Calcium and magnesium hydroxide precipitation from alkaline groundwaters in Oman, and their significance to the process of serpentinization

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 Ca 2 + 2 O H 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 Mg 2+ 2HCO~ surface waters with a Ca2+-2OH groundwater in both surface and groundwater environments. These results indicate that significant differences exist between the processes of mediumand lowtemperature brucite generation. BRUCIXE, Mg(OH)2, is a common but minor component of many alpine ultramafic environments and has generally been regarded as a product of forsterite (Mg2SiO4) hydration (Bowen and Tuttle, 1949; Yoder, 1952; Hostetler et al., 1966). Though difficult to differentiate optically, it occurs typically in the form of fibrous or microcrystalline intergrowths with serpentine, and frequently exhibits petrofabric evidence of early, i.e. synteetonic genesis (e.g. Hostetler et al., 1966). More recently Barnes et al. (1967, 1972, 1978), Barnes and O'Neil (1969), Luce (1971), and Nesbitt and Bricker (1978) have all drawn attention to different aspects of a second, low-temperature phase of serpentinization and brucite generation. The evidence for this is based upon the widespread occurrence of circulating groundwater of meteoric origin which emerges from partly serpentinized ultramafic rocks in the form of highly alkaline Ca 2 + 2 O H springs. Since the emergent spring waters are over-saturated with respect to both serpentine and brucite, the latter t~ Copyright the Mineralogical Society has been assumed to precipitate in amorphous or microcrystalline form following serpentinization reactions. Calcium hydroxide is extremely rare, having been reported from only one ultramafic terrain (Derkovic, 1973), then without any detail; it is, however, known as a rare component of other environments (Tilley, 1933; Minguzzi, 1937; Carobbi, 1940; Hentschel, 1961; Bentor et al., 1963; Tuttle and Gittins, 1966; Gross et al., 1967; Nesbitt and Kelly, 1977; Saines et al., 1980; Barnes et al., 1982). Here a report is given on the new occurrence of magnesium and calcium hydroxide phases and their significance to low-temperature serpentinization reactions from the Northern Oman ultramafic area. One of the calcium hydroxide phases, an amorphous form, has not previously been reported in nature.