Formation mechanisms primary minerals

These two distinct processes lead to the formation of secondary minerals mainly phyl-losilicates such as clays, of soluble products (e.g., carbonates or silica) lixiviated by percolating waters and of colloids usually iron and aluminum sesquioxides complexed by humic acids. While physical degradation involves mechanical (e.g., abrasion, impact) or thermal (e.g., thermal shock) processes, alteration involves only chemical reactions such as hydrolysis influenced by pH conditions and/or the oxidation of primary materials depending on the Eh (redox potential) conditions. Whatever the type of underlying rock, the end product is always a clay except when silica is totally absent from the bedrock, the composition of the clay depending on the type of climate and the time over which the evolution process takes place. These conditions are summarized in Table 14.1. 

The Koongarra U deposit in the Northern Territory of Australia has been studied to evaluate the processes and mechanisms involved in the geochemical alteration of the primary ore zone, and to model the formation of the secondary U ore zone and dispersion fan (Duerden 1991 Duerden Airey 1994). Studies of the distribution of the U in the dispersion fan (Murakami et al. 1991) have provided data on the fixation of U leached from the primary ore deposit. Their work has shown that, for this system, fractures are not only preferential pathways for ground-water movement but also contain secondary minerals with high sorption capacity for elements such as U. Even in the monsoonal climate, in which this deposit is located, a significant proportion of the uranium has not been released from the vicinity of the primary ore body. 

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 ... 

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). 

The most noticeable isotopic difference between saline waters from crystalline rocks and sedimentary formation waters is their position above the meteoric waterline. This is postulated to be due to mineral hydration reactions in a very water-depleted environment (Fritz and Frape, 1982). Several recent smdies have suggested that hydration reactions in low water to rock environments can occur and result in increasing salinity. The incorporation of OH into primary silicate such as amphiboles and phyUosUicates (where OH crystal lattice sites are part of the mineral structure) is suggested as one mechanism for controlling solute concentration (KuUemd, 2000). The formation of secondary OH containing mineral phases such as zeolites and clays can also continue to consume water molecules and concentrate the residual fluids both chemically... 

Phillipsite is the most abundant zeolite in the surface sediments of the Pacific (Boles, 1977 Kastner and Stonecipher, 1978). Although it may be locally abundant (>50 wt.% on a carbonate-free basis, Bonatti (1963)), its etched surface, and the pattern of its decreasing abundance with the burial depth, suggest that it is a metastable phase under deep-sea conditions (Kastner, 1979). The primary mechanism of formation is thought to be alteration of basaltic glass, but it may also form by reaction of biogenic silica and dissolved AP+ (Arrhenius, 1963). Phillipsite is commonly found in association with authigenic smectite, and the combined formation of the two minerals may be represented as... 

The primary mechanism for the adsorption of anionic surfactants on sandstone and carbonate formation material is the ionic attraction between mineral sites and surfactant anion (Zhang and Somasundaran, 2006). The generation of surface charge on the mineral particles is considered to be either due to preferential dissolution or due to hydrolysis of surface species followed by pH-dependent dissociation of surface hydroxyl groups. For oxides such as silica and alumina, the hydrolysis of surface species followed by pH-dependent dissociation is considered to be a major mechanism ... 

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