Petrology EAES2410

The study of rocks and how they form

Petrology

The study of how rocks form

Optical Minerology

recognizing and interpreting textures and relationships between mineral grains

How do petrographic microscopes differ from a regular microscope?

• uses light from below rather than from above

• cross polarized light

  • used to look at thin sections

What is the difference between Grain Mounts and Thin Sections

• Grain Mounts

  • mineral fragments 0.075 - 0.105 mm

  • microscope slide with immersion oil or epoxy

• Thin Sections

  • cemented rock ground down to ~0.03mm

What is the wave length range is visible light?

400-700nm

electromagnetic radiation

light waves have an electric and magnetic component that vibrate at 90* to one another

What does the velocity of light depend on?

1. wavelength

2. density of material it is travelling through

Polarization

Forcing the light to vibrate in a specific direction

Refractive Index

the velocity of light through air

Snell’s Law

the relationship between refractive indices of two materials and the change of angle from one medium to another

sin (i) = n (mineral)

sin r = n (air)

Isotropic

Speed of light is equal in all directions

occurs in Isometric minerals (a=b=c)

Anisotropic

Speed of light non-equal in all directions

triclinic, monoclinic, orthorhombic,tetragonal, and hexagonal

Plane Polarized Light

Unpolarized light passes through polarizer which constrains light to single plane

How do isotropic minerals affect polarization direction

THEY DO NOT !! THEY APPEAR BLACK

Fast Ray vs Slow Ray

Fast Ray

• Lower R.I. (n)

• greater velocity

Slow Ray

• Greater R.I. (n)

• Lower velocity

retardation

the difference between fast ray and slow ray velocities

depends on thickness of mineral

birefringence

a property of minerals that causes light to split into two ways, directional dependence on the speed of light through the mineral

optic axis

every mineral has one or two directions in which light in not split into two perpendicular rays

which crystal systems are uniaxial?

hexagonal and tetragonal

uniaxial and biaxial systems

one and two opix axis

interference colours

light that passes through the upper polarizer.

depends on magnitude of retardation and whether slow or fast rays are in or out of phase

aka: colours shown under cross polarized light

Polychromatic light

all different coloured wavelengths are present going the same EW direction, split into fast and slow rays after passing through anisotropic mineral

pleochroism

when a mineral changes colour as the stage is rotated; different wavelengths of ppl absorbed while rotating

can isotropic minerals display pleochroism

NO! speed of length equal in all directions

dichroic minerals

uniaxial minerals that display 2 colours of pleochoism

trichroic minerals

biaxial minerals that display 3 colours of pleochroism

parallel extinction

extinct when long axis/cleavage of mineral, parallel to crosshairs, (E-W/N-S)

inclined extinction

extinct when long axis/cleavage is at an angle to crosshairs (not parallel)

undulatory extinction

irregular/wavy pattern of extinction. ex. quartz, metamorphic minerals

what is the accessory plate and it’s purpose?

consists of pieces of anisotropic minerals with known retardation such as gypsum or mica

how are accessory plate minerals orientated in regards to fast and slow rays?

slow ray vibrates accross width (E-W)

fast ray vibrates parallel to length (N-S)

what is the purpose of an accessory plate?

help distingush interference colours.

orientate mineral 45* to extinction, then insert accessory plate

if retardatrion gets higher, length slow

if retardation gets lower, length fast