| Description |
Boralsilite Al16B6Si2O37 and Li, Be-bearing "boron mullite" Al8B2Si2O19 were discovered as breakdown products of spodumene from the elbaite-subtype Manjaka granitic pegmatite, Sahatany Valley, Madagascar. Boralsilite assemblages are closely associated with prismatic spodumene crystals, up to 10 cm long, mostly as narrow albite ; K-feldspar zones (albite; K-feldspar; boralsilite; "boron mullite"), up to similar to 0.5 mm thick, or rarely as inclusions in the central parts of spodumene crystals. Long prismatic crystals and fibers of boralsilite, and rare elongated grains of "boron mullite" Al8B2Si2O19, are mostly enclosed in albite and rarely in K-feldspar. The EMP and LA-ICP-MS data yielded relatively variable Al/Si ratios (similar to 5.4-9.8; similar to 3.2-4.6) and concentrations of B (similar to 48500-59500; similar to 24000-28000 ppm), Li (688-1433; 3339-4287 ppm), and Be (288-1082; 1017-2728 ppm), in boralsilite and "boron mullite", respectively. Fluid/melt with very high a(B2O3), a(Na), and low a(H2O), along with high a(CO2) indicated by the absence of significant amount of hydrous phases, very minor extent of exocontact reactions along the pegmatite, and secondary rhodochrosite, are responsible for the origin of boralsilite. Very late solidus or early subsolidus conditions at temperature similar to 350-450 degrees C and pressure similar to 2-3 kbar, estimated for the crystallization of the assemblage albite + boralsilite, are significantly lower than those given for fibrous boralsilite at other localities of mostly anatectic granitic pegmatites (Larsemann Hills, Antarctica; Almgjotheii, Norway; and Horni Bory, Czech Republic).
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