Secondary minerals predicted

Figure 13. Plot of Li isotopic composition vs. inverse concentration for sea floor altered (weathered) mid-Atlantic ridge basalts (Chan et al. 1992). Solid line is the regression of the data (R = 0.927), the dashed line shows the predicted relation of a pure mixture of seawater with unaltered basalt (Teng et al. 2004), underscoring that the altered basalts incorporated Li into secondary mineral with a fractionation factor a -0.981.

Although a number of secondary minerals have been predicted to form in weathered CCB materials, few have been positively identified by physical characterization methods. Secondary phases in CCB materials may be difficult or impossible to characterize due to their low abundance and small particle size. Conventional mineral identification methods such as X-ray diffraction (XRD) analysis fail to identify secondary phases that are less than 1-5% by weight of the CCB or are X-ray amorphous. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), coupled with energy dispersive spectroscopy (EDS), can often identify phases not seen by XRD. Additional analytical methods used to characterize trace secondary phases include infrared (IR) spectroscopy, electron microprobe (EMP) analysis, differential thermal analysis (DTA), and various synchrotron radiation techniques (e.g., micro-XRD, X-ray absorption near-eidge spectroscopy [XANES], X-ray absorption fine-structure [XAFSJ). 

Table 2. Secondary minerals obsened tmd/nr predicted In form in CCU materials exposed to chemical weathering...

Predicted secondary minerals. As discussed above, direct evidence for the formation of secondary trace element phases in weathered ash is rare. Most evidence suggesting precipitation of trace element phases by weathering processes is therefore indirect. For example, geochemical modelling of pore fluids in CCB disposal facilities and ash leachate solutions indicates that aqueous trace element concentrations are controlled by a variety of secondary phases. Table 2 lists numerous secondary trace element phases predicted, but not yet observed, to form in weathering CCB systems. 

The use of chemical modelling to predict the formation of secondary phases and the mobility of trace elements in the CCB disposal environment requires detailed knowledge of the primary and secondary phases present in CCBs, thermodynamic and kinetic data for these phases, and the incorporation of possible adsorp-tion/desorption reactions into the model. As noted above, secondary minerals are typically difficult to identify due to their low abundance in weathered CCB materials. In many cases, appropriate thermochemical, adsorption/desorp-tion and kinetic data are lacking to quantitatively describe the processes that potentially affect the leaching behaviour of CCBs. This is particularly tme for the trace elements. Laboratory leaching studies vary in the experimental conditions used (e.g., the type and concentration of the extractant solution, the L/S ratio, and other parameters such as temperature and duration/ intensity of agitation), and therefore may not adequately simulate the weathering environment (Rai et al. 1988 Eary et al. 1990 Spears Lee, 2004). 

Secondary phases predicted by thermochemical models may not form in weathered ash materials due to kinetic constraints or non-equilibrium conditions. It is therefore incorrect to assume that equilibrium concentrations of elements predicted by geochemical models always represent maximum leachate concentrations that will be generated from the wastes, as stated by Rai et al. (1987a, b 1988) and often repeated by other authors. In weathering systems, kinetic constraints commonly prevent the precipitation of the most stable solid phase for many elements, leading to increasing concentrations of these elements in natural solutions and precipitation of metastable amorphous phases. Over time, the metastable phases convert to thermodynamically stable phases by a process explained by the Guy-Lussac-Ostwald (GLO) step rule, also known as Ostwald ripening (Steefel Van Cappellen 1990). The importance of time (i.e., kinetics) is often overlooked due to a lack of kinetic data for mineral dissolution/... 

The most common elements in the earth s crust (after oxygen) are Si and Al, Avith valences of +4 and +3, respectively. The radius ratios of Si and Al coordinated to are 0.29 and 0.36, respectively. Thus, Si can reside only in tetrahedral holes. The ion is also predicted to occupy tetrahedral holes, and does so in many primary minerals, but the radius ratio is sufficiently close to the limiting value of octahedral coordination that 6-coordination of AF" is common. In fact, under the low temperature and pressure conditions that prevail during secondary mineral formation, 6-coordination of aluminum is more common than 4-coordination. Tetrahe-... 

These results show how geochemical modeling can be used with mineralogical studies to evaluate chemical reactions occurring in tailings pore water and subjacent soils. The identification of gypsum, jarosite, and alunite support the predictions made by PHREEQE that these secondary minerals precipitated from tailings pore water. 

