quartz, topaz, inclusions, gems

Reports microthermometric observations of fluid inclusions in an amethyst of unknown origin and a blue topaz from St Anne’s Mine, Miami, Zimbabwe, extending an earlier study of comparable inclusions in amethyst. Both specimens contain primary negative crystals and irregular inclusions interpreted as pseudosecondary, with liquid and vapour phases occurring in varying proportions. In the amethyst, primary cavities are aligned parallel to the host’s c-axis. After cooling to −150 °C, subsequent warming reveals a phase change described as sublimation at −108 °C in the primary inclusions, followed by ice melting at −1.6 °C; ice in the pseudosecondary inclusions melts at −0.2 °C. The latter homogenise between 153 and 170 °C. Comparison with the previously examined amethyst reveals broadly analogous behaviour, which Gübelin interprets as indicating a common source. He infers water-bearing primary inclusions at low internal pressure and weakly saline aqueous fluid in the pseudosecondary inclusions. Necking down limits the precision with which homogenisation temperatures can be related to formation conditions. A proposed model places amethyst crystallisation in cavities within volcanic rocks, from hydrothermal solutions at approximately 150–200 °C and 20–30 bar. Boiling and entrapment of vapour are invoked to explain some cavities, with an additional gas, possibly carbon dioxide, tentatively suggested. The topaz was cooled to −180 °C and warmed while changes in its inclusions were observed. Ice begins melting near −2 °C, some daughter crystals dissolve between 20 and 100 °C, and many remaining solids dissolve by 232 °C. Heating was stopped at that temperature to avoid cracking, so the proposed homogenisation temperature near 250 °C was estimated rather than measured. Repeated cooling and warming demonstrate spontaneous nucleation and growth on existing crystals. The author interprets the topaz as hydrothermal, consistent with field observations, but notes that the inferred temperature is unusually low compared with published values. Its inclusions appear highly saline, although the salts remain unidentified; their optical behaviour and limited depression of the ice-melting temperature lead him to suggest sulfates rather than chlorides. The interpretations are explicitly presented as working hypotheses requiring further investigation with more abundant material.