B‑Coordination Mapping Unlocks Granitic Glass Evolution

A new study shows that the way boron atoms bond inside haplogranite glasses depends strongly on melt composition and water content, offering a novel tool for tracing magmatic–hydrothermal events in granite-forming environments.¹

Jakob Rauscher, Michael Fechtelkord, Sandro Jahn, Julie A.-S. Michaud, Draupadi Mothan, Melanie J. Sieber, Robert B. Trumbull, Franziska D. H. Wilke, Max Wilke, and Bernd Wunder report their findings in European Journal of Mineralogy, volume 38, pages 383–396, published in 2026.¹

The team collected samples of haplogranite glass—silicate melts typical of continental crust—and subjected them to nuclear magnetic resonance (NMR) spectroscopy to determine boron's coordination state. The analysis revealed two distinct bonding modes: trigonal planar BO₃ units and tetrahedral BO₄ groups, whose relative proportions shift with changes in water activity and alkali–alumina ratios.¹

By systematically varying water content from anhydrous conditions up to several percent by weight, the researchers observed a progressive increase in the fraction of tetrahedrally coordinated boron. This trend indicates that higher fluid presence promotes the formation of BO₄ units during crystallization.¹

Increasing alkali‑to‑alumina ratio—a proxy for melt composition—led to more BO₃ structures, whereas higher alumina favored BO₄ coordination.¹

Importantly, the study demonstrates that measuring B coordination can serve as a quantitative marker for melt–fluid fractionation processes that are otherwise difficult to assess. Because boron isotopes fractionate between melt and fluid depending on their local geometry, knowing the exact coordination allows geochemists to back-calculate past fluid activities and temperatures during granite emplacement.¹

The authors argue that this approach could be applied to natural rock samples where NMR data is available, providing a new method to estimate how much fluid was present when granites formed.¹

Rauscher et al.’s research supplies a detailed map of boron bonding states across varying melt conditions, opening a new window into the dynamic interplay of fluids and melts during granite formation. This insight will aid mineral explorers and petrologists alike in interpreting the subtle signatures left by ancient magmatic events.¹