Geo 406 Lecture 05 - Birefringence, Interference, Elongation
Reading: Ch. 5
next: pp. 53-55, 65
Goals
Understand interference colors
Learn how to tell fast and slow directions
Understand Michel-Levy chart
Understand types of extinction
Understand sign of elongation
Thickness effect
D = d (ns - nf)
quartz wedge demo
d = (ns - nf) = 0.009 (a small value)
shows
change in set of transmitted wavelengths (i.e., color) with increasing
retardation, D
Birefringence effect
D = d (ns - nf)
= d d
can get same set of colors by
varying d at constant d
maximum d is characteristic of
mineral!
e.g., calcite d = 0.172 (a large value)
orientation-dependent
d
(=ns-nf) ranges from 0 to a maximum
0 is looking down optic axis
Interference Color Chart
range of colors - same as quartz wedge
measuring birefringence
Interference Color Chart
Orders
Every 550 nm (≈ lblue)
Interference Color Chart
Two kinds of white
low-order
high-order
Next Lecture
How do you know which “white” you’re looking at?
Wedge effect, gypsum plate
Mineral ID features: sign of elongation, extinction type/angle
Which is the slow ray, w
or e ?
Uniaxial indicatrix, conoscopic
illumination
How are biaxial minerals different?
Biaxial indicatrix, conoscopic
illumination
Questions to think about
How many pleochroic colors would a mineral show that stayed black in XP?
Before polarizing film, microscopes used a Nicol prism, made of two
specially-cut pieces of calcite, glued together. How could you cut calcite to
make this work?
Accessory plates
Tell you fast vs. slow directions
Fig.
7.21, p. 129
Can add or subtract retardation:
Gypsum
plate has D = 550 nm (“l”)
Mica
plate has D = 138 nm (“l/4”)
Short dimension is always slow
Accessory plates
Tell you fast vs. slow directions
Fig.
7.21, p. 129
Can add or subtract retardation:
Gypsum
plate has D = 550 nm (“l”)
Mica
plate has D = 138 nm (“l/4”)
Short dimension is always slow
Using accessory plates
1) Find vibration directions, using extinction
2) Rotate so vibration directions are “diagonal”
3) Insert plate
4) If colors “add”, slowmineral || slowplate,
otherwise, slowmineral || fastplate
Using accessory plates
1) Find vibration directions, using extinction
2) Rotate so vibration directions are “diagonal”
3) Insert plate
4) If colors “add”, slowmineral || slowplate,
otherwise, slowmineral || fastplate
Using accessory plates
1) Find vibration directions, using extinction
2) Rotate so vibration directions are “diagonal”
3) Insert plate
4) If colors “add”, slowmineral || slowplate,
otherwise, slowmineral || fastplate
Using accessory plates
1) Find vibration directions, using extinction
2) Rotate so vibration directions are “diagonal”
3) Insert plate
4) If colors “add”, slowmineral || slowplate,
otherwise, slowmineral || fastplate
Using accessory plates
1) Find vibration directions, using extinction
2) Rotate so vibration directions are “diagonal”
3) Insert plate
4) If colors “add”, slowmineral || slowplate,
otherwise, slowmineral || fastplate