Stability relations have been determined for glaucophane [oNa 2 Mg 3 Al 2 Si 8 O 22 (OH) 2 ] + excess vapor and for quartz + glaucophane + vapor using conventional hydrothermal techniques. The high-temperature stability limit of this amphibole ranges from 850 degrees C. at 175 bars vapor (= total) pressure to 868 degrees C. at 2000 bars P vapor . Neither differential stress nor high pressures are necessary for the formation of glaucophane. The presence of excess silica lowers its high-temperature stability limit only 3 degrees -6 degrees C. Unusually large enthalpy values for the reactions glaucophane --> forsterite + enstatite + albite + vapor, and quartz + glaucophane --> enstatite + albite + vapor (330 + or - 60 and 320 + or - 60 kcal/mol respectively) may be explained only in part by the change in coordination of aluminum from 6 in glaucophane to 4 in albite. The entropy of glaucophane at 864 degrees C. and 1000 bars vapor pressure is 150 + or - 50 cal/deg/mol. Optical properties of synthetic glaucophane agree well with data for natural specimens. Unit cell dimensions of the synthetic material are slightly larger than those of natural glaucophanes. The experimental investigation indicates that glaucophane is stable over a wide range of physical conditions given appropriate chemical conditions. Bulk compositions rich in soda and magnesia and poor in lime relative to alumina should favor production of glaucophane. The rare occurrence of such chemical environments severely restricts the crystallization of glaucophane in nature.