Boron K- edge XANES of borate and borosilicate minerals

Synchrotron radiation B K-edge XANES spectra are reported for 19 borate and borosilicate minerals. The spectra are characterized by three prominent features; a sharp peak (A) at ~194 eV (the edge feature of trigonal B; III B), a broader peak (B) at 197-199 eV (the edge feature of tetrahedral B, IV B), and a broad peak (C) at 200-201 eV for tetrahedral B and 203-204 eV for trigonal B. The area of peak A is very sensitive to content of III B and its position yields information on B-O bond length and linkage of the BO 3 group. The area of peak B is proportional to content of IV B but quantification is limited by overlap with peak C. The width of peak B increases with increasing divergence of B-O bond lengths, responding to splitting of σ*(t 2 ) antibonding orbitals. The tetrahedral component of peak C appears to be a s-shape resonance. For trigonal B minerals, the relative intensity of peak C and its satellite peaks increases with increase in mean size and/or atomic number of next-nearestneighbor cations, C being most intense in vonsenite (Fe 2 Fe 3+ BO 5 ). Priceite (Ca 4 B 10 O 19 ·7H 2 O), of unknown structure, has 80% IV B, and an isolated BX3 group with III B-O = 1.373(5) Å. Comparison of B K-edge XANES spectra collected using total electron yield (sampling depth <60 Å) and fluorescence yield (sampling depth <1100 Å) shows that borates and borosilicates readily reconstruct in surface and near-surface environments. More profound structural damage involving conversion of IV B to III B occurs in minerals with high contents of IV B, and hydrous Mg borates with interstitial H 2 O are unstable in the high vacuum of the spectrometer.