Carbonate minerals chemistry

Despite the vast amount of work on 14C dating which has already been accomplished and despite the fact that it is the best developed method available today, numerous difficulties still exist with its application. First, carbonate geochemistry which helped control 14C concentrations in the past is not simple to reconstruct. Carbonate minerals are commonly in a state of near equilibrium with groundwater, and only slight changes in water temperature or chemistry will promote either dissolution or precipitation of carbonate ions. In this way, the proportion of modern carbon in the water can be changed and some isotope... 

To model the effect of carbonate minerals on drainage chemistry, we continue... 

Dolomite is one of the most abundant sedimentary carbonate minerals but its mode of formation and its surface properties are less well known than for most other carbonate minerals. As we have mentioned, the nucleation of dolomites and its structural ordering is extremely hindered. There is a general trend for the "ideality" of dolomite to increase with the age of dolomite over geological time (Morse and Mackenzie, 1990). Most dolomites that are currently forming in surfacial sediments and that have been synthesized in the laboratory are calcium-rich and far from perfectly ordered. Such dolomites are commonly referred to as "protodolomites . Morse and Mackenzie (1990) have reviewed extensively the geochemistry (including the surface chemistry of dolomites and Mg-calcites. 

The most important feature of mineral chemistry in carbonates of Chehelkureh deposit is the large range in the FeCOsiMgCOs ratio which can indicate large variations in chemistry of hydrothermal fluids. Compositions extend along the magnesite-siderite join, from 22 to 72 mole% FeCOs (Fig. 3). Dolomite and ankerite compositions extend between 0 to 27 mole% FeCOs. 

Mineral chemistry of carbonates of mineralized veins in the Chehelkureh ore deposit show that at least some of these carbonates were deposited from hydrothermal fluids of igneous origin, which is consistent with carbon isotopes results (Maanijou 2007). 

A key question is then the relationship between the isotopic composition of the C02(g) in the soil and the CO2 dissolved in the mineral species growing in the soils. The equilibrium carbon isotope fractionations between C02(g) and carbonate minerals (CaC03 (both calcite and aragonite polymorphs), MgC03 (magnesite)) are known experimentally and can be accurately calculated from quantum chemistry (75). Overall, carbon in carbonate minerals is isotopically heavier than carbon in C02(g) by about... 

Meteorites are generally divided into three broad groups according to their chemistry and mineralogy that is, stones, stony irons, and irons (Chapman, 1999, 353 Dalrymple, 1991, 264). As the name implies, stony irons are meteorites with intermediate compositions between irons and stones (Dalrymple, 1991, 264). Stones mostly consist of carbonate minerals, magnesium- and iron-rich silicates, and/or other nonmetallic... 

Only a few of the reactions summarized in Table 3.3 are actually based on data at subzero temperatures. In most cases, the lower temperature for data is 0°C. This could potentially be a serious limitation for the FREZCHEM model. For example, quantifying carbonate chemistry requires specification of Ah,co2 -ftcb - 2 and Kw all of these reactions are only quantified for temperatures > 0 °C (Table 3.3). Figure 3.9 demonstrates how six of the most important relationships of Table 3.3 extrapolate to subzero temperatures. We were able, based on these extrapolations, to quantify the solubility product of nahcolite (NaHCOa) and natron (Na2CO3 10H2O) to temperatures as low as — 22°C (251 K) (Marion 2001). Even for highly soluble bicarbonate and carbonate minerals such as nahcolite and natron, their solubilities decrease rapidly with temperature (Marion 2001). For example, for a hypothetical saline, alkaline brine that initially was 4.5 m alkalinity at 25 °C, the final alkalinity at the eutectic at —23.6°C was 0.3m (Marion 2001). At least for carbonate systems it is not necessary to extrapolate much beyond about —25 °C to quantify this chemistry, which we believe can reasonably be done using existing equation extrapolations (Fig. 3.9). 

In this chapter, we introduced the reader to some basic principles of solution chemistry with emphasis on the C02-carbonate acid system. An array of equations necessary for making calculations in this system was developed, which emphasized the relationships between concentrations and activity and the bridging concept of activity coefficients. Because most carbonate sediments and rocks are initially deposited in the marine environment and are bathed by seawater or modified seawater solutions for some or much of their history, the carbonic acid system in seawater was discussed in more detail. An example calculation for seawater saturation state was provided to illustrate how such calculations are made, and to prepare the reader, in particular, for material in Chapter 4. We now investigate the relationships between solutions and sedimentary carbonate minerals in Chapters 2 and 3. 

In natural systems, carbonates react with a variety of solutions at different pressures and temperatures. The processes involved in these reactions are complex, but depend significantly on the solubilities of the carbonate minerals, their surface chemistries, and dissolution and precipitation kinetics. In this chapter, we have... 

Most studies of carbonate chemistry in deep sea sediment pore waters have, therefore, focused on the the apparent solubility behavior of carbonates in these sediments. The basic hypothesis is that because of the relatively fast reaction kinetics of carbonate minerals and generally low sedimentation rates in deep sea... 

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (e.g., Berner, 1971). Although the many reactions involved are complex, this process can be schematically represented as ... 

In spite of these major limitations, considerable progress has been made in understanding many of the important factors that influence subsurface water chemistry. Na+, Ca2+ and Cl- account for the major portion of dissolved components in most brines (Figure 8.6). Ca2+, which can comprise up to 40% of the cations, usually increases relative to Na+ with depth (Figure 8.7). Br and organic acids are commonly found at concentrations of 1 to 2 g L1 (Land, 1987). The bicarbonate concentration is largely limited by carbonate mineral solubility, and sulfate is generally found in low concentrations as a result of bacterial and thermal reduction processes. 

We will discuss the chemistry of subsurface waters further in subsequent sections of this chapter as it relates to specific carbonate mineral diagenetic processes such as secondary porosity formation. 

Morse J.W. (1986) The surface chemistry of carbonate minerals in natural waters An overview. Mar. Chem. 20, 91-112. 

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