Calcite coprecipitation

Morse JW, Bender ML (1990) Partition coefficients in calcite Examination of factors irrflnencing the validity of experimental resnlts and their application to natural systems. Chem Geol 82 265-277 Mucci A, Morse JW (1990) The chemistry of low temperature abiotic calcites Experimental studies on coprecipitation, stability and fractionation. Rev Aquatic Sci 3 217-254 Musgrove ML, Barmer JL, Mack LE, Combs DM, James EW, Cheng H, Edwards RL (2001) Geochronology of late Pleistocene to Holocene speleothems from central Texas Implications for regional paleoclimate. Geol Soc Am Bull 113 1532-1543... 

The origin of lead present in individual calcite particles could be ascribed by the LAMMA (laser microprobe mass analysis) technique. At low laser irradiances, the desorption mode, information is gathered on metallic species adsorbed on the surface of the particle. At high irradiances the particle is evaporated, revealing the components that coprecipitated with calcite111. 

Usually, however, the distribution coefficients determined experimentally are not equal to the ratios of the solubility product because the ratio of the activity coefficients of the constituents in the solid phase cannot be assumed to be equal to 1. Actually observed D values show that activity coefficients in the solid phase may differ markedly from 1. Let us consider, for example, the coprecipitation of MnC03 in calcite. Assuming that the ratio of the activity coefficients in the aqueous solution is close to unity, the equilibrium distribution may be formulated as (cf. Eq. A.6.11)... 

Bodine, M. W., H. D. Holland, and M. Borcsik (1965), "Coprecipitation of Manganese and Strontium with Calcite. Symposium Problems of Postmagmatic Ore Deposition", Prague2, 401-406. 

The coprecipitation of Pb2 and Ra2 + with calcite has attracted little attention due to the low concentrations of these elements in most natural waters (22) In contrast the partitioning of Sr2 + into calcite has been intensively investigated over the past two decades and several studies of Ba2 partitioning also have appeared (3, ... 

In summary, Pingitore and Eastman (31) presented a model of lattice and non-lattice incorporation of Sr2+ into calcite, the contribution of each mode of coprecipitation to the overall calculated value of kcSr depending on the specific conditions of the experimental run. 

The range of coprecipitation behavior discussed herein Impacts both experimental and practical studies of partitioning of trace elements Into calcite. Distinguishing lattice from non-lattice coprecipitation emerges as a primary concern. Experiments which explore a wide... 

The B soil horizon largely consists of clay minerals, iron (oxy)(hydr)oxides, and/or calcite. The C horizon, which is composed of sediments and weathered bedrock, usually occurs below the B. Arsenic in oxidizing soils readily sorbs and/or coprecipitates with iron and other (oxy)(hydr)oxides in B and C horizons (Reynolds, Naylor and Fendorf, 1999 Lund and Fobian, 1991). Below the C is the R horizon, which is unweathered bedrock. 

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. 

In the previous chapter, the fact that stoichiometric and apparent constants have been widely used in seawater systems was discussed. Berner (1976) reviewed the problems of measuring calcite solubility in seawater, and it is these problems, in part, that have led to the use of apparent constants for calcite and aragonite. The most difficult problem is that while the solubility of pure calcite is sought in experimental seawater solutions, extensive magnesium coprecipitation can occur producing a magnesian calcite. The magnesian calcite should have a solubility different from that of pure calcite. Thus, it is not possible to measure pure calcite solubility directly in seawater. 

The central question was, how major is the influence of coprecipitated magnesium Berner (1976) used a clever approach to find out. His reasoning was that whereas calcite has a major coprecipitate from seawater (Mg) capable of altering its solubility, aragonite that can be precipitated from seawater is greater than 99% pure, and its solubility in seawater should be measurable. He also pointed out that if the solubility ratio of calcite to aragonite was precisely known, it would... 

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. 

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. 

An additional complication has been demonstrated by Reeder and Grams (1987). They found that coprecipitation of Mg and Mn in calcites is different for different crystal faces. This results in the phenomenon of sector zoning. 

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-. 

The study of the coprecipitation of Mg2+ in calcite has been an active area of research, frequently marked by controversy over experimental results and their applicability to natural systems. The literature on this topic is prolific, and we will not attempt to review all of it (for a general review see Mackenzie et al 1983). The literature is divided into studies where direct measurement of the solids formed from solutions have been made, studies where properties of the solids have been inferred from their interactions with solutions (e.g., Schoonmaker et al 1982), and papers where authors have estimated values of the partition coefficient by deduction (e.g., Lahann and Seibert, 1982 Given and Wilkinson, 1985a). Here the discussion will be confined to the experimental studies where the compositions of the solids have been directly determined. 

Figure 3.2. The influence of temperature on Mg coprecipitation with calcite. A-Katz (1973), B-Fiichbauer and Hardie (1976), C-Oomori et al. (1987-calculated), D-Mucci (1987), and Burton and Walter (1987), E and F-Chilingar (1962).

An important aspect of the coprecipitation of Mg2+ with calcite clearly demonstrated by the studies of Mucci and Morse (1983) is the strong dependence of the partition coefficient on the mole % MgCC>3 in the calcite. The mole % MgCC>3 has also been observed to influence strongly the coprecipitation of Sr2+ and other coprecipitates. 

Experimental studies of the partitioning of sodium into aragonite, calcite, and dolomite have been carried out by White (1977, 1978), who concluded that the mode of substitution was as Na2CC>3 rather than as Na bicarbonate or hydroxide. The behavior of Na+ does not follow that normally described by a simple partition coefficient, but rather coprecipitation is better described by a Freundlich-type adsorption isotherm equation, implying that adsorption is important. 

---