THE CAUSE OF IRIDESCENCE IN NATURAL QUARTZ CRYSTALS FROM THE JALGAON DISTRICT, INDIA

Iridescence is so highly prized in gem materials that gemologists have fabricated techniques that artificially impart a play of colors to solids. Iridescence may be caused by one of two processes: the interference of light by thin films or through diffraction by periodic substructures. Therefore, manmade gems with rainbow effects can be created by coating non-iridescent crystals with thin metal films (as with “flame-aura” quartz), or by synthesizing solids with modulated microstructures. However, naturally iridescent gemstones are rare and therefore highly valued. For this study, we have explored the cause of iridescence in natural quartz crystals from the Jalgaon District, India. The specimens occur as euhedral quartz crystals within chalcedonic geodes that filled cavities in the Deccan Trap basalts. The quartz crystals exhibit strongly expressed terminal faces, and iridescence is only visible on the smaller z {011} faces and not on the r {101} faces.

Our scanning electron microscopy ruled out the existence of a thin film on the iridescent faces and suggested a fine-scale substructure. AFM imaging revealed that the iridescent z faces exhibit periodic ridges, and the distance between the ridges varies from 400 nm to 700 nm, generating a diffraction grating for visible light. On the other hand, the non-iridescent r faces are quite flat with no apparent ridges observable by AFM. We interpret the modulated surface topography on the z faces as the result of preferential dissolution. Previous investigators have hypothesized that the iridescence in quartz is associated with Brazil twinning. Thus, we employed focused ion beam lift-out and transmission electron microscopy to determine whether Brazil twins were concentrated at the ridge boundaries. However, instead of Brazil twin boundaries, we observed periodic planar defects parallel to the c axis. The regularly spaced planar defects might have formed by the episodic injection of silica-rich fluids into the host rock cavities (leading to periods of crystal growth), followed by periods of quiescence and crystal stasis. The planar defects formed by the incorporation of fluid inclusions on crystal faces at the onset of a new growth cycle.