Tuffs in the Green River Formation (Eocene) of Wyoming have been altered by contact with the hypersaline brines of an ancient lake. Authigenic minerals formed in the tuffs include clay minerals, clinoptilolite, modernite, analcime, potassium feldspar, quartz and possibly albite. Si/Al ratios of analcime in the tuffs determined by X-ray diffraction range from 1.95 to 2.58. Authigenic potassium feldspar formed from analcime in the tuffs. Si/Al ratios of analcime associated with potassium feldspar range from 2.0 to 2.44 and statistical tests on these data suggest that there is no relationship between the Si/Al ratio of analcime and the presence or absence of potassium feldspar. The reaction mechanism for the formation of potassium feldspar from analcime did not cause desilication of the analcime. The major parameters of the brines controlling the formation of the authigenic silicate minerals appear to have been the activity of water, the pH, the Si/Al activity ratio, the Na+/K+ activity ratio, and the partial pressure of CO2.