Quantification of H, B, and F in kornerupine: Accuracy of SIMS and SREF (X-ray Single-Crystal Structure Refinement) Data

We use a multiple-analytical approach based on secondary-ion mass spectrometry (SIMS), X-ray single-crystal structure refinement (SREF) and electron-probe micro-analysis (EPMA) to derive the complete crystal-chemical formula of a B-rich kornerupine-group mineral, prismatine, from Hrarigahy, Madagascar: (Ca0.01Li0.02Mg0.20Fe2+0.10) (Mg3.57Fe2+0.06Al5.37) (Si3.84B0.91Al0.26)O21 (OH1.08F0.07). SIMS matrix effects related to crystal structure were investigated by analyzing two grains with a known crystallographic orientation relative to the ion beam. Boron orders at the T3 site. The refined site-scattering for T3, 6.33 eps (electrons per site) agrees well with the mean bond-length for this site (1.512 Å), which indicates nearly complete occupancy by B (85% rel.). B2O3 (~ 4 wt%), derived by SREF, agrees with the SIMS data within analytical uncertainty using Si as the inner reference for the matrix. The occupancy of the X site obtained by combining the SIMS and EPMA data (5.30 eps; electrons per site) agrees with the refined site-scattering value (5.75 eps). Trace quantities of Li and Ca are ordered at this site. SIMS data for H2O is in accord with the stoichiometric value, indicating complete occupancy at O10 by OH. Fluorine (~ 0.17 wt%) orders at O10: it corresponds to ~ 0.07 atoms per formula unit (apfu) vs. 0.15 apfu (atoms per formula unit) by SREF, indicating a slight overestimation of F with SREF, as previously observed in fluoborite. Our data show that SIMS chemical matrix effects are well-calibrated, and emphasize the usefulness of independent micro-analytical techniques in testing the mutual accuracy and consistency of experimental data.