CRYSTAL-FILLED CAVITIES IN GRANITIC PEGMATITES: BURSTING THE BUBBLE

Crystal-filled cavities in pegmatites occur as open pockets, in which crystals have accumulated on the pocket floor, and as densely clay-packed cavities in which crystals of exceptional gem quality, many of which possess no points of attachment, are suspended in the clay. The occurrences of open pockets have fueled a belief, the “bubble hypothesis”, that the crystal-filled cavities in pegmatites were themselves essentially water-filled bubbles at the time of pocket formation. This hypothesis has its roots in the pegmatite-forming model of Jahns and Burnham (1969, Econ. Geol. 64, 843-864), and has been promoted recently by Simmons et al. (2012, Elements 8, 281-287). Arguments against this model include (1) the high viscosity of undercooled granitic melt (~ 10⁸ Pa·s at 450°C) impedes the ascent and coalescence of aqueous bubbles on the time frames of pegmatite formation, (2) most thin, gem-forming pegmatites crystallize along a solidification front in which the fluid medium that fills cavities is the only fluid medium in the pegmatite, (3) an aqueous fluid of low salinity does not dissolve sufficient silicate solute to make the crystalline contents of miarolitic cavities, and in particular the dense clay, (4) Al is essentially insoluble in such fluids, which form quartz-carbonate veins only, and (5) in open pockets, crystals up to tons in weight have detached from the roof and fallen several meters onto the pile of delicate crystals on the floor, yet rarely do any crystals, large or small, show signs of impact and crushing. As an alternative, the pocket-forming medium is a dense hydrosilicate liquid that is enriched in H₂O largely because of its high concentrations of B, P, and F. That fluid, generated by constitutional zone refining of the main pegmatite body, possesses low viscosity at temperatures of ~ 550°C, but its viscosity increases rapidly with falling temperature (analogue liquids in experiments quench to a durable, insoluble glass). Crystallization of gem-forming minerals from that medium liberates H₂O, such that aqueous fluid is the last fluid in contact with the growing crystal surfaces. The dense pocket clay is the remainder of the hydrosilicate fluid that crystallizes when fluxes are removed and water released. Gradual erosion of that clay by groundwater allows giant crystals to settle without damage.