Dr. Helen Lang
Dept. of Geology & Geography
West Virginia University
FALL 2015
GEOLOGY 284:
MINERALOGY
Microscopic Properties of
Minerals and
the Petrographic Microscope
Light
• Visible Electromagnetic Radiation
• Wavelength,  (Greek letter “lambda”)
• Frequency,  (Greek letter “nu”)
• Velocity, v = 
• Velocity of light in a vacuum c = 2.998 x
108 meters per second
The Electromagnetic Spectrum
Color and Wavelengths of Visible Light
Fig. 4.4 text
violet
blue
green
yellow
orange
red
Polarization of
Light
Fig. 4.3 text
from Nikon’s microscopyU.com
• Electric vectors of
unpolarized light
vibrate in all directions
• Light can be
constrained to vibrate
in only a single plane
by a polarizing filter
• Such light is said to be
Plane Polarized Light
(PPL)
The Petrographic
(polarizing)
Microscope
Analyzer (NS)
Objectives
Rotating Stage
Polarizer (EW,
perpendicular to
Analyzer)
New Leica Microscopes
The Petrographic
(polarizing)
Microscope
Analyzer (NS
or EW)
Rotating Stage
Polarizer (EW or
NS, perpendicular
to Analyzer)
Older Olympus Microscopes
Non-opaque Minerals are either
• Isotropic
or • Anisotropic
• having the
• having different
same properties properties in
in all directions
different
directions
Why Polarized Light?
• Light interacts differently with anisotropic minerals
depending on the light’s vibration direction relative
to planes in the mineral structure
Calcite Structure
Mica Structure
Differences between Petrographic
and Biological Microscopes
• Petrographic microscopes use
polarized light
• The stage of petrographic microscopes
can rotate
• Why? So you can see the variation in
properties as you rotate the stage
(which equals rotating the mineral)
Properties observable in Plane
Polarized Light (PPL):
Relief
• Relief is determined by the difference between
the refractive index (n) of the mineral and the
refractive index of its surroundings
• Refractive index, n = velocityvacuum/velocitymineral
• nminerals mostly between 1.5 and 2.0
Refractive Index is directly
related to Density
forms of SiO2
Examples of Relief in Thin Sections
Garnet, hi relief
moderate relief
low relief (Qtz, feldspar)
Shape in PPL
Euhedral stubby prisms of Nepheline
Hornblende Cleavage 60o & 120o
Pyroxene Cleavage ~90o
Examples of Cleavage in PPL
Color and Pleochroism
• Color in
transmitted light
results when some
wavelengths
(colors) of white
light are absorbed
more than others
• Pleochroism is
when anisotropic
minerals absorb
polarized light
differently along
different directions
in the mineral
Pleochroism: Color
tourmaline is darkest
polarizer
change depends on
orientation of grain
relative to polarizer
biotite is darkest // polarizer
polarizer
Minerals observed in
Crossed-polarized light (XPL)
• Viewed between two perpendicular polaroid filters
– the Polarizer below the sample
– the Analyzer above the sample (insert using rod or switch)
Some microscopes (our binocs,
including new Leicas)
NS
NS
anal.
EW
pol.
anal.
pol.
Other microscopes (our monocs)
EW
When an isotropic substance is
viewed in Crossed Polarized
Light (XPL) it appears dark
Why?
Because the polarized light that
passes through it is unchanged,
and when it hits the analyzer it
is blocked.
Double Refraction (separation of light into two rays
that travel at different speeds and in slightly different
paths) happens in all Anisotropic Minerals
• Calcite Displays Double Refraction Most Dramatically
When polarized light enters an anisotropic
mineral, it is split into two “rays” which
vibrate perpendicular to eachother
• The “fast” ray travels faster
• The “slow” ray travels slower
vfast>vslow
• Remember velocity (v) = 
• Wavelength (changes, but frequency
( remains the same, therefore
fast>slow
=retardation
In an anisotropic min
the “slow” ray lag
“slow” ray
behind the “fast” ra
“fast” ray,
If recombined wave
is parallel to the
Analyzer, all light
passes, mineral
appears brightest
“fast” ray,
long 
“slow” ray
short 
se, 2004)
If recombined
wave is perpendicular to
Analyzer, no
light passes,
mineral is
dark
short  long 
When light passes thru an anisotropic mineral
Constructive and Destructive
Interference
constructive
(brightest)
destructive
(darkest)
Lagging of the “slow” ray behind
the “fast” ray is called Retardation
• When the two rays recombine at the
Analyzer, they interfere (constructively or
destructively) with each other and there is
generally a component of light parallel to
the Analyzer
• Different colors of light experience different
amounts of Retardation
Retardation and Interference
Quartz Wedge
between Crossed
Polaroid Films in
Monochromatic
(NaD; =590nm)
Light
Note constructive
and destructive
interference
Interference Colors
Constructive
(bright) and
Destructive
Interference
(black) for
different colors
sums to the
interference
colors (at the
bottom) for
white light
(Phillips, 1971)
Interference Colors depend on two things:
• How strongly anisotropic the mineral is in a
certain direction
• The thickness of the mineral (typically 30
m in a thin section)
Interference Colors
Plag
Qtz
birefringence
Cpx
Olivine
Musc
Calcite
Thickness (mm)
0.03
retardation
1st order, 2nd order, 3rd order
Interference Colors
B. Sørensen 2012, European Journal of Min.
Is anyone
colorblind?
Journal of Geoscience Education, March 2007
All anisotropic minerals go
extinct (black) 4 times as you
rotate the stage 360o
• Why?
• Because at those positions, each of the two
perpendicular “allowed” vibration
directions is parallel to the polarizer or the
analyzer.
Quartz (in sandstone) has low birefringence
PPL
XPL
Plagioclase has low (white to gray)
birefringence and polysynthetic twinning
Microcline has low birefringence
and plaid twinning
Pyroxene has Moderate
Birefringence (in XPL)
Olivine has High Relief (in PPL) and
Moderate Birefringence (in XPL)
Muscovite also has Moderate Birefringence
From WebMineral-French Web site
Calcite and Titanite/Sphene have
Extremely High Birefringence
Interference Colors
Plag
Qtz
birefringence
Cpx
Olivine
Musc
Calcite
Thickness (mm)
0.03
retardation
Properties best (or only) observed in XPL
• Is mineral isotropic or anisotropic?
• Birefringence or interference colors
– Mineral color may obscure this
• Extinction
• Twinning
• Special properties like “bird’s-eye” extinction in
micas
• Grain boundaries of similar relief minerals
• Other properties?
“Bird’s-eye”
Extinction in
Biotite
(typical of all
micas)
The Electromagnetic Spectrum
Leica Petrographic
Microscope
Olympus Petrographic
Microscope
Why Polarized Light?
• Light interacts differently with anisotropic minerals
depending on the light’s vibration direction relative
to planes in the mineral structure
Calcite Structure
Mica Structure
When light passes thru an anisotropic
mineral
If recombined wave
If recombined wave
is perpendicular to
Analyzer, no light
passes, mineral
appears dark
is parallel to the
Analyzer, all light
passes, mineral
appears brightest
“fast” ray,
long 
“slow” ray
short 
=retardation
“fast” ray,
long 
“slow” ray
short 
(Nesse, 2004)
In an anisotropic mineral,
the “slow” ray lags
=retardation
behind the “fast” ray
“fast” ray,
long 
“slow” ray
short 
(Nesse, 2004)