| Description |
The system B2O3-Al2O3-SiO2 (BAS) includes two ternary phases occurring naturally, boromullite, Al9BSi2O19, and boralsilite, Al46B6Si2O37, as well as synthetic compounds structurally related to mullite. The new mineral vranaite, a third naturally occurring anhydrous ternary BAS phase, is found with albite and K-feldspar as a breakdown product of spodumene in the elbaite-subtype Manjaka granitic pegmatite, Sahatany Valley, Madagascar. Boralsilite also occurs in this association, although separately from vrnnaite; both minerals form rare aggregates of subparallel prisms up to 100 p.m long. Vranaite is monoclinic, space group 121m, a= 10.3832(12), b= 5.6682(7), c= 10.8228(12) angstrom, beta = 90.106(11)degrees; V= 636.97(13) angstrom(3), Z= 1. In the structure [R-1, = 0.0416 for 550 F-o> 4 sigma F-o], chains of A106 octahedra run parallel to [010] and are cross-linked by Si207 disilicate groups, B03 triangles, and clusters of A104 and two A105 polyhedra. If all sites were filled (Al4 and Al5 to 50%), the formula becomes Al16B4Si4O38, close to Li1.08Be0.47Fe0.02Al14 65B3.89Si3.88O36.62 calculated from the analyses assuming cations sum to 24. The compatibility index based on the Gladstone-Dale relationship is 0.001 ("superior"). Assemblages with vranaite and boralsilite are inferred to represent initial reaction products of a residual liquid rich in Li, Be, Na, K, and B during a pressure and chemical quench, but at low H2O activities due to early melt contamination by carbonate in the host rocks. The two BAS phases are interpreted to have crystallized metastably in lieu of dumortierite in accordance with Ostwald Step Rule, possibly first as "boron mullite," then as monoclinic phases. The presence of such metastable phases is suggestive that pegmatites crystallize, at least partially, by disequilibrium processes, with significant undercooling, and at high viscosities, which limit diffusion rates.
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