Reactions secondary mineral formation

All of the Type A and B inclusions studied are surrounded by a layered rim sequence of complex mineralogy [21] which clearly defines the inclusion-matrix boundary. Secondary alteration phases (grossular and nepheline, especially) are also a common feature of these inclusions, suggesting that vapor phase reactions with a relatively cool nebula occurred after formation of inclusions. Anorthite, in particular, is usually one of the most heavily altered phases the relationship between Mg isotopic composition and alteration is discussed below. (See [12] for striking cathodoluminesce photographs of typical Allende alteration mineralogy.) Inclusion Al 3510 does not fit the normal pattern as it has no Wark-rim and does not contain the usual array of secondary minerals. 

Table I. tmporiuni geochemical reactions resulting in the formation of secondary minerals in CCB wastes...

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). 

Nonlinear Precipitation of Secondary Minerals from Solution. Most of the studies on dissolution of feldspars, pyroxenes, and amphiboles have employed batch techniques. In these systems the concentration of reaction products increases during an experiment. This can cause formation of secondary aluminosilicate precipitates and affect the stoichiometry of the reaction. A buildup of reaction products alters the ion activity product (IAP) of the solution vis-a-vis the parent material (Holdren and Speyer, 1986). It is not clear how secondary precipitates affect dissolution rates however, they should depress the rate (Aagaard and Helgeson, 1982) and could cause parabolic kinetics. Holdren and Speyer (1986) used a stirred-flow technique to prevent buildup of reaction products. 

The primary focus of research on secondary porosity formation has been on mechanisms for generating undersaturated formation waters. Because reactions that may result in undersaturation of waters with respect to carbonate minerals by consumption of calcium are unlikely to be quantitatively important, emphasis has been placed on reactions that may lower the carbonate ion concentration. Although not clearly documented in deep subsurface environments, mixing of waters of dissimilar composition can result in undersaturation with respect to calcite (see Chapter 7), and lead to secondary porosity formation. Acidic waters associated with igneous intrusions and thermal metamorphism can also cause carbonate dissolution that results in secondary porosity (e.g., deep Jurassic carbonates in Mississippi, U.S.A. Parker, 1974). 

There are approximately 3000 different minerals found in rock formations however, much of the Earth s crust consists of only 50 dominant mineral forms (Degens, 1989). The primary and secondary minerals commonly found in soils are listed in table 6.1. As mentioned above, chemical weathering processes in soils are important in the transformation of primary to secondary minerals. For example, when examining the weathering of feldspars (this most abundant group of minerals in the Earth s crust), K-feldspar is transformed into kaolinite through the following reaction ... 

At lower temperatures, as discussed in Chapter 5.16, congruent and incongruent weathering reactions and neoformation of clays and zeolite minerals, which are common secondary mineral phases in crystalline rocks, are postulated to alter fluid and isotopic chemistry (Fritz and Frape, 1982) and increase salinity during hydration reactions (Bucher and Stober, 2000). As an example, reaction (5) represents the formation of zeolites. These well-documented fracture-filling minerals have been found by several authors at Fennoscandian sites, and at research sites in Europe (Bucher and Stober, 2000) ... 

The formation of secondary minerals in soils generally results from the combination and addition of ions and molecules from the soil solution to the solid phase. This mechanism was originally given little consideration, because aluminium and silicon in solution did not appear to combine during laboratory experiments. Only relatively recently has the slow kinetics of such reactions been appreciated. Experiments that take slow reactivity into account and provide nucleation centers for crystal formation have shown that secondary minerals can precipitate from solutions containing the proper constituent ions and Si(OH)4. 

In this reaction, K and Si enter the solution, while protons are eonsumed. The solubility of feldspar ineieases when proton activity increases or the removal of K or Si is fast, e.g. by seeondary mineral formation. Secondary kaolinite may dissolve to form gibbsite, a reaction that does not neutralize aeid ... 

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