V OTHER OPTICAL PROPERTIES
V.1 Pleochroism
Pleochroism, the occurrence of various colours of light because the absorption spectrum changes with the vibrational direction, is often observed in coloured double refractive minerals. It can be observed in plane polarised light when we rotate the stage. We haven't discussed this any earlier because it does not interfere with the other optical properties when one knows the orientation of the indicatrix. It is not necessarily so that when the indicatrix axes are not related by the crystallographic axes that the pleochroit colours are the brightest in the directions of the indicatrix axes. Still, it is common practice to report the pleochroitic colours in the directions of these axes.
Uniaxial minerals generally display two colours and are sometimes referred to as dichroic. In biaxial minerals the pleochroism is determined in the direction of the three axes α, β and γ. Common examples are biotite and amphiboles.
Fig. 5.1 pleochroism of glaucophane
Optical activity can be characterised as a rotation of the polarisation plane of linear polarised light when the light travels through an active medium. Formerly one can explain this effect by assuming that linear polarised light can be seen as the resultant of left-circular and right-circular polarised light (Fig. 5.2). We can see this effect when left and right are not the same, so we can expect this in crystals belonging to the holo-axial point groups (with only symmetry axes, i.e. 1, 2, 3, 6, 23, 222, 422, 32, 622 and 432). Besides these point groups it can also be observed in m, mm2, -4 and -42m. The rotation of the polarisation plane is proportional to thickness of the crystal section and can sometimes show large dispersion.
Examples are:
NaClO3, isometric, point group 23. Between crossed polarisers the crystal is always light.
Quartz, trigonal, point group 32. For λ = 396 nm wright = 1.55810 and wleft = 1.55821. A section perpendicular to the optical axis is thus between crossed polarisers not dark (when it is thick enough) and the interference figure shows at the position of the optical axis a special polarisation colour. The isogyrs show an hiatus.
In all cases there are left and right rotating crystals.
Fig. 5.2 resultant of two vibrations perpendicular to one another, for which the one along the y-axis has a phase difference of φ relative to the one parallel to the x-axis.
V.3 Linear, elliptical and circular polarised light
Light that has passed through a polariser and a double refracting mineral can be described as two transversal vibrations, with the vibrational planes perpendicular to one another and with a certain phase-difference. These two vibrations can now be combined again. When one vibration follows the x-axes and the other the y-axis and the phase difference is zero, then the resulting vibration is a linear polarised ray (Fig. 5.2 a). Is the phase difference 1/4, then we get an elliptical polarised ray as shown in Fig. 5.2b. At a phase difference of 1/2, we get a linear polarised ray again (Fig. 5.2c). Similarly with a phase difference of 3/4 we get an elliptical polarised ray (Fig. 5.2d). With any other phase difference between 0 and 1/4 we get elliptical polarised rays (Fig. 5.2e). When the amplitudes of both vibrations are the same, then we get at a phase difference of 1/4 or 3/4 a circular polarised ray (Fig. 5.2f).
General:
Bloss, F.D. (1999) Optical crystallography. MSA Monograph 5, Mineralogical Society of America, Washington DC.
Ehlers, E.G. (1987) Optical mineralogy, theory and techniques. Volume 1, Blackwell Scientific Publications, Melbourne.
Gribble, C.D. and Hall, A.J. (1985) Practical introduction to optical mineralogy, George Allen & Unwin Ltd., UK.
Kerr, P.F. (1977) Optical petrology, 5th ed. McGraw-Hill, New York.
Nesse, W.D. (1986) Introduction to optical mineralogy, 1st ed., Oxford University Press, Oxford.
Phillips, W.R. (1971) Mineral optics, principles and techniques. San Francisco, Freeman.
Shelly, D. (1985) Optical mineralogy, 2nd ed., Elsevier, New York.
Wahlstrom, E.E. (1979) Optical crystallography, New York, 5th ed.
Winchell, A.N. (1947) Elements of optical mineralogy: I. Principles and methods, Wiley, New York
Determination tables, systematics:
Kordes, E. (1960) Optischen Daten zur Bestimmung anorganischer Substanzen mit dem Polarisationmikroskop, Verlag Chemie.
Perkins, D. and Henke, K.R. (2000) Minerals in thin sections, Prentice Hall, Upper Saddle River, New Jersey.
Tröger, W.E. (1971) Optischen Bestimmung der gesteinsbildenden Minerale. I. Bestimmungstabellen, Schweizerbart, Stuttgart.
Tröger, W.E. (1967) Optischen Bestimmung der gesteinsbildenden Minerale. II. Textband, Schweizerbart, Stuttgart.
Winchell, A.N. and Winchell, H. (1951) elements of optical mineralogy. II. Descriptions of minerals, Wiley, New York
Winchell, A.N. and Winchell, H. (1939) elements of optical mineralogy. III. Determinative tables, Wiley, New York