GEOCHEMICAL TRENDS OF BERYL TRACE ELEMENTS: EXPECTATIONS MET AND UNMET

Beryl (Be₃Al₂Si₆O₁₈) is one of the first rare element minerals to form during granitic fractionation progression; it is most common in pegmatites and evolved granites. It can also occur in a variety of hydrothermal systems, metamorphic environments, and topaz rhyolites. While tourmaline is often called a trashcan mineral for its ability to contain a very wide variety of elements in a range of geologic environments, beryl is better described as a bucket mineral. Beryl can accommodate a moderately diverse range of elements at minor- and trace-element levels, hence it can be very useful to trace petrogenetic processes and geologic events within the upper portion of Earth’s crust. However, beryl is comparatively understudied relative to other gem minerals such as garnet, tourmaline, apatite, and zircon. This study explores the geochemical compositional range of beryl and evaluates geologic and economic applications where beryl can be utilized.

Beryl can be separated structurally or by color (thus chromophores) – these are often complementary features. Structurally, beryl has two dominant drivers: substitutions occurring at the octahedral Al-site, or substitutions occurring at the tetrahedral Be-site. The simplest visual criteria for beryl classification/separation are by its color, largely corresponding to its gemstone varieties. Considering the structural and color driven groups together, four broad practical groupings emerge: emeralds, non-emerald octahedral type substitution dominant beryl (aquamarine, heliodor, green beryl), Li-driven beryl (goshenite and morganite), and a natural kind subset of non-emerald octahedral type substitutions: red beryl.

We evaluate beryl as a geochemical tracer of petrogenetic processes and gain understanding of how geographic origin determination can be conducted for gem specimens beyond the current economic deposits of emerald. We use a data-driven approach, with thousands of new LA-ICP-MS analyses acquired from hundreds of samples, to ensure we represent the widest range of chemistry yet known in natural beryl. Classical geochemical approaches, statistical analyses, and machine learning reveal new compositional trends in beryl according to their classifications, verify or defy current expectations, and demonstrate that beryl is a tracer mineral in broader geologic research involving fractionated environments.