SOLUBILITY AND STABILITY OF BERYLLIUM-SILICATES IN HAPLOGRANITIC MELTS

Despite its low crustal abundance of ~2.8 ppm, beryllium is a major structural component for more than 90 minerals. In this study, an experimental petrological approach was employed to determine the solubility of beryl and stability within haplogranitic melts. Beryl saturation was evaluated using seven haplogranite compositions with varying concentrations of SiO₂ (64-74%), F (0.02-2.57%), and ASI (0.84-1.25) (ASI= Al/Na+K). The haplogranites of varying compositions were mixed with 10 wt.% beryl and 10 wt.% H₂O, then run at 900-950°C and 200 MPa (H₂O) in a cold-seal apparatus, in order to extend the temperature range used in the beryl stability experiments performed by Evensen et al. (1999). The average calculated BeO content of the haplogranite + beryl powder from 10 runs was ~1.3 wt. %. The glass products were analyzed using EMP and BSE imaging in order to delineate phase stabilities of bertrandite and beryl within the system. Bertrandite is the only stable Be-bearing mineral in 6 runs; no mineral phases are present in 3 runs. Corundum and nickel-spinel are the only stable minerals in 1 run, and is likely prompted by capsule rupture and contamination from the Ni-alloy reaction vessel. The crystallization of bertrandite and absence of beryl may be explained by either: 1) bertrandite is the only stable phase following the breakdown of beryl, or 2) relic beryl hydrated to bertrandite at relatively low temperatures (<300°C). Bertrandite commonly replaces beryl, with a sheet silicate taking up the aluminum, which correlates to an overall increase of Al₂O₃ (wt. %) and decrease of SiO₂ (wt. %) within 5 of the 6 bertrandite-bearing glass products. Although bertrandite has surpassed beryl as the most important source of domestic source of beryllium, the beryl + bertrandite stability relation within the BeO-Al₂O₃-SiO₂-H₂O (BASH) system is not well understood. The effects of relatively high temperature (900-950°C) conditions on beryl solubility and stability within haplogranite melts may provide further insight into the BASH system.