Hambergite-rich melt inclusions in morganite crystals from the Muiane pegmatite, Mozambique and some remarks on the paragenesis of hambergite

Abstract

Raman spectroscopic studies of daughter crystals of hambergite [Be2BO3(OH, F)] in primary melt and secondary fluid inclusions in morganite crystals from the Muiane pegmatite, Mozambique, show that the inclusions have extremely high beryllium concentrations, corresponding to as much as 10.6 % (g/g) in melt inclusions and 1.25 % (g/g) BeO in fluid inclusions. These melt and fluid inclusions were trapped at about 610°C and 277°C, respectively. We propose two possible mechanisms for the formation of the hambergite crystals: (i) direct crystallization from a boron- and beryllium-rich pegmatite-forming melt or (ii) these are daughter crystals produced by the retrograde reaction of the boron-rich inclusion fluid with the beryl host, after release of boric acid from the primary trapped metastable volatile-rich silicate melt during cooling and recrystallization. Although we favour the second option, either case demonstrate the extent to which Be may be concentrated in a boron-rich fluid at relatively high temperatures, and in which species of Be may be transported. One important constraint on the stability of the hambergite paragenesis is temperature; temperatures of >650°C (at 2 kbar) hambergite is not stable and converts to bromellite [BeO].