Mineralogy of Beryllium in Granitic Pegmatites

Abstract

Beryllium is one of the most widespread rare elements in granitic pegmatites. These rocks have been historically the sole industrial source of this metal (e.g., Norton et al. 1958), and they still contribute a significant proportion of the global output of beryllium ores. Hand-cobbed beryl constitutes a substantial proportion of beryllium ore concentrates in Africa, Asia and South America, although non-pegmatitic, rhyolite-related bertrandite ores are virtually the single source in North America (Petkof 1975). The mineralogy of beryllium in granitic pegmatites is strongly diversified, but very “imbalanced” in terms of numbers of species per mineral class on one hand and of the paragenetic role, distribution and abundances on the other. Only a very few Be minerals form at the magmatic stage of pegmatite consolidation, with beryl absolutely dominant among them. Phosphates constitute most of the late subsolidus phases, with silicates a close second, but the number of the phosphate occurrences is very low and volumes are negligible. A few oxide, hydroxide and borate minerals complement the spectrum. So far, no other mineral classes are represented, although the occurrence of some arsenates is considered possible. Part of the reason for the pattern above is the crystal-chemical behavior of Be, one of the classic amphoteric elements, which acts as a cation in acidic environments but participates in complex anions under alkaline conditions. Thus beryllo(alumino)silicates and beryllo-phosphates of alkali and alkaline-earth cations are widespread, in contrast to silicates or phosphates of beryllium with no other cations, or Al only. Be2+ is always tetrahedrally coordinated with oxygen (BeO4)6−, or with oxygen and hydroxyl (BeO3OH)5−. The divalent charge on Be renders substitutions for other tetrahedrally coordinated oxycomplex-forming cations, such as Si, Al or P, difficult. It is possible only by charge-balancing via additional alkali or alkaline-earth …