Abstract: The occurrence and genesis of Ca(OH)₂ and Mg(OH)₂ deposits associated with hyperalkaline groundwater from partly serpentinized ultramafic rocks in Northern Oman is reported. Two types of Ca(OH)₂ deposit are differentiated; crystalline portlandite formed by the evaporation of Ca²⁺-2OH⁻ spring waters, and a form of amorphous gel, not previously known in nature. The latter is a product of present-day low-temperature serpentinization. Mg(OH)₂ (brucite) in both holocrystalline and gel forms is produced by mixing of Mg2+−2HCO3− surface waters with a Ca²⁺-2OH⁻ groundwater in both surface and groundwater environments. These results indicate that significant differences exist between the processes of medium- and low-temperature brucite generation.