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The composition of the hydrous vapor in equilibrium with synthetic granite and water-saturated synthetic granite magmas has been determined at pressures up to 10,000 bars at temperatures above and below the solidus. At temperatures above the solidus, the silicate constituents of the vapor phase are depleted in potassium feldspar relative to “normal” granites. The anhydrous composition of the vapor phase in equilibrium with alkali feldspar and quartz at 2 kb is, in contrast, very rich in silica; at 10 kb the anhydrous composition of the vapor phase is approximately the same as that of the liquid phase at the beginning of melting.

The addition of sodium disilicate to a synthetic granite composition at 5 kb, 650°C was found to greatly enhance the solubility of water in the granitic liquid. The solubility of silicates in the aqueous vapor phase was also increased considerably. This relationship suggests that peralkaline granitic magmas may show continuous solubility with respect to water. That is, a continuous transition from magma to a hydrothermal solution.

These experiments bear on the possible metasomatic changes to be expected in the immediate vicinity of a granite pluton, as well as on the process of forming “granite looking” rocks. The continuous transition from quartz veins to pegmatite veins, as the granite contact is approached, is to be expected on the basis of the experimental study. Certain features of the concentrically zoned pegmatites, notably the quartz cores, may be explained in terms of the data presented here.

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