BORALSILITE, AL16B6SI2O37, A SILLIMANITE-LIKE MINERAL OF GRANITE PEGMATITES: COMPOSITIONAL VARIATION AND ASSOCIATED PHASES IN NATURE AND EXPERIMENT

Boralsilite, the only natural anhydrous ternary B2O3-Al2O3-SiO2 (BAS) phase, is a minor constituent of pegmatites cutting granulite-facies rocks at Rogaland, Norway and Larsemann Hills, Prydz Bay, East Antarctica. It has been synthesized from BASH gels with Al/Si ratios of 8:1 and 4:1 but variable B2O3 and H2O contents at 700-800ºC, 1-10 kbar. A poorly crystallized material was obtained at 450ºC, 10 kbar. Rietveld refinement of synthetic boralsilite gives monoclinic symmetry, C2/m, a=14.797(1), b=5.5800(3), c=15.095(1) Å, beta=91.751(4)º, V=1245.8(2)Å3 with ~25% Si replaced by B. Mullite was synthesized at 750-800°C, 1-2 kbar and commonly accompanied boralsilite. Powder XRD of one mullite gave a =7.505(2), b=7.640(2), c=2.8330(5) Å, V=162.44 Å3; its cell volume is 3% smaller than that of 3:2 mullite, most likely due to B incorporation. We could not confirm the existence of a compound that was synthesized at 800-830°C, 1-4 kbar and reported to have doubled a and c cell parameters and composition Al8B2Si2O19. Instead, we could index its powder pattern with a mullite unit cell to give a=7.513(9) b=7.658(9) c=2.841(4)Å, V=163.5(3)Å3 except for one peak that could be due to quartz or boralsilite impurity. Sillimanite, andalusite, and dumortierite, but not mullite, are found with natural boralsilite. Where associated with the structurally related werdingite and grandidierite, borasilite contains up to about 30% of a werdingite component, (Mg,Fe)2Al14B4Si4O37. Otherwise, boralsilite contains 0.2-0.6 wt% FeO and 0.01-0.02 wt% MgO; inverse B-Si variation suggests up to ~5% Al2SiO5 substitution. Garnet, cordierite, and/or prismatine are also present in boralsilite-bearing pegmatites, but not in association with boralsilite. Key to boralsilite formation is localized concentration of boron, depletion of Mg and Fe and destabilization of tourmaline. Activities of H2O and Na during original crystallization of the Norway pegmatite were too low for tourmaline, so that B was available for other borosilicates. In the Antarctic pegmatites, crystallization of abundant tourmaline+quartz symplectite during undercooling left a residual liquid depleted in the components necessary for further tourmaline formation, but with sufficient B to stabilize boralsilite.