gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1984.048.347.07 Calcium and Magnesium Hydroxide Precipitation from Alkaline Groundwaters in Oman, and Their Significance to the Process of Serpentinization Neal Colin Institute of Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, Oxon. OX10 8BB Stanger Gordon Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA 6 1984 48 347 237 241 10 6 1983 7 10 1983 Copyright © 1984, The Mineralogical Society 1984 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.