Mineralogical Characteristics and Cause of Color of a Red Fire Opal

摘要

The gemological conventional testing, Fourier Transform Infrared Spectroscopy (FTIS), X-ray spectrofluorimetry and scanning electron micrograph analysis were adopted to conduct a systematic research of regular characteristics, spectroscopic characteristics, chemical components and microstructure of the red opal. The gemological regular conventional testing results suggested that the infrared absorption characteristic of the red fire opal, the natural fire opal and the newlycombined fire opal were basically similar. The X-ray spectrofluorimetry showed that Fe and Cu might be the major coloring elements, which were evenly distributed in the fire opal in the microcrystalline form of the hematite (Fe_2O_3) and the cuprite red (Cu_2O). The scanning electron micrograph analysis suggested that the red fire opal was made up of silicon dioxide (SiO_2) balls of irregular shapes. The diameter of these SiO_2 balls was only dozens of nm, thus being unable to diffract and intervene in the visible light. Consequently, the red fire opal cannot generate the chatoyance effect. All the above gemological conventional test results and the test results of the large-scale instruments suggested that the red fire opal adopted by this research was the natural fire opal.