Publications

The speciation of boron in H2O+H3BO3±NaCl and H2O+Na2B4O7 fluids was studied in situ at temperatures between 22 and 600°C and pressures from 0.1 MPa to about 2 GPa using Raman spectroscopy and a hydrothermal diamond anvil cell. Additionally, we determined the frequency shifts of the 877 cm^-1 Raman line of [B(OH)3]⁰ in aqueous fluids with temperature (dv877/dT)P=0.1 MPa = -0.02532 cm^-1K^-1 and pressure (dv877/dP)T=22°C = 4.06 cm^-1GPa^-1. The observed species in acidic fluids were [B(OH)3]⁰ and smaller amounts of a four-coordinated boron species which may be attributed to dissolved metaboric acid HBO2(aq). The ratio of this B^[4]-O species to [B(OH)3]⁰ increases with temperature and decreases slightly with addition of NaCl. In alkaline solutions, polyboric ions depolymerize rapidly with temperature. Thus, [B(OH)3]⁰ and [B(OH)4]^- were the only remaining detectable species at 500 and 600°C. The Raman spectra showed an increase of [B(OH)3]⁰ relative to [B(OH)4]^- with temperature and an increase of [B(OH)4]^- relative to [B(OH)3]⁰ with pressure.
The general trend in the boron speciation is a higher stability of simpler complexes with temperature. The experimental observations strongly indicate that planar three-coordinated [B(OH)3]⁰ is the predominant boron species in the aqueous phase over a wide range of P-T-pH conditions. This supports the validity of previous assumptions on boron coordination in crustal and mantle wedge fluids.