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In the presence of water vapor under pressure, soda feldspar and potash feldspar crystallize readily from their glasses. At high temperatures they appear to form a complete series of solid solutions showing continuous variation of lattice-spacing as measured by X-rays. Studies of the dry melts had indicated that the solid solutions are of the type having a minimum-melting mixture. This conclusion is reinforced upon investigation in the presence of water. At a pressure of 1, 000 kg/cm^2 of water vapor the minimum lies at the composition 70 per cent NaAlSi₃O₈, and the temperature is 843° C. This temperature is 220° below that observed in dry melts, the lowering being induced by the presence of some 7. 5 per cent H₂O in the liquid phase or melt. Increase of pressure to 2, 000 kg/cm^2 produces a further lowering of only about one-third as much. When crystallized at still lower temperatures, the complete solid-solution relation no longer obtains, and two feldspars form side by side. Homogeneous synthetic feldspars formed at high temperatures unmix when held at lower temperatures, in confirmation of the accepted theory of most natural perthites. The composition of both members in natural cryptoperthites can be determined by means of X-ray data established for our synthetic feldspars of known composition.
Bowen et al. (Fri,) studied this question.