Chapter 10 - A
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Transcript of Chapter 10 - A
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Chapter 10 - AIdentification of minerals
with the petrographic microscope
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Content Sample preparation Microscope alignment Determination of the refractive
index Use of interference colors Conoscopic observation of
interference figures
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Microscopy Transmitted light microscopy
Transparent crystals Light transmits through mineral grains Common rock-forming minerals
Reflected light microscopy Opaque crystals Light reflects from highly polished surface Usually ore minerals
This course: transmitted light microscopy
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Sample preparation:Transmitted light
microscopy Grain mount:
Finely ground fragments; immersed in oil and scattered on glass plate; covered by thin sheet of glass
Thin section: 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
Rock-forming minerals now transparent
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Why use a cover glass?
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Microscope alignment Important in order to:
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
Oculars – one or both adjusted for each eye; cross-hair in focus
Stage – center exactly in the optic axis; object not to move during stage rotation
Condenser – when switched on light beam should be centered around cross-hair
Polarizer – one set at 0º and one at 90º
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Other settings Brightness of light – comfortable for your eyes –
very bright will give headaches and burns out filaments
Iris – determines the diameter of the light beam coming from the source – different setting for different magnifications
Condenser lens – use to get high resolution at high magnification
Focusing – to avoid collision: first bring sample close to objective lens (not against) and increase distance until sample in focus
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Determination of the refractive index
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: Change liquids until two adjacent
liquids defines the range for the index of the mineral
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Determination of refractive Index
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Birefringence (δ) 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
True maximum birefringence value (δ) of mineral Isotropic: δ = n – n = 0 Uniaxial: δ = nε – nω
Biaxial: δ = nγ – nα
Under the microscope: Observed under crossed polarized light as:
Interference colors Only in anisotropic minerals
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Birefringence/double refraction
Doubly refracted waves are polarized but separate, vibrating in different planes – no interaction
Need interference – study interference colours and properties To get interference – a second polarizer
inserted – the analyzer: Crossed polarizer/upper polarizer/crossed
nichols Used to analyze the interference effects of light
in minerals
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Interference colours
First order colors Second order colors Third order colors
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Birefringence A characteristic that all anisotropic minerals
have, intensity differs 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: 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
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Interference colours
Determine order of colour and so value for birefringence – interference color chart
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Use of interference colorsTrue birefringence
In sample: crystals in random orientations each grain different interference colors, each with corresponding birefringence
Minimum birefringence 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
True birefringence 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
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Use of interference colors:Accessory plates (compensators)
Accessory plate is a crystal with known birefringence and orientation
Determine unknown mineral optical orientation by comparing with known crystal plate orientation
Crystal orientation in plate parallel with mineral orientation Plate colors interfere constructively with colors of
mineral Addition – Positive (Red plate + color of mineral = blue)
Crystal orientation in plate perpendicular with mineral orientation Plate colors interfere destructively with colors of
mineral Subtraction – Negative (Red plate - color of mineral =
yellow)
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Use of interference colors:Accessory plates (compensators)POSITIVE NEGATIVE
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Use of interference colors:
Extinction As an anisotropic crystal is rotated a
full turn under crossed polarized light, it goes into extinction 4 times 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
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Use of interference colors:Extinction angle
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 Parallel extinction Inclined extinction Symmetrical extinction No extinction angle
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Observation of interference figures using convergent light
– conoscopic view Insert condenser
lens Gives convergent
light Enters sample at
50º - 90º angles See image of light
source
Interference effects atdifferent angles
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Conoscopic observation of interference figures
Isotropic No image
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Conoscopic observation of interference figures
Uniaxial Perpendicular to
optical axis
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Conoscopic observation of interference figures
Uniaxial At an angle to the
optical axis
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Conoscopic observation of interference figures
Uniaxial Parallel to the
optical axis
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