The primary borate minerals in nonmarine bedded borate deposits - those of the playa type, and their deformed derivatives - are the high hydrates. Both field and laboratory data indicate that the high hydrates are the borate minerals that form stable phases at the lowest temperatures: under the surface conditions of playas. From a study of hand specimens of borate minerals from the new Turkish borate deposits, Meixner suggested that the borate minerals having the lowest specific gravity, and hence, highest water content, are the primary minerals. The role of temperature in the formation of the hydrous borates is shown by solid phases in the system Na ₂ B ₄ O ₇ -H ₂ O. As temperature increases, hydration of the solid phase decreases successively from borax (10H ₂ O) through tincalconite (5H ₂ O), kernite (4H ₂ O), to metakernite (2H ₂ O). This suggests that in the temp- erature ranges considered in the present paper - those of playas - the high hydrate (borax) is the one most likely to form as the primary mineral in nature. Studies of the Ca, Mg, and Ca-Na hydrous borate systems suggest that the same statement holds in these systems. Until recently, only borax and ulexite (Na ₂ O . 2CaO . 5B ₂ O ₃ . 16H ₂ O) have been known as primary minerals of playa deposits. However, inderite (2MgO . 3B ₂ O ₃ . 15H ₂ O) has recently been found as a primary mineral I 5i in Argentina and primary inyoite (2CaO . 3B ₂ O ₃ . 13H ₂ O) has been found in Peru. These 4 minerals are the high hydrates of their respective mineral series. None of the lower hydrates are known as primary minerals in nonmarine bedded deposits. On burial and deformation of the primary minerals, the changes in temperatures and pressures are almost certainly in such a direction as to encourage minerals of higher density and lower hydration to become stable phases. Field evidence bears this out; in all deformed deposits secondary minerals of low hydration are quantitatively important.