Carbonate minerals coprecipitation

Carbonate minerals in natural systems precipitate in the presence of various other solutes This trace amounts of all components present in the solution may get incorporated into the solid carbonate minerals ("coprecipitation"). 

Coprecipitation is a partitioning process whereby toxic heavy metals precipitate from the aqueous phase even if the equilibrium solubility has not been exceeded. This process occurs when heavy metals are incorporated into the structure of silicon, aluminum, and iron oxides when these latter compounds precipitate out of solution. Iron hydroxide collects more toxic heavy metals (chromium, nickel, arsenic, selenium, cadmium, and thorium) during precipitation than aluminum hydroxide.38 Coprecipitation is considered to effectively remove trace amounts of lead and chromium from solution in injected wastes at New Johnsonville, Tennessee.39 Coprecipitation with carbonate minerals may be an important mechanism for dealing with cobalt, lead, zinc, and cadmium. 

Cadmium is found naturally deep in the subsurface in zinc, lead, and copper ores, in coal, shales, and other fossil fuels it also is released during volcanic activity. These deposits can serve as sources to ground and surface waters, especially when in contact with soft, acidic waters. Chloride, nitrate, and sulfate salts of cadmium are soluble, and sorption to soils is pH-dependent (increasing with alkalinity). Cadmium found in association with carbonate minerals, precipitated as stable solid compounds, or coprecipitated with hydrous iron oxides is less likely to be mobilized by resuspension of sediments or biological activity. Cadmium absorbed to mineral surfaces (e.g., clay) or organic materials is more easily bioaccumulated or released in a dissolved state when sediments are disturbed, such as during flooding. 

Chemisorption raises basic questions for the carbonate geochemist about the boundary between sorption and coprecipitation. If the adsorption reaction takes place in a solution that is also supersaturated with respect to the carbonate mineral substrate, then the adsorbed ions can be buried in the growing layers of the mineral and become coprecipitates. This mechanism can result in distribution coefficients that are dependent on growth rates. Also, when chemisorption is involved, an entirely new phase or a coprecipitate can form in the near-surface region of the carbonate (e.g., see Morse, 1986 Davis et al 1987). A classic example is apatite formation on calcite in dilute solutions (e.g., Stumm and Leckie, 1970). 

Investigations of the adsorption of inorganic ions on carbonate mineral surfaces have been carried out in a much less systematic manner than for many other mineral systems such as iron oxides and clays. The work has been largely confined to calcite, and in many studies the data were obtained in such a way that it is not clear whether adsorption or coprecipitation were being measured. Considering the number of major processes that are allegedly controlled by adsorption reactions, this is surprising. 

Coprecipitation Reactions and Solid Solutions of Carbonate Minerals... 

Natural carbonate minerals do not form from pure solutions where the only components are water, calcium, and the carbonic acid system species. Because of the general phenomenon known as coprecipitation, at least trace amounts of all components present in the solution from which a carbonate mineral forms can be incorporated into the solid. Natural carbonates contain such coprecipitates in concentrations ranging from trace (e.g., heavy metals), to minor (e.g., Sr), to major (e.g., Mg). When the concentration of the coprecipitate reaches major (>1%) concentrations, it can significantly alter the chemical properties of the carbonate mineral, such as its solubility. The most important example of this mineral property in marine sediments is the magnesian calcites, which commonly contain in excess of 12 mole % Mg. The fact that natural carbonate minerals contain coprecipitates whose concentrations reflect the composition of the solution and conditions, such as temperature, under which their formation took place, means that there is potentially a large amount of information which can be obtained from the study of carbonate mineral composition. This type of information allied with stable isotope ratio data, which are influenced by many of the same environmental factors, has become a major area of study in carbonate geochemistry. 

In this chapter we will examine the basic chemical concepts of coprecipitation and solid solutions, and the partition coefficients of different elements and compounds in major sedimentary carbonate minerals will be presented. A brief summary of information on oxygen and carbon isotope fractionation in carbonate minerals will also be presented. A major portion of this chapter is devoted to... 

