BORON K-EDGE XANES OF BORATE AND BOROSILICATE MINERALS

Abstract

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; IIIB), a broader peak (B) at 197–199 eV (the edge feature of tetrahedral B, IVB), 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 IIIB and its position yields information on B-O bond length and linkage of the BO3 group. The area of peak B is proportional to content of IVB 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*(t2) antibonding orbitals. The tetrahedral component of peak C appears to be a σ-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-nearest-neighbor cations, C being most intense in vonsenite (Fe2Fe3+BO5). Priceite (Ca4B10O19·7H2O), of unknown structure, has 80% IVB, and an isolated BX3 group with IIIB-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 IVB to IIIB occurs in minerals with high contents of IVB, and hydrous Mg borates with interstitial H2O are unstable in the high vacuum of the spectrometer.