

• show double refraction

• non-isometric

• have birefringence

• uniaxial and biaxial

isotrophic materials

• identical light velocity in all directions- stays black when the stage is rotated in XPL

  • isometric minerals

  • materials with no internal structure- glass

uniaxial minerals

have two different refractive indices

• crystal systems where a = b ≠ c

  • tetragonal, trigonal, hexagonal

biaxial minerals

have three different refractive indices

• monoclinic, orthorhombic, triclinic

birefringence

Double refraction of light in a transparent ordered material resulting in orientation-dependent differences in refractive index.

parallel extinction

extinction parallel to cleavage planes

• only occurs in minerals with 90 degree crystal axis angles (trigonal, hexagonal, tetragonal)

retardation

Phase shift between the polarization component projected along the fast axis and the component projected along the slow axis.

• the amount by which the slow wave falls behind the fast wave during passage through an isotropic crystal

• = thickness x birefringence

optic sign

• birefringence (B) = |nε-nω|

• is positive or negative depending on which is more

uniaxial negative

ε<ω

uniaxial positive

ε>ω

biaxial negative

γ-β<β-α

biaxial positive

γ-β>β-α

melatope

Point in an interference figure corresponding to direction of an optic axis in the crystal section ( middle of cross)

isogyre

Dark shadow in an interference figure, usually as a cross (uniaxial) or bands (biaxial)

isochromes

Curve in an interference pattern where colour is identical (rings around the cross/ coloured sections)

Mineral ID in thin section

1. Hand sample and thin section broadly: What type of rock? Can you visually ID the mineral?

2. Examination in plane polarized light

• If opaque, use reflected light

• Color, Pleochroism

• Crystal shape, habit

• Cleavage

• Relief

3. Examination in cross-polarized light

• Isotropic vs Anisotropic (if the latter, continue)

•Maximum birefringence

• Extinction angle

• Twinning

4. Interference figure (Bertrand lens)

• Uniaxial or Biaxial

• Optic sign (+ or -)

• If biaxial, estimate 2V angle