Carbonate rocks precipitation

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

Pressure management, where fluid is injected into oil fields in order to maintain adequate fluid pressure in reservoir rocks. Calcium carbonate may precipitate as mineral scale, for example, if pressure is allowed to deteriorate, especially in fields where formation fluids are rich in Ca++ and HCO3 and CO2 fugacity is high. 

Another type of economically important Fe ore occurs in so-called shams. Skarn is an old Swedish name for a gangue formation from the Archean age produced by me-tasomatic replacement of carbonate rocks by solutions. If rich in Fe, these solutions led to the precipitation of Fe ores containing hematite and magnetite as the main Fe oxides. 

The interplay of factors leading to the formation of carbonate rocks is extremely complicated, but several avenues of investigation have yielded fruitful results. These approaches include studies of surface energies, isotopic composition and radiochemistry (including the use of carbon-14 to study the rates of carbonate deposition), solubilities under ambient conditions, and the effect of magnesium-calcium ratios in the solutions from which carbonate minerals are precipitated. 

Let us now consider the problem from the standpoint of calcite precipitation kinetics. At saturation states encountered in most natural waters, the calcite reaction rate is controlled by surface reaction kinetics, not diffusion. In a relatively chemically pure system the rate of precipitation can be approximated by a third order reaction with respect to disequilibrium [( 2-l)3, see Chapter 2]. This high order means that the change in reaction rate is not simply proportional to the extent of disequilibrium. For example, if a water is initially in equilibrium with aragonite ( 2c=1.5) when it enters a rock body, and is close to equilibrium with respect to calcite ( 2C = 1.01), when it exits, the difference in precipitation rates between the two points will be over a factor of 100,000 The extent of cement or porosity formation across the length of the carbonate rock body will directly reflect these... 

Because sedimentary carbonates represent primarily chemical and biochemical precipitates from seawater, and because they make up 20% of the common sedimentary rock record, these rock types have been particularly good sources of chemical and mineralogical data for interpretation of the secular and cyclic evolution of the Earth s surface environment. This carbonate rock record as a function of geological age is now explored as are age trends in other rock types and sediment properties. With this information as background material, we can then discuss what these relationships tell us about the history of carbonates and the exogenic system throughout geologic dme. 

Chert is another organic marine sediment, less common than carbonate rocks, but found in huge deposits in some parts of the world. It initially consists of the skeletons of billions of tiny, single-celled animals called radiolaria. These skeletons are composed of microcrystalline quartz or chalcedony (Si02). Dense layers of this material accumulate on the ocean floor, where they are buried and compressed over time. The term chert is sometimes also applied to any compact, very fine-grained siliceous sediment that has resulted from precipitation or consolidation of silica gel. There may be chert lenses or very thin layers within other types of sediments, such as limestone. 

Chemical bonding as a means of solidihcation is very widely observed in nature. Formation of sedimentary rocks, such as carbonate rocks, is an excellent example. Carbonate rocks are formed by the reaction of calcium oxide with the carbon dioxide from the sea water [14]. Sea organisms also use this process and construct seashells. The organisms that flourish in calcium-saturated solutions of sea water change the alkalinity of the solutions slightly and precipitate calcium carbonate, which is used to form protective shelters such as shells and conches. 

There are two common stratigraphic occurrences of chert as bedded cherts associated with shales or iron formations and as nodules in carbonate rocks (Blatt et al., 1980). The bedded cherts are predominant in Precambrian time, reaching a maximum extent 2-3 Ga, when they represented as much as 15% of the sedimentary record. The Precambrian bedded cherts contain microspheres of quartz, suggesting that they may have precipitated inorganically. Commonly, bedded cherts are associated with ophiolite sequences, which may have hydrothermal or metasomatic sources of silicate. The co-occurrence of bedded cherts and shales (typically dark in color) suggests that many cherts form in a hemi-pelagic or deep-sea, open-ocean setting, far from sources of coarse clastic material. In... 

