Abstract

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. The pegmatite mineral assemblage is relatively simple (Ab + Qz > Kfs > Tur + Spd > Ap + several accessory B-rich phases). This pegmatite forms an E–W-trending elongated body emplaced in an Mg-rich calc-silicate rock; the pegmatite exocontact locally contains thin reaction zones (Tur+Phl±Brl). 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 ∼0.5 mm thick, or rarely as inclusions in the central parts of spodumene crystals. Long prismatic crystals and fibers of boralsilite, up to 200 μm long, and rare elongated grains of “boron mullite” Al8B2Si2O19, up to 100 μm long, are mostly enclosed in albite and rarely in K-feldspar. The EMP and LA-ICP-MS data yielded relatively variable Al/Si ratios (∼5.4–9.8; ∼3.2–4.6) and concentrations of B (∼48500–59500; ∼24000–28000 ppm), Li (688–1433; 3339–4287 ppm), and Be (288–1082; 1017–2728 ppm), in boralsilite and “boron mullite”, respectively. Textural relations of the boralsilite assemblage assuming immobile Al suggest the hypothetical reaction: 46 LiAlSi2O6 + 30 Na+ (melt/aq.) + 6 H3BO3 = Al16B6Si2O37 + 30 NaAlSi3O8 + 46 Li+ (melt/aq.) + 9 H2O + 4 O2; part of the Li may have entered rare secondary clay minerals and partly also the “boron mullite”, but most of the Li was mobilized. 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 ∼350–450 °C and pressure ∼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 Horní Bory, Czech Republic).

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