Mucci and Morse (1989) have reviewed much of the research on coprecipitation reactions with calcite and aragonite, and the interested reader is referred to their paper for a detailed discussion of this literature. Here we present examples of the complex coprecipitation behavior of some of the most important ions in natural systems with carbonate minerals. The ions that we have selected are Mg2+, Sr2+, Na+, Mn2+, and SO42-. 

Sulfate The coprecipitation of relatively few anions with carbonate minerals has been studied and, with the exception of sulfate, these studies have generally not been as detailed as many of those with cations. However, coprecipitation reactions can be important for the removal of ions such as fluoride, borate, and phosphate from seawater (e.g., Morse and Cook, 1978 Okumura et al., 1983). It is also probable that anions will eventually gain a greater stature in the study of diagenesis... 

For models to be useful in studies of natural carbonate minerals, they must also be able to deal with the problem of multiple coprecipitates. The importance of this statement in terms of Sr2+, Na+ and SO42- concentrations has previously been discussed in this chapter. This problem is made particularly formidable by the observations of Angus et al. (1979). They measured the partition coefficients of several ions in calcite and aragonite using electron spin spectroscopy (GSR),... 

Methods need to be developed to measure and determine the influences of factors such as strain, crystal geometry, dislocation density, low concentrations of coprecipitates, etc. on subtle solubility and reactivity differences of carbonate minerals. 

The Fe and Mn that diffuse downward are subject to precipitation as carbonate and sulfide minerals in which the metals are present in reduced form. These minerals do not undergo any further chemical changes unless tectonic processes (uplift) cause them to come into contact with O2. As with the oxide phase, other metals tend to coprecipitate into the sulfide minerals, such as cadmium, silver, molybdenum, zinc, vanadium, copper, nickel, and uranium. 

Sedimentary rocks with the highest arsenic concentrations largely consist of materials that readily sorb or contain arsenic, such as organic matter, iron (oxy)(hydr)oxides, clay minerals, and sulfide compounds. Arsenian pyrite and arsenic-sorbing organic matter are especially common in coals and shales. Ironstones and iron formations are mainly composed of hematite and other iron (oxy)(hydr)oxides that readily sorb or coprecipitate arsenic. Iron compounds also occur as cements in some sandstones. Although almost any type of sedimentary rock could contain arsenic-rich minerals precipitated by subsurface fluids (Section 3.6.4), many sandstones and carbonates consist almost entirely of minerals that by themselves retain very little arsenic namely, quartz in sandstones and dolomite and calcite in limestones. 

With very few exceptions, surface and near surface waters contain an excess of Rn (Table I l-III) compared to Ra (Table 11-lV) and U (Table 11-VI). This Rn must come from Ra in solids such as rocks, soils and sediments. The solubility product of Ra salts is seldom reached in natural waters, because invariably it is adsorbed onto sulphates and carbonates at the surfaces of rocks and minerals. In the zone of oxidation it is also coprecipitated by hydrous oxides of Fe and Mn. Only in the vicinity of strong sources of very saline waters do Ra concentrations rise to 10 or even 10 g/L. 

Although formation of arsenic oxoanion minerals may be uncommon in the subsurface, many other minerals in which oxoanions are the fundamental structural unit (most commonly sulfates, phosphates, and carbonates) can assimilate trace amounts of arsenic via adsorption and coprecipitation processes. By virtue of their ubiquitous nature in sediments, these phases... 

Adsorption plays important role in the formation of groimd water composition. It conduces coprecipitation of some ions in the process of mineral formation. At that, according to the Fajam-Paneth rule adsorption way are coprecipitated those ions, which form poorly soluble salt with the oppositely charged ion of the precipitating mineral. The lower the solubility of a given salt, the more active is coprecipitation. That is why, for instance, along with the formation of the calcite, Zn and Pb well coprecipitate (their carbonates are poorly soluble) and MoO ", WO , PO " (their compounds with Ca are poorly soluble). 

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