Geochemical interest

Biogeochemistry- Study of microbially mediated chemical transformations of geochemical interest, such as nitrogen or sulfur cycling. 

Friedman I, O Neil JR (1977) Compilation of stable isotope fractionation factors of geochemical interest. U S Geol Surv Prof Paper 440-KK... 

Langmuir, D., Techniques of estimating thermodynamic properties for some aqueous complexes of geochemical interest, in Chemical Modeling in Aqueous Systems Speciation, Sorption, Solubility and Kinetics, Jenne, E.A., Ed., ACS Symposium, American Chemical Society, Washington, DC, 1979, pp. 353-387. 

Unfortunately, phases of geochemical interest are not ideal. As well, aqueous species do not occur in a pure form, since their solubilities in water are limited, so a new choice for the standard state is required. For this reason, the chemical potentials of species in solution are expressed less directly (Stumm and Morgan, 1996, and Nordstrom and Munoz, 1994, e.g., give complete discussions), although the form of the ideal solution equation (Eqn. 3.4) is retained. 

In this chapter we consider the problem of the kinetics of the heterogeneous reactions by which minerals dissolve and precipitate. This topic has received a considerable amount of attention in geochemistry, primarily because of the slow rates at which many minerals react and the resulting tendency of waters, especially at low temperature, to be out of equilibrium with the minerals they contact. We first discuss how rate laws for heterogeneous reactions can be integrated into reaction models and then calculate some simple kinetic reaction paths. In Chapter 26, we explore a number of examples in which we apply heterogeneous kinetics to problems of geochemical interest. 

Direct measurement of the change in interfacial potential difference at the oxide-electrolyte interface with change in pH of solution can be measured with semiconductor or semiconductor-oxide electrodes. These measurements have shown d V g/d log a + approaching 59 mV for TiC (36, 37). These values are inconsistent with the highly sub-Nernstian values predicted from the models with small values of K. (Similar studies 138.391 have been performed with other oxides of geochemical interest. Oxides of aluminum have yielded a value of d t)>q/A log aH+ greater than 50 mV, while some oxides of silicon have yielded lower values.)... 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

Water of various degrees of purity is the normal heat transfer fluid employed and a number of important problems with modern boiler water circuits are markedly influenced by solution composition. Most problems arise where solutions can concentrate and the compositions of such solutions can only be obtained by calculation from thermodynamic data. This paper concentrates on the kind of aqueous phase data which are currently most needed. Many of the needs overlap with those of geochemical interest. However, since Barnes (3) has recently reviewed the latter field, specifically geochemical needs will not be discussed. "High temperature" in this paper is generally taken to mean within about 100°C of the critical point of water (374 C), though some important problems which occur at lower temperatures are also considered. 

Fortier SM, Cole DR, Wesolowski DJ, Riciputi LR, Paterson BA, Valley JW, Horita J (1995) Determination of the magnetite-water equilibrium oxygen isotope fractionation factor at 350°C a comparison of ion microprobe and laser fluorination techniques. Geochim Cosmochim Acta 59 3871-3875 Friedman I, O Neil JR (1977) Compilation of Stable Isotope Fractionation Factors of Geochemical Interest. US Geol Surv Prof Paper 440-KK... 

Franklin et al. 1986). None of these techniques has enjoyed long term success. Measurement of Li isotopes by mass spectrometry faces the primary problem of controlling mass fractionation from the emitter. Ironically, the very property that makes Li geochemically interesting makes quantifying its isotopic composition with precision extraordinarily challenging. For this reason, mass spectrometric measurements of Li must be compared directly to a standard material. As long as all laboratories make use of the same standard material, its isotopic composition is academic, as the measured isotopic composition of the standard drops out of the arithmetic of normalization. 

In more complex solutions of high ionic strengths with more than one electrolyte at significant concentrations, e.g., (Na, Mg, Ca " ) (Cl, SOl ), Pitzer s equation may be used to estimate the osmotic coefficient the necessary interaction coefficients are known for most systems of geochemical interest. 

