Weathering, 296 alkalinity feldspars

Gays are an essential component of soils, to which we owe our survival, and they are also the raw materials for some of mankind s most ancient and essential artefacts pottery, bricks, tiles, etc. Clays are formed by the weathering and decomposition of igneous rocks and occur typically as very fine panicles e.g. kaolinite is formed as hexagonal plates of edge. 1-3 p m by the weathering of alkaline feldspar... 

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

Most kaolinite is formed by the acid leaching of alkaline rocks, primarily the feldspars and micas however, practically any silicate rock or mineral will alter to kaolinite if leaching conditions are suitable for a sufficiently long period of time. Kaolinites that are formed by weathering and remain in place are called residual kaolinites. Those that are transported and sedimented are called sedimentary kaolinites... 

Of course, once the ore is obtained from its deposit, the actual work of extracting the desired metal has yet to be accomplished. In addition to metals, a variety of other substances comprise natural minerals. Since aluminum and silicon are the most prevalent elements in the Earth s crust, most of the metals exist naturally as aluminates, silicates, or aluminosilicates. The most common minerals are feldspars and clays. These materials have been used since ancient times for the production of materials such as pottery, brick, and china. An example of a feldspar is K2Al2Si60i6, which corresponds to a mixture of potassium superoxide, alumina, and silica (K20-Al203 6Si02). Upon contact with water and carbon dioxide, a weathering reaction results in kaolinite, an aluminosilicate clay (Eq. 1). However, in addition to these oxidized sources of metals, there are substances such as alkaline carbonates, sulfates, phosphates, as well as organic matter that need to be removed to yield the desired metal. As you would expect, the yield for this process is quite low ores typically possess less than 1 % of the desired metal ... 

Feldspars weather directly to ionic components by hydrolysis and H attack however, secondary minerals may form as products of the weathering reaction, depending on the chemical environment in the soil. In a closed system where feldspar is not exposed to acidity generated within or outside the soil, hydrolysis is the dominant reaction, producing an alkaline solution ... 

By this hypothesis, Millot argued that aluminum tends to direct silica toward a two-dimensional (sheet) rather than three-dimensional (framework) structure under conditions usually prevalent in soil solutions (although this assumes that conditions are not strongly alkaline). Thus, feldspars are not known to neoform (by, say, the reverse of reaction 6.26) except perhaps under geological conditions favorable to metamorphosis. Yet thermodynamically less stable minerals such as zeolites do form under the alkaline conditions that are presumably favorable to the formation of feldspar. Zeolites are framework silicates with an arrangement of tetrahedra unlike that of feldspar, and are considered to be metastable with respect to feldspar. We find, then, that in weathering processes, the final mineral products may not be, in the thermodynamic sense, the most stable products. 

The material source of the watershed fault zone is both from host rock and overlying siliceous rocks. The fault zone is an open system with strong chemical weathering, and alkaline condition. Calcium in limestone is leached, the cohesive force of host rock is reduced and formed smectite. It is easy for feldspar transforms to kaolinite under alkaline condition during faulting. Hence, more geochemistry needed to study in the future, to establish qualitative model between chemical compositions and strength of fault rocks. 

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