The structure and optical behaviour of iridescent agate

Summary

A detailed study has been made by optical and X-ray methods of two plates of banded agate which display iridescence over part of their area. The light transmitted by the banded areas is found to be completely polarised with the vector normal to the planes of banding. On the other hand, the wavy superposition patterns exhibited by the iridescent areas disappear for the same vibration direction. From these facts and the observed optical characters of the diffusion and diffraction phenomena it is deduced that the crystallites of quartz form fibres elongated in the direction of a crystallographica-axis, while their principal orc-axes lie in the planes of banding but are orientated in a periodic manner in these planes so as to build up a structure which functions as a diffraction grating. The X-ray results support these findings. More generally also, they indicate that the banding of agate is a consequence of the presence in it of groups of crystallites of quartz having common specific orientations.