Afghanite: A New Silicate‐Matrix Reference Material for In Situ Halogen and Sulfur Microanalysis

Sulfur (S) and the halogens (F, Cl, Br, I) play key roles in geochemical processes that govern volatile cycling, redox evolution, phase reactions and ore formation in the Earth's crust and mantle. Quantifying these elements in situ requires well‐characterised reference materials for calibration and quality control, yet such materials for silicate matrices remain scarce. In this study, we present new data for a suite of gem‐quality crystals of the feldspathoid mineral afghanite and evaluate their potential as a silicate reference material for halogen and S microanalysis. Afghanite incorporates up to ~ 4.5% m/m Cl and S each within its silicate framework. Our results demonstrate that the investigated afghanite crystals are compositionally homogeneous and yield highly precise measurements with excellent agreement between multiple laboratories and analytical methods. LA‐ICP‐MS measurements calibrated with afghanite determine Cl mass fractions in silicate glass reference materials GSE‐1G and NIST SRM 610 within 5% of compiled literature values without any corrections applied. In contrast, when calibrated with scapolite, Cl mass fractions in afghanite and the same silicate glasses are systematically overestimated. To quantitatively evaluate this bias, we introduce the matrix sensitivity factor (MSF), which demonstrates that matrix effects affecting scapolite result in systematic Cl overestimations of 8–13% in other silicate matrices. Sulfur mass fractions in the scapolite control reference material Sc‐1 determined by LA‐ICP‐MS using afghanite as calibrator are within 5% of results obtained by EPMA and matrix‐matched calibrated LA‐ICP‐MS analyses. We propose afghanite as a robust silicate reference material for in situ determination of Cl and S for geochemical and petrological applications.