EVOLUTION OF THE MINERALS OF BERYLLIUM, AND COMPARISON WITH BORON MINERAL EVOLUTION

Beryllium and boron are quintessential upper crustal elements, but differ in abundance (2.1 and 17 ppm, respectively), species diversity, and chemical behavior. Most of the 106 Be minerals are found in granitic pegmatites (49 species) and alkalic rocks (29 plus 9 in both) or in hydrothermal deposits associated with volcanic and shallow-level plutonic rocks and skarns. In contrast to B minerals, non-metasomatic metamorphic occurrences are minor and sedimentary occurrences unknown except for placers. Minerals containing essential Be appear only after extensive differentiation, which explains their relatively late first reported appearance in the geologic record. The oldest are beryl and phenakite in the Mesoarchean Gravelotte emerald deposit, South Africa (2970 Ma), which was followed by the appearance of 16 more species through the earliest Paleoproterozoic (2485 Ma) in granitic pegmatites associated with greenstone belts (e.g., 2860 Ma, Pilbara Craton, Australia), the Sakharjok alkali complex, Russia (2680 Ma), and granulite-facies rocks. There was a burst of 28 new species in the late Paleoproterozoic (1850-1715 Ma) in complex Mn skarn deposits of the Långban type in Sweden and granitic pegmatites. From 1700 to 1 Ma, the number of new Be minerals steadily increased, with surges of 13 species at 1160 Ma (Ilímaussaq peralkaline complex, Greenland) and 7 species at 560 Ma (granitic pegmatites, Brazil). Although B minerals appeared 700 Ma earlier, much of the Precambrian trajectories for Be and B minerals are similar, resulting in but 3 more B species at 1600 Ma, and 12 more at 540 Ma. Only in the Phanerozoic did species numbers greatly diverge, largely due to the preservation of ephemeral evaporitic and fumarolic B minerals. It is very likely these minerals also formed in the Precambrian, only to be destroyed by later geologic activity. Although none of the Be minerals are ephemeral, Barton & Young (2002, Rev. Mineral. Geochem., v. 50, p. 654) cautioned that older occurrences could have been lost to erosion because most Be deposits are concentrated at shallow levels in the Earth’s crust. Nonetheless, the increasing diversity of Be minerals, as well as of surviving B minerals, could have resulted from the increasing diversity and mixing of geologic materials as the Earth’s crust is recycled by tectonic processes.