Formation of Apollo 15 green glass beads

Abstract

Size frequency distributions of the cross‐sections of green glass beads in a thin section of regolith breccia 15427 have been determined. Cross‐section medians of vitrophyric and glassy beads are 0.22 mm and 0.094 mm, respectively. Vitrophyric beads contain exclusively olivine crystals of three crystallographically different morphologies. With a synthetic melt of green glass composition, free flight cooling rates have been determined. Spherules of 0.22 mm and 0.094 mm in diameter cool at rates of 1500°C/s and 4200°C/s, respectively, in the temperature range 1050°C–1000°C. The critical cooling rate for green glass formation, measured under controlled conditions, is about 1°C/s, indicating that lunar green glass beads have not been cooled in free flight but in a hot gaseous medium. By controlled cooling of synthetic green glass melt droplets from above the liquidus temperature, olivine morphologies have been produced that are identical to those in lunar vitrophyric beads. On the other hand, after heating synthetic glass spherules, textures have been observed that do not occur in lunar green glass. It follows that the lunar beads have been continuously cooled without later reheating. It is inferred that green glass melt droplets have been erupted from the lunar interior together with a large mass of gas, the cooling history of which is recorded by the internal textures and the size distributions of the green glass beads.