Transcript 1. OPTICS AND SPECTROSCOPY OF MINERALS

Study of Rocks
1) Field outcrop
observe relationship between rocks
preliminary identification of large minerals
generalized rock composition and type
take samples
2) Microscopic determination
mineralogy
textural relationships
rock composition, type
origin and history
3) Other analytical techniques such as
Electron Microprobe, ICPMS,
Scanning Electron Microscope
X-ray diffraction
Isotopic analysis
Mineral spectroscopy
More detailed understanding of origin and
history of rock
NMR
PPM
-84.0-92.0-100.0
Petrographic
Microscope
Ocular Lens
Upper Polarizer
Objective Lens
Focus
Stage
Substage Assembly
Including lower polarizer
Light and blue filter
Thin section
Thin rectangular slice of rock that light can pass
through.
One side is polished smooth and then
stuck to a glass slide with epoxy resin
The other side is ground to 0.03 mm thickness, and
then polished smooth.
May be covered with a thin glass cover slip
0.03 mm
Properties of Light
Light travels as an electromagnetic wave
In a solid, liquid or gaseous medium the
electromagnetic light waves interact with the
electrons of the atom.
(wavelength)
(Amplitude)
Direction of Travel
Plane Polarized light (PPL)
In air, light normally vibrates in all possible directions
perpendicular to the direction of travel (A)
Plane Polarized Light vibrates in one plane (B)
PPL is produced by substage polarizer which stops all other
vibration directions
Crossed Polars
A second polarizer can be inserted above the stage,
perpendicular to the substage polarizer.
In air or an isotropic medium, it will stop light from first
polarizer
Isotropic garnet in XPL
Isotropic garnet in PPL
Passage of Light
(1) Reflection from an external or internal
surface.
Angle of incidence (i) = angle of reflection (r)
i r
(2) Refraction
The velocity of light depends on the medium through which it passes
Light is an electromagnetic wave which interacts with electrons
The distribution of electrons are different for each material and
sometimes for different directions through a material
When light passes from one medium to another there is a difference
in velocity
Light rays apparently bend at the contact
Angle of incidence ≠ Angle of Refraction.
i
r
i
r
Refractive Index
The amount of refraction is related to the difference in
velocity of light in each medium.
Refractive index (R.I.) for air is defined as 1
The absolute refractive index for a mineral (n) is the
refraction relative to that in air.
depends on the atomic/crystal structure
is different for each mineral
is constant for a mineral
is a diagnostic property of the mineral
between 1.3 and 2.0
There may be one, two or three values of R.I. depending
on the atomic structure of the mineral.
Deer, Howie and Zussman
Refractive Indices are listed
for rock- forming minerals in
D.H.Z. as n (isotropic), ε ω
(uniaxial) or α β γ (biaxial).
δ (birefringence) is the
maximum difference
between values of R.I.
Garnet Group
Opaque Mineral
Sulphides and oxides
PPL does not pass through
Minerals looks black in PPL regardless of orientation of
mineral or polarizers
Mineral cannot be identified in transmitted light; needs
reflected light
Opaque mineral in granite
Rotated 45o in PPL
Transparent mineral
PPL passes through the 30μm thickness of the thin section
The electromagnetic light waves interact with the electrons
in the minerals and slow down
The higher the density of electrons the slower the light
wave travels
CPX in gabbro
PPL
Becke Line
A white line of light between two minerals
allows the Relative Refractive Index
(R.R.I.) to be measured
This is relative to an adjacent medium
which can be glass, epoxy, or another
mineral
R.I. epoxy: 1.54 to 1.55
Perthite:
Microcline with
exsolved albite
showing Becke
Line between the
two minerals
(PPL)
The edge of the
grain acts like a lens
distorting the light
To measure relative refractive index of
two touching minerals or mineral/epoxy
Use PPL (upper polarizer out)
Partly close the substage diaphragm, reducing light by 50-75%
Slightly raise and lower the microscope stage, observing the movement of
the Becke Line at boundary of grain.
When decreasing the distance between the ocular and the stage, (raising
the stage) the line moves into the material of lower R.I.
Relief
Apparent topographic relief of mineral grains caused by
differences in R.I.
Positive relief - high R.I.
Negative relief - low R.I.
R.I. epoxy = 1.54 to 1.55
Apatite
R.I.= 1.624, 1.666
In quartz
R.I. = 1.544, 1.553
PPL
Cleavage
Parallel cracks in mineral related to crystal
structure, often diagnostic of a mineral
In thin sections cleavage is developed
during grinding of thin section
Note how many directions of cleavages
are present
Measure the angle between cleavages or
between cleavage and some mineral
feature e.g. edge of grain, extinction.
Pyroxene e.g. augite ~ 90o;
Amphiboles
e.g. hornblende ~ 54o/126o
Plagioclase: ~90o
Fracture:
Irregular cracks not related to atomic structure e.g. olivine
Olivine in gabbro (PPL)
Metamict Texture
Intense fracturing cause by radiation
Disruption of crystal lattice can decrease optical properties
The mineral may appear isotropic
Zircon
Allanite
Colour in PPL
Due to absorption of selective wavelengths of light by electrons e.g
absorption of red gives a green colour
May be diagnostic of the mineral e.g. green chlorite
Beware: biotite and hornblende may be either brown or green
Brown
biotite
in
granite
Green
chlorite
in
granite
Green/blue
hornblende
in
amphibolite
Isotropic Minerals
Isometric (cubic) minerals e.g.
garnet, halite
Amorphous materials: glass, epoxy
resin, air
NaCl
a1
a2
Atomic structure is the same is all
directions
Light travels through the mineral with
equal velocity in all directions
Refractive Index: one value (n)
regardless of orientation
a3
a1 = a2 = a3
α = β = γ = 90o
Between crossed polars
Isotropic minerals always look black regardless of
orientation of crystal or rotation of stage
Garnet
rotated
in XPL
Indicatrix
An imaginary figure which indicates the
vibration directions and size of
refractive index
The length of a semi-axis shows the
size of R.I. in that direction through
the mineral
For isotropic minerals, R.I. (n) and
hence the length of the indicatrix
semi-axes are the same for all
directions through the mineral
Therefore, the indicatrix for isotropic
minerals is a sphere with only one
value of R.I. (n)
Isotropic Indicatrix
n
n
Isotropic Minerals
Colour in PPL may be diagnostic
Absorption of light is the same in all directions so the colour
will be the same regardless of orientation of crystal and
remains constant when stage is rotated
Cleavage: rare but fracture common
Always in extinction between crossed polars
Garnet in metasediment
XPL
PPL