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The phase equilibrium diagram of the condensed system LiF-YFg is presented. Data were obtained from thermal analysis of heating and cooling curves and by identifying the phases present in small samples which were quenched after equilibration at high temperatures. Within the system two invariant points occur, the eutectic at 19 mole % YF3 and 695°, and the peritectic at 49 mole % YF3 and 819°. The single intermediate compound, LiF-YF», is formed. It melts incongruently to YF3 and liquid at 819°, and is tetragonal, o» = 5.26 ± 0.03 A., and c0 = 10.94 ± 0.03 Á., space group I4i/a. The compound LiF-YF3 is colorless, uniaxial (+), with refractive indices = 1.454, N< = 1.472. No solid solutions are formed among the equilibrium solids in the system LiF-YFa. The melting point of YF3, 1148°, determined by other workers, was corroborated in the present study. A solid state transition in YFS was shown to occur at 1052°.

Kaolinite was synthesized by heating coprecipitated AJ20 3-Si02 gels and mixtures of Al20 3 and Si02 with water in a pressure bomb at 310 0 C. Similarly dickite was prepared by treating coprecipitated Ah03-Si02 gels at 350 0 C and 365 0 C, beidellite from mixtures of Ab03 and Si02 at 350 0 C and 390 0 C, and also from co precipitated gels and kaolinite at 390 0 C. Nontronite was madc by treating a coprecipitated Fe203-2Si02 gel at 350 0 C. Beidellite was formed in these experiments in the presence of soda by transport of Si02 in solution to the Alz03 and reaction with Al20 3 in situ.The products were identified by X-ray patterns. Their optical properties were also consistent with those of the natural minerals. The stability ranges of kaolinite, dickite, and beidellite probably occur in the order named, with increasing temperature. This relation between kaolinite and dickite is consistent with geological evidence as to their formation in nature.

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