The seepage water concentrations that result from the model calculation are a worst-case prediction, as secondary mineral reactions are not included in DifiMod . Neglecting buffer reactions does not introduce a great error in this case, because the acid neutralizing capacity of the overburden material is very low. 

Amino acids, cell wall components (other than crude fibre) and secondary minerals are quite frequently measured, but less so than the previous parameters. There are many variations in the analytical methods used for these parameters. However, it was usually possible to establish equations (applicable within a group of feed materials) to predict the final values of amino acids and cell wall components. 

The distances, from the column inlet, at which the dissolution of the primary minerals and precipitation of secondary minerals occur should give a clear indication of the differences between the local equilibrium and kinetic assumptions. If the distances observed in the experiments are close to the distances predicted assuming equilibrium and the fronts relatively sharp, then the rate of reaction of the minerals must be relatively fast. If fronts that are more diffuse were observed in the experiments, this would suggest that the rates of reaction of the minerals are slow. 

While prediction of the exact secondary mineral phase may not be crucial, the models should be capable of predicting the broad types of mineral (e.g. zeolites or CSH phases). In this context, minerals of the same type will have broadly similar properties in terms of sorption behaviour and the tendency to block or increase porosity. The models will have been considered to perform acceptably if the minerals predicted are of the same type as those observed. 

Fig. 4. Predicted profiles of secondary minerals at time intervals of 1 x lO s ( =s 11.5 days) during the simulated reaction of quartz with the evolved fluid. (PRECIP modelling).

In the predictions for the reaction of calcite with both young and evolved fluids, a small amount of calcite dissolution was predicted at the inlet end of the column. In the evolved fluid case, this was associated with a porosity increase where no secondary mineral precipitation was... 

Generally in the PRECIP predictions, porosity was reduced over the whole of the reaction zone but with an increase of porosity associated with the dissolution of the primary minerals. CHEQMATE generally gave similar predictions but with larger increases in porosity due to the prediction of the total dissolution of the primary minerals in some cases. The experimental observations showed a reduction in porosity close to the inlet of the columns whilst close to the outlet of the column an increase in porosity was observed. This was due to the dissolution of primary minerals coupled with little secondary mineral precipitation. In general, the predicted porosity variations obtained with the PRECIP model are closer to the observations. The CHEQMATE model predicts the total dissolution of the primary minerals. This results in large porosity increases, which were not observed in the experiments. 

Govett, G.J.S. 1976. Detection of deeply buried and blind sulphide deposits by measurement of H+ and conductivity of closely spaced surface soil samples. Journal of Geochemical Exploration, 6, 359-382. Hall, G.E.M., Hamilton, S.M., McClenaghan, M.B., Cameron, E.M. 2004. Secondary geochemical signatures in glaciated terrain. Extended Abstracts, SEG 2004 - Predictive Mineral Discovery Under Cover Symposium, no. 33, University of Western Australia, Perth. 

Kelley, D.L., Cameron, E.M., Southam, G. Secondary geochemical dispersion through transported overburden. SEG 2004 abstracts Predictive Mineral Discovery Under Cover", September 27, 2004 Perth, Western... 

Oxidative coupling of CH4/CD4 mixtures over natural Mn mineral catalysts was found to be comparable with that carried out over a synthetic Mn oxide catalyst547. Theoretical predictions of secondary D IEs for ring opening in the reactions cis, cis, c -l,3,5-cyclooctatriene has been made using ab initio MO theory540. A secondary DIE of 1.3 at 99.5 °C has been observed in the extrusion of ethylene-D4 from rhenium(V)... 

Thus, surface effects and adsorption equilibria can dramatically influence the relative reactivity of photoelectrochemical transformations. Not only does the surface effectively control the movement of reagents from the electrolyte to the photo-activated surface and the desorption of products (avoiding overreaction or complete mineralization), but it also influences the stability and accessibility of photogenerated intermediates toward secondary intermolecular reactions [87]. Because the efficiency of diffusion and mass transfer to and from the photocatalyst surface depends on the solvent and catalyst pretreatment, quantitative predictions of photocatalytic reactions have proved to be difficult, although the qualitative principles governing each step of these events can be easily recognized. 

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