To appreciate the predictive properties of Kieffer s model, it is sufficient to compare calculated and experimental entropy values for several phases of geochemical interest in table 3.1, which also lists entropy values obtained through apphcation of Debye s and Einstein s models. One advantage of Kieffer s model with respect to the two preceding formulations is its wider T range of applicability (Debye s model is appropriate to low frequencies and hence to low T, whereas Einstein s model is appropriate to high frequencies and hence to high T). 

Silica has 22 polymorphs, although only some of them are of geochemical interest—namely, the crystalline polymorphs quartz, tridymite, cristobahte, coesite, and stishovite (in their structural modifications of low and high T, usually designated, respectively, as a and jS forms) and the amorphous phases chalcedony and opal (hydrated amorphous silica). The crystalline polymorphs of silica are tectosilicates (dimensionality = 3). Table 5.68 reports their structural properties, after the synthesis of Smyth and Bish (1988). Note that the number of formula units per unit cell varies conspicuously from phase to phase. Also noteworthy is the high density of the stishovite polymorph. 

In theory, once the activity of an electrolyte in solution is known, the activity of the solvent can be determined by the Gibbs-Duhem integration (see section 2.11). In practice, the calculation is prohibitive, because of the chemical complexity of most aqueous solutions of geochemical interest. Semiempirical approximations are therefore preferred, such as that proposed by Helgeson (1969), consisting of a simulation of the properties of the H20-NaCl system up to a solute... 

Figure 9.2 shows how fugacity coefficient of H2, O2, and CO2 behaves with increasing P at various T (0 °C, 100 °C, 200 °C). Tables 9.1 and 9.2 list discrete values attained by F for H2O and CO2 gases under various P and T conditions. Values of F for gases of geochemical interest for which experimental data are... 

Table 9.3 Critical temperatures (TJ and critical pressures (P ) for gaseous species of geochemical interest (from Garrels and Christ, 1965).

Values of critical temperature and critical pressure for gaseous species of geochemical interest are listed in table 9.3. 

Table 9.5 Thermodynamic parameters for main gaseous species of geochemical interest.

Friedman I, O Neil JR (1977) Compilation of stable isotope fractionation factors of geochemical interest. In Data Geochem, 6th ed Geol Surv Prof Pap 440KK Friedman I, Scholz TG (1974) Isotopic composition of atmospheric hydrogen (1967-1969). J Geophys Res 79 785-788... 

The chief question of geochemical interest is how the radicals came to be in the macerals in the first place and why the change with rank is as found. Although it has been known for some years that whole coals contain appreciable concentrations of free radicals, this question appears not to have been discussed previously in any general way. Three possible answers suggest themselves. 

As opposed to ketones, the same low-temperature action of sulfur and ammonia on aldehydes received very little attention presumably because the results of the initial exploration discouraged utilization in synthetic chemistry. Again in reference to unpublished work, the product mixture was claimed to contain 3-thiazoline, formed in 10% yield (3). The low-temperature synthesis of aldehyde-generated thiazolines (2, Rj = R2 = H), as depicted in Figure 1, possibly may have some geochemical interest when viewed in the Tight of reports that such thiazolines are dehydrogenated to thiazoles by sulfur at 130 6C ( -10). 

There is a wealth of both theoretical treatment and empirical data for the phenomenon of solution, including empirical data for noble gas solution. As with adsorption, however, data for solution in geochemically important materials are sparse. A prominent exception, the only one, is water, extensive data for which are presented in Chapter 4. Most of the available data for other materials of principal geochemical interest are summarized in Table 2.3. 

Ore metals and their crystal guests may accumulate in soils and then move up the food chain. In organisms they will bind to the SH-groups of proteins (including enzymes) and thereby denature them. Thus they are toxic to organisms, and metals like Cd are well known for their pronounced toxicity. The trace constituents of the sulphide ore minerals are therefore of more than geochemical interest. 

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