Some Observations on Pleochroism and Idiophany in Mineral Plates

T. Crook
Scientific and Technical Department, Imperial Institute, Dublin

8. Summary: An examination of pleochroic plates in parallel rays shows that polarization by reflection and refraction is an important factor in determining the behaviour of the plates in some cases. Under certain conditions, which are exemplified in a remarkable manner by brown phlogopite, axial pleochroism arising from reflection-polarization is the predominant feature in the effects to be observed when the plate is examined in parallel rays of sufficiently oblique incidence. Reflection-polarization not only influences the phenomena to be observed in genuinely pleochroic plates; but under certain conditions, again exemplified by mica, it is capable of producing strong pseudo-pleochroism.

Since absorption-figures (Absorptionsbüschel) represent the effects of normal and oblique incidence simultaneously, it is desirable to consider them in the light of the behaviour of pleochroic plates in parallel rays. The two sets of phenomena fall naturally together under the head of the pleochroism of plates in light of variable incidence. Hence reflection-polarization is a factor to be reckoned with in dealing with absorption-figures, as is proved by the detailed consideration of the behaviour of brown phlogopite in a convergent beam. We have, therefore, to admit its action as a possible cause of the interference-effects observed when absorption-figures are examined in polarized light.

These interference-effects are theoretically inevitable as a result of reflection-polarization, whether the absorption-figures be examined in ordinary or polarized light. In practice, however, they are imperceptible with a convergent beam of ordinary light when the axis of the beam is normal to the plate; but they become perceptible when examined with a nicol. If the axis of the convergent beam of ordinary light is sufficiently inclined to the plate they may be seen without the agency of any polarization other than that arising by reflection and refraction at the surface of the plate.

If we make due allowance for this action of polarization by reflection and refraction, the explanation of idiophanous interference-effects demands no extraordinary assumption, such as that made by Mallard of a transitional polarizing atmosphere around crystals; or that made by Voigt with regard to the condition of the rays traversing the crystal. Both absorption-figures and idiophanous figures can be explained in a comparatively simple manner, on exactly the same basis as that employed for the explanation of the ordinary effects of pleochroism and interference.

Mineralogical Magazine; May 1911 v. 16; no. 73; p. 1-29; DOI: 10.1180/minmag.1911.016.073.03
© 1911, The Mineralogical Society
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