inclusions, gems

Gübelin investigates mineral and fluid inclusions in phenakite from Brazil to reconstruct stages of crystal growth and assess the nature of the mineral-forming solution. Three phenakite crystals containing unusual acicular aikinite inclusions were supplied by Pete Dunn following an earlier report of the occurrence. The largest specimen contained two adjacent, parallel aikinite crystals, one projecting into a slender cavity and occupying approximately 98% of its volume. The remaining space held liquid, a gas bubble, and an unidentified anisotropic solid. Further examination revealed small green needles and numerous fluid inclusions along a partially healed fracture. Electron-microprobe analysis of the green needles detected lead, copper, bismuth, and sulphur, supporting their identification as altered aikinite. Their green, disintegrated condition is interpreted as evidence that they existed and underwent alteration before incorporation into the growing phenakite, making them protogenetic inclusions. Fresh aikinite needles and the associated primary fluid cavity represent inclusions formed during host growth, whereas the fracture-related fluid inclusions are classified as pseudosecondary because they were trapped before growth ended. Microthermometric examination involved cooling to −140 °C and subsequent controlled warming. In the primary inclusion, ice began melting at −2.5 °C, carbon dioxide hydrate dissociated near +8 °C, and minute solid grains disappeared between +20 and +50 °C. The anisotropic solid remained undissolved at +60 °C. Gübelin interprets the fluid as a dilute aqueous saline solution containing 4.2 wt% sodium-chloride equivalent and a detectable carbon dioxide component below 6 wt%. The pseudosecondary inclusions, measuring approximately 5–350 µm, contained liquid, gas, and several solid phases. Ice melting began at −2.1 °C and hydrate dissociation occurred near +9 °C, yielding an estimated salinity of 3.5 wt% sodium-chloride equivalent and a similarly low carbon dioxide content. Their comparable fluid compositions suggest limited chemical change between the two trapping episodes, possibly separated by only a short interval. The daughter solids indicate a more complex original solution, although their identities remained unresolved. Together, the three inclusion generations document successive events before and during phenakite growth and support a hydrothermal origin for the host crystal.