Carbonate minerals solid solutions

The main alteration minerals surrounding Kuroko ore body are K-mica, K-feldspar, kaolinite, albite, chlorite, quartz, gypsum, anhydrite, and carbonates (dolomite, calcite, magnesite-siderite solid solution), hematite, pyrite and magnetite. Epidote is rarely found in the altered basalt (Shikazono et al., 1995). It contains higher amounts of ferrous iron (Fe203 content) than that from midoceanic ridges (Shikazono, 1984). 

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"). 

Carbonates. The carbonate minerals, in general, vary widely in composition because of the extensive solid solution of calcium, magnesium, iron, manganese, etc. that is possible within them. There is also a wide range of mineral compositions for the carbonate minerals in coals. The relatively pure end members, calcite (CaC03) and siderite (FeC03),... 

Also, calculate the holding time index. If the holding time is more than, say, 48 hours, expect potential problems with the dispersion and removal of suspended solids and sludges. Also expect potential problems with regard to the holding up of supersaturated minerals in solution, such as calcium carbonate. Again, some innovative solution may need to be found. 

The only compounds in Table 3.2 that do not form solid phases within our model are Mg(NOs)2 and Ca(NOs)2. On Earth, common nitrate salts such an NaNC>3 and KNO3 typically form in arid, alkaline environments. Under these environmental conditions, Mg and Ca concentrations are low because of the insolubility of their respective carbonate minerals. Mg(N03)2 and Ca(N03)2 Pitzer-equation parameters were added to the model to account for trace concentrations of Mg and Ca in such nitrate environments. It would be a serious misuse of the model to calculate solution properties in systems where Mg(NC>3)2 and Ca(N03)2 are present at high concentrations. 

A basic premise of solubility considerations is that a solution in contact with a solid can be in an equilibrium state with that solid so that no change occurs in the composition of solid or solution with time. It is possible from thermodynamics to predict what an equilibrium ion activity product should be for a given mineral for a set of specified conditions. As will be shown later in this chapter, however, it is not always possible to obtain a solution of the proper composition to produce the equilibrium conditions if other minerals of greater stability can form from the solution. It shall also be shown that while it is possible to calculate what mineral should form from a solution based on equilibrium thermodynamics, carbonate minerals usually behave in a manner inconsistent with such predictions. 

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... 

Solid solution theory The chemical theories of primary importance to understanding factors controlling carbonate mineral compositions in natural systems are associated with solid solutions. Carbonate minerals of less than pure composition can be viewed as mixtures of component minerals (e.g., SrCC>3 and CaSC>4 in CaCC>3). If the mixtures are of a simple mechanical type then the free energy of formation of the resulting solid will be directly proportional to the composition of the aggregate. Thus, for a two component, a and b, mixture ... 

Mackenzie F.T., Bischoff W.B., Bishop F.C., Loijens M., Schoonmaker J. and Wollast R. (1983) Magnesian calcites Low-temperature occurrence, solubility and solid-solution behavior, In Carbonates Mineralogy and Chemistry (ed. R.J. Reeder), pp. 97-144. Mineral. Soc. Amer., Chelsea, Mich. 

Mixed solid (or solid solution) formation can occur, for example, when secondary minerals, such as carbonates, metal oxides, or clay minerals, precipitate from the soil solution during weathering. These solids often are characterized by a wide range of isomorphic substitutions, in which both cations... 

Artificial pyrrhotite has also been obtained by passing hydrogen sulphide over heated ferrous chloride, air having been previously expelled by passage of carbon dioxide9 and by decomposing marcasite or iron pyrites with hydrogen sulphide at temperatures above 575° C.8 The reaction may be first detected at about this temperature and proceeds fairly rapidly at 665° C. The mineral is not a definite compound, but in all probability a solid solution of sulphur m iron sulphide.10... 

Heterogeneous exchange of radionuclides on carbonates has already been mentioned. Exchange on other sparingly soluble minerals (e.g. halides, sulfates, phosphates) may lead to rather selective separation of radionuclides. Following the exchange at the surface, ions may be incorporated into the solids in the course of recrystallization, which is a very slow but continuous process. Anomalous solid solutions with radioactive ions of different charges may also be formed. 

The numerator of the right side is the product of measured total concentrations of calcium and carbonate in the water—the ion concentration product (ICP). If n = 1 then the system is in equilibrium and should be stable. If O > 1, the waters are supersaturated, and the laws of thermodynamics would predict that the mineral should precipitate removing ions from solution until n returned to one. If O < 1, the waters are undersaturated and the solid CaCOa should dissolve until the solution concentrations increase to the point where 0=1. In practice it has been observed that CaCOa precipitation from supersaturated waters is rare probably because of the presence of the high concentrations of magnesium in seawater blocks nucleation sites on the surface of the mineral (e.g., Morse and Arvidson, 2002). Supersaturated conditions thus tend to persist. Dissolution of CaCOa, however, does occur when O < 1 and the rate is readily measurable in laboratory experiments and inferred from pore-water studies of marine sediments. Since calcium concentrations are nearly conservative in the ocean, varying by only a few percent, it is the apparent solubility product, and the carbonate ion concentration that largely determine the saturation state of the carbonate minerals. 

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