Chapter 10 - A
Identification of minerals with the
petrographic microscope
Content
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Sample preparation
Microscope alignment
Determination of the refractive index
Use of interference colors
Conoscopic observation of interference figures
Microscopy
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Transmitted light microscopy
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Transparent crystals
Light transmits through mineral grains
Common rock-forming minerals
Reflected light microscopy
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Opaque crystals
Light reflects from highly polished surface
Usually ore minerals
 This course: transmitted light microscopy
Sample preparation:
Transmitted light microscopy
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Grain mount:
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Finely ground fragments; immersed
in oil and scattered on glass plate;
covered by thin sheet of glass
Thin section:
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Cut slab from rock sample – area of
interest
Bottom - polished and cemented
onto glass slide
Top - ground to desired thickness;
covered with balsam and thin cover
glass
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Rock-forming minerals now transparent
Why use a cover glass?
Microscope alignment
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Important in order to:
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have light going through the center of all lenses, of the stage, the
condenser
get two polarizers filtering light at vibration directions perpendicular to
each other
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Oculars – one or both adjusted for each eye; cross-hair in focus
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Stage – center exactly in the optic axis; object not to move
during stage rotation
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Condenser – when switched on light beam should be centered
around cross-hair
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Polarizer – one set at 0º and one at 90º
Other settings
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Brightness of light – comfortable for your eyes – very bright will
give headaches and burns out filaments
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Iris – determines the diameter of the light beam coming from the
source – different setting for different magnifications
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Condenser lens – use to get high resolution at high magnification
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Focusing – to avoid collision: first bring sample close to objective
lens (not against) and increase distance until sample in focus
Determination of the refractive index
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Grain mount
Edges of crystal – act as small prisms which
concentrate light as a ring of light – the Becke
line
When increasing the distance from sample to
objective (defocusing), the Becke line is always
refracted in the direction of a medium of higher
RI
In practice:
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Change liquids until two adjacent liquids defines the
range for the index of the mineral
Determination of refractive Index
Birefringence (δ)
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When a ray of light is split into two separate polarized
rays – each with a single vibration direction
perpendicular to that of the other ray
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True maximum birefringence value (δ) of mineral
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Isotropic: δ = n – n = 0
Uniaxial: δ = nε – nω
Biaxial: δ = nγ – nα
Under the microscope:
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Observed under crossed polarized light as:
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Interference colors
Only in anisotropic minerals
Birefringence/double refraction
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Doubly refracted waves are polarized but
separate, vibrating in different planes – no
interaction
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Need interference – study interference colours
and properties
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To get interference – a second polarizer inserted –
the analyzer:
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Crossed polarizer/upper polarizer/crossed nichols
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Used to analyze the interference effects of light in minerals
Interference colours
First order colors
Second order colors
Third order colors
Birefringence
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A characteristic that all anisotropic minerals have, intensity
differs
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High birefringent minerals – third/fourth order interference colours
Med birefringent minerals – second order interference colours
Low birefringent minerals - first order interference colours
For specific mineral birefringence depends on orientation:
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Maximum birefringence - orientation of grain shows highest
possible interference colour for the specific mineral
Minimum or no birefringence – orientation of grain shows lowest
or no interference colour for specific mineral
Intermediate birefringence – orientation of grains shows
interference colours intermediate between minimum and maximum
Interference colours
Determine order of colour and so value for birefringence –
interference color chart
Use of interference colors
True birefringence
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In sample: crystals in random orientations
 each grain different interference colors, each with corresponding
birefringence
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Minimum birefringence
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True birefringence
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Circular section (perpendicular to optical axis) give lowest order or no interference
colours – refractive indices on both axes equal or almost equal
Also referred to as the isotropic section
Longest elliptical section (parallel to optical axis) give highest order colors
Refractive index on major axis = largest; on minor axis = smallest
THUS: to determine the true birefringence of mineral – choose grain with
highest interference colors and read of the value of birefringence from the
color chart
Use of interference colors:
Accessory plates (compensators)
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Accessory plate is a crystal with known birefringence and
orientation
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Determine unknown mineral optical orientation by comparing
with known crystal plate orientation
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Crystal orientation in plate parallel with mineral orientation
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Plate colors interfere constructively with colors of mineral
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Addition – Positive (Red plate + color of mineral = blue)
Crystal orientation in plate perpendicular with mineral
orientation
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Plate colors interfere destructively with colors of mineral
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Subtraction – Negative (Red plate - color of mineral = yellow)
Use of interference colors:
Accessory plates (compensators)
POSITIVE
NEGATIVE
Use of interference colors:
Extinction
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As an anisotropic crystal is rotated a full turn
under crossed polarized light, it goes into
extinction 4 times
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I.e. – at every 90° rotation the mineral goes dark
This happens every time the two perpendicular
vibrating directions falls parallel with the two
polarizer directions
Use of interference colors:
Extinction angle
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When optical axis vertical (circular section) – mineral
dark during rotation
When inclined – mineral go dark once every 90º
Angle of extinction can be measured for elongated
minerals or minerals with strong cleavage
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Parallel extinction
Inclined extinction
Symmetrical extinction
No extinction angle
Observation of interference figures using
convergent light – conoscopic view
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Insert condenser
lens
Gives convergent
light
Enters sample at
50º - 90º angles
See image of light
source
Interference effects at
different angles
Conoscopic observation of
interference figures
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Isotropic
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No image
Conoscopic observation of
interference figures
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Uniaxial
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Perpendicular to
optical axis
Conoscopic observation of
interference figures
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Uniaxial
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At an angle to the
optical axis
Conoscopic observation of
interference figures
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Uniaxial
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Parallel to the
optical axis