Chert is defined here, following Folk (1980, p. 79), as a chemically precipitated sedimentary rock, essentially monomineralic and composed chiefly of microcrystalline and/or chalcedonic quartz, with subordinate megaquartz and minor amounts of impurities. Chalcedony, which consists of sheath-like bundles of thin, radiating fibers of SiOi, is rare in most Precambrian chert. Precambrian chert occurs as distinct beds or lenses, particularly in Archean rocks, as nodules or silicified laminae in carbonate rocks, or as a sihceous end-member, as granules or cements in iron formation. 

The overall cycling rates of carbon have always depended upon geothermal forces that cause volcanic CO2 exhalation and carbonate uplift, independently of life processes. The hydrosphere s carbonate buffer system kept the oceans at near saturation, with respect to CaC03, at all times (Holland, 1972). Rates of deposition and accumulation of carbonate rocks per unit of geological time have also been within the range of fluctuation observed for the Phanerozoic (Garrels et al., 1976). The form of carbonate deposits, however, was certainly different in the Precambrian from those of today, i.e. it was a predominantly chemical precipitate, rather than biogenic skeletal carbonate (Monty, 1973). 

Anderson, G. M., 1983, Some geochemical aspects of sulfide precipitation in carbonate rocks in International conference on Mississippi Valley Type lead-zinc deposits, Rolla,... 

Beachrock is a friable to well-cemented sedimentary rock that results from rapid lithification of sand and/or gravel by calcium carbonate cement precipitation in the intertidal zone. It occurs predominantly on tropical ocean coasts, but is also found in temperate realms that extend up to 60° latitude. In contrast to the implications of the name, beachrock precipitation phenomena are not restricted to beaches but also occur on reef ridges, tidal flats and in tidal channels. Intertidal beachrock may be confused with other sediments lithified in the intertidal and subtidal zones, such as hardened crusts or certain reef limestones. 

The soluble form of calcium can be precipitated in the marine environment to form rock by some physical conditions such as warming of the water (carbon dioxide is less soluble in warm water than in cold water and thus calcium carbonate is precipitated), by the use of carbon dioxide by marine plants, or by alterations in the pH of water by ammonia-producing bacteria which also lowers the solubility of calcium carbonate. However, the majority of calcium carbonate deposits are formed from skeletal fragments of organisms living in the marine environment. Some of these organisms inhabit reefs but the majority float free in water. Figure 2.13 shows various shapes of shells formed by Coccolithophorides which can be spherical coccospheres some, such as dicoaster, are star shaped. 

In the Floridan aquifer, traces of gypsum are present in the carbonate rock. The high calcium concentration from gypsum dissolution exceeds its value at saturation with calcite, leading to precipitation of the carbonate and the production of additional CO2 (reaction 8). Concurrently, anaerobic decay of buried organic matter (reaction 3) and sulfate reduction (reaction 6) take place. The combination of these processes has caused an increase in the CO2 pressure of the groundwater from 10" bar in the recharge zone to 10" bar downdip as the pH decreases from 8.0 to 7.4 over a map distance of 115 km (Fig. 5.3) (cf. Back and Hanshaw 1970 Plummer et al. 1983). 

Figure 6.11 Stability relations among the calcium-magnesium carbonates at 25 C and 1 bar total pressure as a function of the mCa VmMg ratio and CO2 pressure. The stippled field is that of ordered, well-crystallized dolomite. The plot shows that when dolomite is kinetically inhibited from precipitating, nesquehonite, hydromagnesite, or brucite can coexist with calcite under metastable conditions. Huntite (not shown) is stable relative to crystalline dolomite for mCa VmMg < 10 . From D. Langmuir, Physical and chemical characteristics of carbonate water. In Guide to the hydrology of carbonate rocks, P. E. La-moreaux, B. M. Wilson, and B. A. Memeon, eds. 1984 by UNESCO. Used by permission.

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