Calcite Supersaturation Research Articles

Precipitation of calcite from flashed geothermal waters in Iceland

Unflashed geothermal waters in Iceland are invariably just saturated with calcite. Upon flashing the water always becomes supersaturated, especially during the early stages of flashing. Thus flashing will lead to calcite precipitation. It is believed that the rate of precipitation bears a positive relation to the degree of supersaturation and the water temperature. Accordingly, precipitation will always be most pronounced at the beginning of flashing. The degree of calcite supersaturation, which is produced by flashing, only depends on the temperature and the ionic strength of the unflashed water. Strongest supersaturation is produced at low temperature and at a high ionic strength of the water. Calcite precipitation is known to be troublesome for the exploitation of many geothermal fields in the world. The study of the factors which control the magnitude of this precipitation is therefore of interest to those engaged in the exploitation of geothermal fluids.

The microbiological formation of carbonate concretions in the Upper Lias of NE England

Carbonate concretions from the Jet Rock (Upper Lias, Lower Jurassic) of NE England grew in uncompacted sediment, close to the sediment surface. Microbiological activity created isolated microenvironments in which dissolved carbonate and sulphide species were produced more rapidly than they could be dispersed by diffusion, so establishing the localised supersaturation of calcite and metastable iron sulphides. Precipitation of these minerals in the microenvironment formed a single concretion. Mass-balance calculations demonstrate that at least two different microbiological processes participated in concretionary growth. The early growth stages had an unidentifiable microbiological source of carbonate which declined in importance relative to sulphate reduction as growth proceeded. It is suggested that the diffusion of dissolved organic material was important in sustaining microbiological activity. Mineralogical zonations in the concretions result from changes in the chemistry of the microenvironment due to variations in the rates of addition/removal Ca 2+, Fe 2+, HCO − 3 and HS − by microbiological activity, the crystallization of authigenic minerals and diffusion between the microenvironment and surrounding pore waters. Such changes are of only local significance and the resulting mineralogical zonations in a concretion cannot be used to deduce successive stages of diagenesis in the whole sediment.

Geochemical controls on a calcite precipitating spring

A small spring fed stream was found to precipitate calcite by mainly inorganic processes and in a nonuniform manner. The spring water originated by rainwater falling in a 0.8 km2 basin, infiltrating, and dissolving calcite and dolomite followed by dissolution of gypsum or anhydrite. The Ca2+/Mg2+ indicates that calcite is probably precipitated in the subsurface from a supersaturated solution. This water emerges from the spring still about 5 times supersaturated with respect to calcite and continues calcite precipitation. When 10 times supersaturation is reached, due to CO2 degassing the precipitation is more rapid. The calcite accumulation from the stream with a flow of 5 l/s is calculated to be 12600 kg/yr with the highest rates in areas where CO2 degassing is the greatest. The non-equilibrium, as shown by the high calcite supersaturation, is also reflected in a variable partitioning pattern for Sr2+ between the water and calcite.