Causes of iridescence in natural gem materials

Open Access
Author:
Lin, Xiayang
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
October 06, 2015
Committee Members:
  • Peter J Heaney, Thesis Advisor
Keywords:
  • iridescence
  • quartz
  • hematite
  • gemstone
  • diffraction grating
Abstract:
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 a gem quality hematite from João Monlevade, Minas Gerais, Brazil and natural quartz crystals from the Jalgaon District, India. Iridescent hematite, also known as “rainbow hematite”, was investigated with field-emission scanning electron microscopy, X-ray energy-dispersive spectroscopy, atomic force microscopy, and synchrotron X-ray diffraction. This study reveals that rainbow hematite has a microstructure that consists of spindle-shaped hematite nanocrystals with minor aluminum and phosphorus. The nanorods are 200-300 nm in length and 50-60 nm in width, and they are arranged in three orientations rotated by 120º with respect to each other and stacked layer by layer to form the bulk crystal. The distances between adjacent parallel spindle-shape particles in the same layer are in the range of 280 – 400 nm, generating a diffraction grating for visible light. The sub-structure is apparent on all freshly fractured surfaces, indicating that it is not merely an exterior surface coating. Rather, we interpret the periodic sub-structure as the result of crystal growth by oriented aggregation of hematite nanorods. The iridescent quartz 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 visible only 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.