PhD Bertha Aguilar-Reyes

"Microstructural study of opals: application to whitening destabilization"

Bertha Aguilar Reyes, 29 November, 2004.


This study focused on the 3 following aspects:

  1. Microstructure of opal, in particular common opal.
  2. Raman properties of opal.
  3. Study of whitening destabilization.

Many samples from several localities have been characterized by SEMin order to adress the following issues: do specific origins show specific microstructures? Do opals of a single color but from several localities show similar properties? What is the origin of color of fire opal? In this study we used FT-Raman in order to better interpret the signification of the main, large Raman band in the silica region.

The origin of whitening destabilization of opal is a main contribution of this study. We first documented how miners and opal dealers consider this problem as a crucial limiting factor for opal business. destabilization is a major problems for opal producers such as in Mexico, Ethiopia or Australia. some specimes, sometimes of very high price, must be refund because of destabilization. We first investigated the whitening phenomenon in order to understand the physical processes behind it. We used FT-Raman, X-ray diffraction, Differential Thermal Analysis, porosity measurement by BET, Nuclear Magnetic Resonance, and Scanning Electron Microscopy. We crossed results of all experiments on single samples and determined that a loss of water and a development of micron-size porozity are involved in whitening destabilization.

 

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