The hydrothermal single crystal growth of topaz

Abstract

A general description of the structure, composition and occurrence of natural topaz is given. Previous syntheses are also discussed, together with possible methods of topaz synthesis. A brief description of the hydrothermal process is given, including a discussion of transport and reaction mechanisms in hydrothermal solutions, the role of mineralisers in hydrothermal systems and the role of ammonium and fluoride ions in natural hydrothermal systems. Topaz was grown successfully under retrograde solubility conditions. Factors affecting growth rates and quality of topaz growth were investigated. Temperature was found to be the most important single factor affecting crystal growth rates. Topaz was grown most successfully in NH4F mineralisers, but other mineralisers were also tried. The effects of pressure, temperature and concentration of mineralisers on the solubility of topaz were investigated under supercritical conditions. Doping of the synthetic topaz was attempted but proved unsuccessful. Properties of the colourless synthetic topaz were found to be almost identical to those of naturally formed colourless crystals.