Ore samples must be carefully prepared for reflected light microscopy by grinding, polishing, and casting in epoxy resin mounts to create flat, scratch-free surfaces. Grinding uses progressively finer abrasives to remove surface irregularities. Polishing achieves a optical-quality surface using abrasives as fine as 6 micrometers. Thin sections only 30-40 micrometers thick can also be doubly polished on both sides for transmitted light analysis. Careful sample preparation is essential for clear observation of mineral properties and identification in reflected light microscopy.

1. Reflected light Optics and
Preparation of polished ore samples

2. Violet (380 nm) ----------------------Red (780 nm)
 Opaque minerals are studied in reflected light & that
is termed as ore microscopy.
 Commonly limited to visible spectrum of light
Light produced by a single wave wavelength is called monochromatic and light
containing variety of wavelengths are polychromatic.
white light from sun or artificial source comprises of all the visible spectrum.
 Frequency
 wavelength
 speed of light

3. Vibration direction
Propagation direction

5. Refractive Index
It is the relation between the velocity of light in vaccum to that in material.
Refractive Index (RI), n =
Vvaccum
Vmaterial
C= Vvaccum = 3.00x108 m/s which is constant
So, the RI of the material is characteristic of the light velocity in
that material.
n ∝
1
Vmaterial

6. Snell’s Law
ni
nR
As the RI of light in air or
vaccum is 1
nR
sin i
sin R
=
n ∝
1
Vmaterial
As
In this case the light ray
would bend towards the
normal. In other words
when the ray bends
towards the normal the
velocity decreases and
the medium has higher RI.
nR sin i
sin R
ni
= =
Vi
VR

7. Normal Light vs Polarised Light
When light vibrates in
only one plane of
vibration, it is called
Polarized light.
Normal light
vibrates in
all possible planes
that contain the
light ray
When light vibrates in
all possible planes of
vibration, it is called
Unpolarised light.
light source
light ray
waves travel
from source
to eye
wavelength, l
amplitude, A
plane of
vibration
vibration
direction
propagation
direction

8.  Polarisation can be achieved by:
 Reflection
 Refraction
 Absorbation
 Transmission using polaroids
The process of transforming
unpolarized light into polarized
light is known as
polarization
When angle between reflected and refracted rays is 90° the reflected and refracted rays are
completely plane polarised. Remember the Snell’s Law.

10. Isotropic and Anisotropic minerals
Isotropic minerals show optical
behaviour that is independent of the
direction of light propagation i.e.,
they are identical in all directions .
c-
c+
b+
b-
a-
a+
Isotropic minerals
a=b=c
&
α=β=γ=90°
α
β
γ

11. Anisotropic minerals have more than one characteristic RI which are
directionally dependent.
Anisotropic minerals
Uniaxial
minerals
Biaxial
minerals
This
classification is
based on optic
properties
Hexagonal
(Quartz, calcite, apatite,
tourmaline)
Orthorhombic
(Opx, Orthoamphiboles (Anthophyllite-
Gedrite), Olivine, Sillimanite,
Andalusite, Barytes)
Tetragonal
(Zircon, rutile)
Monoclinic
(Kfs, micas, Cpx, clinoamphiboles)
Triclinic
(Plagioclase, Kyanite)

12. c
ε
ω
Double Refraction
Mutually perpendicular vibrations of
variable RI and thus velocity
The ray with refractive Index, ε is the ray that
travels parallel to c-axis and is called as
extraordinary ray.
The ray with refractive index, ω is the ray that
travels perpendicular to the c-axis and is called as
ordinary ray.
InTransmitted Light Microscopy

14. In reflected light microscopy
There are three different but related types of polarization:
1. plane polarization
2. circular polarization and
3. elliptical polarization.
Plane Polarized

15. Circular polarized Light
 Two waves with equal amplitude vibrate at right angles to each other with
one wave retarded ¼ λ relative to the other.
Vector sum of these two waves is a helix surface that resembles the threads
on a screw, whose cross section is circular (When viewed along the direction
of propagation).
Circular Polarized

16. Circular polarized Light

17. Elliptical polarized Light
 produced in the same manner as circular polarized light except the two
waves that produce it are retarded relative to each other by a value different
than ¼ λ.
 The result is still a helix, but instead of being circular in cross section, the
helix is elliptical.
Elliptical Polarized

18. Elliptical polarized Light

19. Light path in the reflected light microscope

20. Preparation of samples for Ore Microscopy
Prime criteria for preparation of sample for ore
microscopy:
Should be:
 free of scratches
 Thermal and mechanical modifications of sample
surface
 Relief (uneven surface due to hard phases being worn away less
than soft)
Casting of samples
 Samples are cut with wet
diamond saw to avoid heating of
sample
 They are then grinded to provide
a flat surface

21. Samples are then casted generally in
circular molding (2.5 to 5 cm
diameter) material
These molds (made of plastic,
polyethene or metal) are given a thin
coating of non-reactive lubricating
agent (e.g., silicon stopcock grease
or vaseline).
 Samples are then set in cold-
setting epoxy resins or plastics
mounting media were no heat or
pressure is required.
Casting of samples

22. Grinding and polishing of samples
 Once the molded samples are ready after drying they are
grinded on rotary equipment.
 Grinding removes surface irregularities, casting resin
over the surface of the sample, reduce thickness, prepare
smooth surface.
 Grinding is done using emery or carborundum powder.
Grinding and Polishing Machine
 Abrasives with grain size
120, 240, 400, 600 mesh are
used successively for few
minutes.
 Followed by longer duration
of grinding using 1000-1200
mesh carborundum on glass

23.  Polishing is done on a hard napless cloth without
excessive weight pressuring down with surface
sufficiently just lubricated with 15 μm
 6 μm abrasives of aluminum oxide/chromic
oxide/magnesia/cerium oxide can be used for polishing on
napped cloth (e.g., velvet).
Thin-cum-Polished sections
 Polishing of uncovered surface of a thicker thin section
(30-40 microns)
Doubly Polished thin section
 Samples are polished and then glued and then again
polished on other side.