Presented by – Student’s name
Contents
Introduction
Polarised light
Different types of transmitted microscope
studies
Properties under plane polarised light
Properties under cross nicol condition
Birefringence
Retardation
Isotropic indicatrix
Biaxial indicatrix
Interference figures
optical mineralogy– branch of mineralogy dealing with optical
properties of minerals.
Optical properties of minerals are important for their identification.
Optical properties are determined with the help of polarising
microscope.
Double refraction -Light separates into two rays which makes
images seen through the crystal appear to be doubled.
Ordinary light – ordinary light travels in straight lines with a
tranverse motion. It vibrates in all directions at right angles to the
direction of propagation.
Polarized light – when the vibrations
of the wave motion is confined to a
single plane only, the light is called
polarised light.
Trinocular
head
Analyzer, upper
polarizer, nicols
lens
Objectives
conoscope
Internal light
source, polarized
Reflected
light source
Accessory
plate
Vernier
scale
Determined under
Plane polarized light
Determined under
crossed polars &
orthoscopic
illumination
Determined from
interference colours
obtained under
crossed polars &
conoscopic
illumination
Colour & pleochroism
Form, inclusion,
alteration
Isotropic or anisotropic Uniaxial or biaxial
Polarisation colours
Interference figure
Extinction & Extinction
angle
Cleavage, twinkling
Elongation
Optic sign ( + or -)
Refractive indices,
Relief
Sign of elongation
(length slow or length
fast)
2v
Pleochroic haloes
Twinning & zoning
Properties under plane polarised lightColour – reflection of light from any surface of mineral or any
object.
Form – The shapes of commonly occuring crystals and/or of
aggregates of crystalline grains.
Inclusion – It is the smaller minerals within the larger host minerals.
Alteration – when mineral subjected to weathering & it altered into
secondary mineral.
Eg. Olivine – Serpentine,
Fluid inclusions in quartz in alkali granite
Olivine altered in serpentine
Relief - Relief is a measure of the relative difference between a mineral
grain and its surroundings.
Quartz has low relief
Garnet has high relief
Cleavage – This is the property that some minerals exhibit of
breaking along definite smooth planes.
In hornblende 2 set cleavage in ppl & ucn conditions
Twinkling – Twinkling effect is observable in anisotropic minerals with
widely differing refractive indices on rapidly rotating the stage under
plane polarised light.
Eg. Calcite.
In biotite twinkling is present.
Refractive indices – is a ratio between the sine of the angle of
incidence and the sine of the angle of refraction, which is always
constant for the two media concerned.
E.g. Quartz – 1.55, Halite – 1.54
The mineral biotite changes color from dark brown to black when
the thin section is rotated.
Pleochroic haloes – these are
curious little circular spots
characteristically present in a
few minerals that tend to be
strongly pleochroic.
Eg. Biotite, tourmaline,
amphiboles muscovite.
Isotropic minerals – the interaction of light with minerals
in every direction is constant.
Eg. Garnet, diamond.
Anisotropic minerals - the minerals which changes their
optical properties when oriented in different direction.
Polarisation colour - When an anisotropic mineral is
placed between crossed nicols, it exhibits vivid colours
as a consequence of light being split into two rays on
passing through the mineral. These are interference
colours.
Birefringence – It is difference between the refractive
indices of two rays i.e., ordinary & extraordinary rays.
Extinction – When minerals are seen under the cross nicol position and when
the field remain dark is called the extinction.
Types – Straight extinction - orthopyroxene
Inclined extinction – clinopyroxene
wavy extinction - Quartz
Extinction Angle – Crystal edges or prominent cleavages are used to find the
angle at which extinction occurs and is known as extinction angle.
c
extin
ction
angle
Z
c=Z
n
n
b
Y
a=X
b=Y
a
X
UCN
Twinning – Two or more crystals
intergrow in each other.
Zonning - Some plagioclase
feldspars will have one
composition in the interior of
the crystal, and a gradually or
sharply changing composition
toward the outer edge of the
crystal,This is called zoning.
Zoned feldspar
Polysynthetic twinning in calcite
Retardation 
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When slow ray emerges from a anisotropic crystal, fast
ray must have already emerged & travelled some
distance. This DISTANCE is called Retardation (∆)
Retardation is proportional to thickness (t) of the crystal
and to the birefringence () in the direction light is
travelling:
∆=tx
Isotropic indicatrix
All minerals belonging to the cubic
crystal system are isotropic with
respect to their optical properties.
Uniaxial indicatrix
In the tetragonal,
trigonal, and
hexagonal crystal
systems (a=b =c; or
a1=a2=a3=c ).
R.I. in the plane
perpendicular to the
main symmetry axis
(z) must be constant.
R.I. parallel to C can
be different.
Positive & Negative uniaxial mineral
(+) crystal:
 (c)>  (a)
(-) crystal:
>
 oblate
Calcite
Quartz
Biaxial mineral
Orthorhombic,
monoclinic, and
triclinic crystal
systems have a
triaxial ellipsoid
indicatrix, defined by
semi axes with
length, a b, and g.
Elongated along Z
axis but flattened
along X axis.
Z
X
Y
b
By convention we
define a < b < g.
(medium)
a
(short)
OPTIC AXES – through which no double refraction
occurs
 OPTIC PLANE – includes optic axis/axes
 OPTIC NORMAL – perpendicular to optic plane
 2V or OPTIC ANGLE – angle between optic axes
 Axis is ACUTE BISECTRIX – if 2V is bisected by it (Z or X)
 Axis is OBTUSE BISECTRIX – if obtuse angle between
optic axes is bisected by Z or X.
 If ACUTE BISECTRIX IS X mineral is NEGATIVE
 If ACUTE BISECTRIX IS Z mineral is POSITIVE
 2V is present only in BIAXIAL MINERALS!
 2V is always <90o.
 If 2V = 90 then mineral is optically neutral.
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To determine OPTIC SIGN the best position is to look down optic axis .
How to Know that position?
Mineral appears isotropic in that position.
Grains should be oriented so that optic axis should be vertical or near
vertical.
Resulting figure is called OPTIC AXIS UNIAXIAL figure.
A typical figure has a BLACK CROSS which do not move when stage is
rotated. Centre of cross is MELATOPE (direction of optic axis).
Dark bands are ISOGYRES (=orientation of LP & UP).
Surrounding colour rings (if present) are ISOCHROMES.
They are interference colours of equal retardation.
Minerals with low birefringence do not show ISOCHROMES.
THIS IS CALLED A CENTRED OPTIC AXIS FIGURE.
Centred
Off Centred
SIGN DETERMINATION USING ACCESSORY PLATE
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They are obtained in the same way as
uniaxial figures.
Very complicated & difficult to find grains
oriented as desired.
Interpretation is difficult.
The interference colours shown by them are
dependent on
Birefringence
Thickness
Grain orientation
Four types of biaxial interference figures can
be obtained:
Optic normal figure (max interference colours)
Obtuse bisectrix figure (high interf. Colours)
Acute bisectrix figure (relativ low int.Col)
Optic axis figures (no interference colors)
Last two give max. Optical information.
Optic plane is parallel
to polarizer
Optic plane is not parallel
to polarizer CROSS SEPARATES
INTO 2 ISOGYRES (NW-SE)
2V determination in acute bisectrix figure
Separation of melatopes
is a measure of 2V
Optic Axis figure
It is difficult to identify crystals in the correct orientation to
give an acute bisectrix figure.
It is much easier to obtain an optic axis figure, since in this
orientation the mineral appears isotropic (or very low order
birefringence).