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Mineral color and pleochroism

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Mineral color and pleochroism
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Color
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True color
Not interference colors
Observed in plane polarized light
Not crossed nicols
Most minerals are colorless
Pleochroism
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Property of having two or more true colors
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Occurs only in anisotropic minerals
Each principal vibration direction has a unique
color
Preferentially absorbs selected wavelengths of
light
Pleochroism – Glaucophane
(amphibole – Na, Mg, Fe – Silicate)
Pleochroism – orthopyroxene – (Ca,
Fe, Mg – Silicate)
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Color depends on which vibration direction
parallels polarized direction
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Slow ray has one color
Fast ray another color
Color intermediate if neither direction parallel
to polarized direction
Pleochroic formula
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Relationship of color to index of refraction (a,
b, g, e, w) that shows the color
Pleochroic formula
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Multiple types of formulas:
1.
2.
3.
Color of e, w, a, b, or g rays
Greater absorbance e.g. w > e or e > w
“strongly” or “weakly” pleochroic
Determination of formula –
uniaxial minerals
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Find grain with d = 0
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This is value of w color
Find grain with maximum d
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This has both e and w
Already know w, so other color must be e
Direction of
polarized
light
Grains seen in plane
polarized light (not crossed
nicols)
•1st grain (not shown) – complete extinction
•Viewed in plane polarized light gives w color
•2nd grain (shown) – provides w and e colors
• Since know w already, the other color is e
•Determine fast and slow with accessory plate
Fig. 7-30
Biaxial Pleochroism
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Biaxial minerals may have three colors:
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One for a, b, and g
Procedure similar to uniaxial minerals, but
more complex
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Find extinct section – b color
Find maximum d – this grain has a and g
colors
Determine fast and slow direction with
accessory plate
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Vibration directions
parallel to
accessory plate
If addition, color
associated with ng
If subtraction, color
associated with na
Remember – check
color without
analyzer in
na
ng
Fig. 7-31
Extinction
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Four Categories:
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Parallel extinction – feature (usually cleavage)
parallel to cross hairs at extinction
Inclined extinction – extinction when feature
is at an angle to cross hairs
Symmetrical extinction – occurs in minerals
with two cleavages: bisect cleavage
No extinction angle – minerals with no
elongation or cleavage
Parallel
Symmetrical
Inclined
No extinction
angle
Fig. 7.32
Extinction may not be uniform
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Physically deformed minerals
Minerals with variable chemical
composition (chemically zoned)
Undulatory Extinction
Zoned Extinction
Extinction angle
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Inclined extinction - angle between
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long axis of mineral grain
prominent cleavage
Twins
Other crystallographic feature
Extinction angle
Long direction, also
parallel to cleavage
•Rotate stage until crystallographic
feature is parallel to cross hairs
•Record angle on goniometer
•Rotate stage until mineral is extinct
•Now mineral vibration direction is
parallel to polarized light direction
•Amount of rotation is extinction angle
Fig. 7-31
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Possible to determine chemical
composition from extinction angle
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Michel-Levy technique
Michel-Levy Technique
Section cut
perpendicular to
{010}
Albite twin
lamellae
b
c
b
a
Cut of mineral must be
with {010} plane vertical,
b crystallographic axis
horizontal
b
NaAlSi3O8
CaAl2Si2O8
Characteristics:
High plagioclase = volcanic
•Sharp boundaries between twins
Low plagioclase = plutonic
•Twin lamellae have same interference colors
Fig. 12.15 & 12.17
Albite
Na-feldspar
NaAlSi3O8
Z
b
X
b
Z
X
An0 to An10
An30 to An50
Z
Z
b
X
An50 to An70
b
X
An90 to An100
Feldspars - Triclinic
minerals:
•Two cleavages
•Many types of twins
Extinction angles show
relationship between X-YZ axes (indicatrix axes)
and a-b-c axes
(crystallographic axes)
Anorthite
Ca-feldspar
CaAl2Si2O8
p. 245
Sign of Elongation
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Length fast: elongate direction of mineral
parallels fast vibration direction
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Length slow: elongate direction of mineral
parallels slow vibration direction
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Also called negative elongation
Also called positive elongation
Length fast and length slow depends on
cut of grain
Determination
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Orient grain with vibration direction and
length about 45Вє to polarized direction
Use accessory plate to determine addition
or subtraction of retardation
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Determines if fast or slow ray
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Vibration directions
parallel to
accessory plate
If addition, length
slow (positive
elongation)
If subtraction,
length fast
(negative
elongation)
Length fast
Length slow
Fig. 7-31
Orthorhombic
Minerals (biaxial)
ng = elongate
Always
length slow
na = elongate
nb = elongate
Always
length fast
Either length
slow or length
fast
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