quartz, inclusions, gems

Microscopic and freezing-stage observations are used to investigate conspicuous single-phase and two-phase inclusions in a small amethyst crystal, with particular attention to their contents and unusual crystallographic shapes. Numerous negative crystals occur in two parallel arrays beneath prism surfaces and are sufficiently large to be visible without magnification. Some retain conventional quartz-like forms, whereas many are markedly elongated along the host’s horizontal crystallographic axes. Most appear to contain a single fluid or gaseous phase; a smaller number contain liquid and a bubble. The article introduces heating- and freezing-stage microscopy as methods for examining individual inclusions and obtaining information about fluid composition and mineral formation. Heating can establish homogenisation temperatures, although interpretation requires consideration of pressure, and carbon-dioxide-rich inclusions may rupture before homogenisation. Freezing followed by controlled warming permits observation of melting, sublimation and other phase changes that can constrain salinity, gas identity and fluid density. In the amethyst studied, cooling to −155 °C produced solid bodies that subsequently disappeared at −99 °C and −90 °C. Ice in aqueous inclusions melted at −0.4 °C and −0.6 °C, corresponding to reported sodium chloride concentrations of approximately 0.7 and 1.0 wt%, respectively. These observations establish a dilute aqueous component, but the gaseous phase could not be identified conclusively. It did not liquefy even at the lowest temperature reached, and the solid material formed during cooling could not be analysed directly. Methane is considered the most plausible interpretation because the solid’s morphology resembles that observed in methane-bearing inclusions; nitrogen is discussed as an alternative. If methane was present, its behaviour implies trapping at very low pressure. The author also relates the distorted negative-crystal forms to previous observations that methane can influence quartz growth habits and cavity morphology. Comparisons with methane-bearing quartz from the Swiss Alps support this interpretation without removing the uncertainty over gas composition. The coexistence of different inclusion fillings is taken to suggest that the amethyst did not grow in a purely methane environment. Instead, the author proposes that an aqueous solution and supercritical methane coexisted following a rapid reduction in temperature and pressure. The study combines a methodological explanation with a cautiously interpreted case, demonstrating both the information available from inclusion phase changes and the limitations of identification without direct chemical analysis.