1. Olaf Medenbach, Ruhr-Universität Bochum Atlas of Optical Crystallography Olaf Medenbach Institut für Geologie, Mineralogie und Geophysik Ruhr-Universität Bochum 44780 Bochum, Germany olaf.medenbach@rub.de © by Olaf Medenbach Interference figures, part III: dispersion and symmetry

2. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m 2/m 2/m no distinctive dispersion, crystal orthorhombic, monoclinic or triclinic mirror planes m 2-fold axes

3. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m 2/m 2/m, orthorhombic crystal, strong dispersion of 2V, r>>v, cerussite, PbCO 3 m mirror planes 2-fold axes

4. Olaf Medenbach, Ruhr-Universität Bochum Dispersion of 2V Why are color effects in the 45° position reverse of the true axial dispersion? Melatopes (optic axes) for blue and red light Melatopes and isogyres for blue light (points of zero retardation and extinction, respectively, for blue light), remaining wavelengths combine to red Melatopes and isogyres for red light (points of zero retardation and extinction, respectively, for red light), remaining wavelengths combine to blue

5. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m 2/m 2/m orthorhombic crystal, crossed axial planes, brookite, TiO 2 m mirror planes 2-fold axes

6. Olaf Medenbach, Ruhr-Universität Bochum Dispersion of the Indicatrix in Orthorhombic Crystals crossed axial planes for blue and red light Brookite, TiO 2 optic plane for blue light: (010) uniaxial for green light optic plane for red light: (001)

7. Olaf Medenbach, Ruhr-Universität Bochum Brookite crossed axial planes for blue and red light The animation starts a few seconds after the next click.

8. Olaf Medenbach, Ruhr-Universität Bochum (010) (001) 420 nm440 nm460 nm480 nm500 nm520 nm540 nm560 nm580 nm600 nm620 nm640 nm660 nm680 nm Brookite crossed axial planes for blue and red light

9. Olaf Medenbach, Ruhr-Universität Bochum (001) (010) 420 nm440 nm460 nm480 nm500 nm520 nm540 nm560 nm580 nm600 nm620 nm640 nm660 nm680 nm Brookite crossed axial planes for blue and red light

10. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m monoclinic crystal, extreme inclined dispersion 2-fold axis mirror plane

11. Olaf Medenbach, Ruhr-Universität Bochum Dispersion of the Indicatrix in Monoclinic Crystals n y = b, optic plane  (010), extreme inclined dispersion, 2V v >>2V r MgPt(CN) 4 5H 2 O C 3 H 8 O 3, synthetic uniaxial for red light intermediate axial angle for green light large axial angle for blue light

12. Olaf Medenbach, Ruhr-Universität Bochum monoclinic extreme inclined dispersion The animation starts a few seconds after the next click.

13. Olaf Medenbach, Ruhr-Universität Bochum b (010) 420 nm440 nm460 nm480 nm500 nm520 nm540 nm560 nm580 nm600 nm620 nm monoclinic extreme inclined dispersion

14. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m monoclinic crystal, extreme horizontal dispersion 2-fold axis mirror plane

15. Olaf Medenbach, Ruhr-Universität Bochum Dispersion of the Indicatrix in Monoclinic Crystals obtuse bisectix ׀׀ b, optic planes  (010), extreme horizontal dispersion SrPt(CN) 4 3H 2 O, synthetic axial plane for blue light axial plane for green light axial plane for red light

16. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m monoclinic crystal, extreme crossed dispersion 2-fold axis m mirror plane

17. Olaf Medenbach, Ruhr-Universität Bochum Dispersion of the Indicatrix in Monoclinic Crystals acute bisectix ׀׀ b, optic planes  (010), extreme crossed dispersion n-propylamine picrate, synthetic axial plane for blue light axial plane for green light axial plane for red light

18. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: no symmetry in the interference figure triclinic crystal

19. Olaf Medenbach, Ruhr-Universität Bochum Dispersion of the Indicatrix in Triclinic Crystals no symmetry elements present Diisopropylamine picrate, synthetic axial plane for bluegreen light axial plane for green light axial plane for red light

20. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m crystal's symmetry: monoclinic diagnostic color effects m mirror plane 2-fold axis

21. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m crystal's symmetry: monoclinic crossed dispersion, borax, Na 2 [B 4 O 5 (OH) 4 ] 8H 2 O diagnostic color effects 2-fold axis m mirror plane

22. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m crystal ' s symmetry: monoclinic crossed dispersion, heulandite, Ca[Al 2 Si 7 O 18 ] 6H 2 O diagnostic color effects 2-fold axis m mirror plane

23. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m crystal's symmetry: monoclinic inclined dispersion, high sanidine, K[AlSi 3 O 8 ] 2-fold axis mirror plane diagnostic color effects

24. Olaf Medenbach, Ruhr-Universität Bochum 2-fold axis mirror plane Find the Symmetry: symmetry of the interference figure: 2/m crystal's symmetry: monoclinic inclined dispersion

25. Olaf Medenbach, Ruhr-Universität Bochum Find the Symmetry: symmetry of the interference figure: 2/m crystal's symmetry: monoclinic horizontal dispersion, low sanidine, K[AlSi 3 O 8 ] 2-fold axis mirror plane diagnostic color effects

26. Olaf Medenbach, Ruhr-Universität Bochum Nesse (2001) after Steward & Ribbe (1983). Feldspar mineralogy, Rev. Mineral., MSA Optical behaviour of K-feldspar as a function of the degree of Si-Al order Nesse (2001) after Su et al. (1984). Am. Mineral. 69, 440-448 high sanidine: inclined dispersion microcline: horizontal dispersion (010) medium degree of order: pseudo-uniaxial