GEOL 303A MINERALOGY

1
GEOL 303A MINERALOGY LECTURE 8 Quick preface
of optical mineralogy
2
Introduction Although optical properties of
minerals may be more difficult to determine
compared to physical properties, they are
nonetheless an extremely important tool in
mineral identification
OLIVINE
www.geology.neab.net
3
Introduction Although optical properties of
minerals may be more difficult to determine
compared to physical properties, they are
nonetheless an extremely important tool in
mineral identification
OLIVINE in BASALT
www.geology.neab.net
4
Introduction Although optical properties of
minerals may be more difficult to determine
compared to physical properties, they are
nonetheless an extremely important tool in
mineral identification
OLIVINE in BASALT
www.geology.neab.net
www.saharamet.com
5
Introduction Although optical properties of
minerals may be more difficult to determine
compared to physical properties, they are
nonetheless an extremely important tool in
mineral identification
OLIVINE in BASALT
www.geology.neab.net
www-odp.tamu.edu
6

• THE NATURE OF LIGHT
• In order to account for all properties of light,
  we must think of it in terms of 2 theories Wave
  Theory and Particle Theory
• In optical mineralogy, however, Wave Theory
  provides sufficient explanation

www.mhhe.com
The Electromagnetic Spectrum
7

• THE NATURE OF LIGHT (cont.)
• Visible light travels in straight lines with
  transverse wave motion
• That is, it vibrates at a right angle to the
  direction of propagation

www.mhhe.com
The Electromagnetic Spectrum
8

• THE NATURE OF LIGHT (cont.)
• Visible light travels in straight lines with
  transverse wave motion
• That is, it vibrates at a right angle to the
  direction of propagation

www.acs.appstate.edu
micro.magnet.fsu.edu

• The wavelength (?) of such a wave is the distance
  between successive crests
• The amplitude is the displacement, or height of
  the wave from a position of equilibrium
• The frequency (f) is the of waves/sec passing a
  fixed point (CONSTANT)
• The velocity (V) is the frequency multiplied by
  the wavelength

9
THE NATURE OF LIGHT (cont.) If two (or more)
waves vibrate in the same plane and travel along
the same path, they interfere with each other.
The distance that one wave lags behind another
is called the retardation (?). If ? is an
integer, the waves are in phase (constructive
interference). If the ? is not an integer, the
waves are out of phase (destructive
interference).
10

• THE NATURE OF LIGHT (cont.)
• When light passes from one medium through
  another
• The velocity slows down 2. The wave is reflected
• 3. The wave is refracted

How much will light be refracted? Snells
Law sin ?1 n2

sin ?2 n1
11

• THE NATURE OF LIGHT (cont.)
• Isotropic versus Anisotropic Minerals
• Isotropic minerals The chemical bonds are the
  same in all directions so the electron clouds can
  oscillate the same in every direction.
• Anisotropic minerals The chemical bonds are
  different in many (and sometime all) directions
  so oscillation is different in most directions.
  So, an anisotropic mineral is one in which the
  velocity of light is different in different
  directions

www.brocku.ca/earthsciences
12

• THE NATURE OF LIGHT (cont.)
• Polarized light
• Ordinary light (sunlight or incandescent light)
  is unpolarized, in that it vibrates in all
  directions at right angles to the direction of
  propagation
• A polarizer constrains the vibration to one
  plane

13
OPTICAL PROPERTIES Relief the degree to which
mineral grains stand out from the mounting
medium. High relief means that the mineral
stands out strongly.
14
OPTICAL PROPERTIES Birefringence the resulting
interference colors observed in polarized light
as a result of light being split in two rays
during passage through a mineral
15
OPTICAL PROPERTIES Birefringence the resulting
interference colors observed in polarized light
as a result of light being split in two rays
during passage through a mineral
16
www.uwgb.edu/dutchs/petrolgy
jgutmann.web.wesleyan.edu
people.uncw.edu
www.microscopy.fsu.edu
17
OPTICAL PROPERTIES Extinction, Extinction Angle
Anisotropic minerals (unless the c-axis is
perfectly parallel to the direction of light
propagation) go dark, or extinct, every 90 of
rotation of the microscope stage. The angle
between the length or cleavage of a mineral and
the minerals vibration directions
www.gly.bris.ac.uk
18
OPTICAL PROPERTIES Pleochroism The change in
color of anisotropic minerals with stage rotation
www-odp.tamu.edu