Boralsilite, Al16B6Si2O37, and "boron-mullite:" Compositional variations and associated phases in experiment and nature

Abstract

Boralsilite, the only natural anhydrous ternary B2O3-Al2O3-SiO2 (BAS) phase, has been synthesized from BASH gels with Al/Si ratios of 8:1 and 4:1 but variable B2O3 and H2O contents at 700-800 °C, 1-4 kbar, close to the conditions estimated for natural boralsilite (600-700 °C, 3-4 kbar). Rietveld refinement gives monoclinic symmetry, C2/m, a = 14.797(1), b = 5.5800(3), c = 15.095(2) Å, β = 91.750(4)°, and V = 1245.8(2) Å3. Boron replaces 14% of the Si at the Si site, and Si or Al replaces ca. 12% of the B at the tetrahedral B2 site. A relatively well-ordered boralsilite was also synthesized at 450 °C, 10 kbar with dumortierite and the OH analogue of jeremejevite. An orthorhombic phase ("boron-mullite") synthesized at 750 °C, 2 kbar has mullite-like cell parameters a = 7.505(1), b = 7.640(2), c = 2.8330(4) Å, and V = 162.44(6) Å3. "Boron-mullite" also accompanied disordered boralsilite at 750-800 °C, 1-2 kbar.