1. Atlas of Optical Crystallography
Interference figures, part II: biaxial crystals
Olaf Medenbach
Institut für Geologie, Mineralogie und Geophysik
Ruhr-Universität Bochum
44780 Bochum, Germany
© by Olaf Medenbach

2. Interference figures of biaxial crystals
cut perpendicular to the acute bisectrix
acute bisectrix (nX or nZ)
ny = optical normal (ON)
extinction position
45° off extinction position

3. Interference figures of biaxial crystals
cut perpendicular to the acute bisectrix
trace of optic plane
emergence point of the optic axes (melatopes)
extinction position
45° off extinction position

4. Interference figures of biaxial crystals
cut perpendicular to the acute bisectrix
extinction position
45° off extinction position
isogyres
(location of vibration directions in extinction positions)

5. Interference figures of biaxial crystals
cut perpendicular to the acute bisectrix
after BLOSS, 1999
isochromes (Bertin's surfaces)
(curves of equal retardation = equal interference color bands)

6. Interference figures of biaxial crystals
cut perpendicular to the acute bisectrix
nx'
nz= acute bisectrix (AB)
ny = optical normal (ON)
nx= obtuse bisectrix (OB)
nz'
nZ' compensator
Red I
addition
subtraction
optic sign (+)

7. Interference figures of biaxial crystals
cut perpendicular to the acute bisectrix
nx'
nz'
nx= acute bisectrix (AB)
ny = optical normal (ON)
nz= obtuse bisectrix (OB)
nZ' compensator
Red I
addition
subtraction
optic sign (-)

8. Interference figures of biaxial crystals
cut perpendicular to the acute bisectrix, in circular polarized light
the isogyres disappear,
the melatope und the isochromes are still visible

9. Interference figures of biaxial crystals
cut perpendicular to the acute bisectrix, for crystals with different axial angles
extinction position
45° off extinction position
2E = 114,5°
Topaz
2E = 71,5°
Anhydrite
2E = 17,4°
Cerussite
2E = 30,8°
Aragonite
2E = 7,7°
KNO3
trace of optic plane (OP)

10. Interference figures of biaxial crystals
The animation starts a few seconds after the next click.
nZ' compensator
Red I
perpendicular to the acute bisectrix, optic sign (-), 2V ≈ 20°

11. Interference figures of biaxial crystals
The animation starts a few seconds after the next click.
nZ' compensator
Red I
perpendicular to the acute bisectrix, optic sign (-), 2V ≈ 40°

12. Interference figures of biaxial crystals
The animation starts a few seconds after the next click.
perpendicular to the acute bisectrix, large axial angle

13. Interference figures of biaxial crystals cut perpendicular to one axis
ny = optical normal (ON)
nz'
nx'
nZ' compensator
Red I
addition
subtraction
optic sign (+)

14. Interference figures of biaxial crystals cut perpendicular to one axis
ny = optical normal (ON)
nz'
nx'
nZ' compensator
Red I
addition
subtraction
optic sign (-)

15. Interference figures of biaxial crystals
cut perpendicular to one axis, in circular polarized light
the isogyres disappear,
the melatopes und the isochromes are still visible

16. Interference figures of biaxial crystals
The animation starts a few seconds after the next click.
perpendicular to one axis, 2V small

17. Interference figures of biaxial crystals
The animation starts a few seconds after the next click.
nZ' compensator
Red I
perpendicular to one axis, optic sign (-), 2V large

18. Interference figures of biaxial crystals
The animation starts a few seconds after the next click.
perpendicular to one axis, 2V ≈ 90°
(the isogyre is not curved in 45° off extinction position!)

19. Interference figures of biaxial crystals
The animation starts a few seconds after the next click.
semi-random orientation, large axial angle