Primary mineral weathering

Notice that in step 2, the weathering of an aluminosilicate mineral by the aggressive attack of "H2S04" waters may result in the release of dissolved silica, as well as cations. The ratio of silica to cations released depends on the composition of the primary mineral weathered and its alteration product. The weathering of a carbonate mineral gives cations and bicarbonate. 

Simple steady-state models have been used to determine critical loads for lakes and forest soils. The basic principle is that primary mineral weathering in the watershed is the ultimate supplier of base cations, which are required elements for vegetation and lake water to ensure adequate acid-neutralizing... 

Silica, alumina, iron, and the various base cations that are dissolved by primary mineral weathering can precipitate as new low-temperature minerals—a process known as neoformation. This should be distinguished from weathering processes, termed alteration, in which part of the parent mineral structure is inherited by the weathering product. Examples of alteration are given in the previous section. 

It is necessary to conclude that many of the mineral stability diagrams that are commonly constructed to explain primary mineral weathering (such as the one for feldspar in Figure 6.12) have no quantitative value—they are useful only to the extent that they gauge the tendency of the weathering reaction to proceed in a forward direction. If, for example, feldspar and kaolinite coexist in a soil, overall equilibrium between the two minerals is not possible when a realistic temperature and time frame is considered. The reaction is irreversible if it proceeds, feldspar must decompose and kaolinite must precipitate. It is true that back reactions such as the... 

The proof of reversibility in primary mineral weathering would be instances where primary mineral structures have formed under earth-surface conditions. There are reports that secondary quartz can slowly precipitate at room temperature from solutions supersaturated with monosilicic acid. More typically, however, precipitated silica in soils is structurally disordered, in the form of chalcedony or opal. In fact, as long as alumina is present, silica does not precipitate as a separate phase, reacting instead to form aluminosilicates (layer silicates, imogolite, or allophane). 

During chemical weathering, rocks and primary minerals become transformed to solutes and sods, and eventually to sediments and sedimentary... 

Copper ore minerals maybe classified as primary, secondary, oxidized, and native copper. Primaryrninerals were concentrated in ore bodies by hydrothermal processes secondary minerals formed when copper sulfide deposits exposed at the surface were leached by weathering and groundwater, and the copper reprecipitated near the water table (see Metallurgy, extractive). The important copper minerals are Hsted in Table 1. Of the sulfide ores, bornite, chalcopyrite, and tetrahedrite—teimantite are primary minerals and coveUite, chalcocite, and digenite are more commonly secondary minerals. The oxide minerals, such as chrysocoUa, malachite, and azurite, were formed by oxidation of surface sulfides. Native copper is usually found in the oxidized zone. However, the principal native copper deposits in Michigan are considered primary (5). 

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

Once a layer-silicate clay forms, it does not necessarily remain in the soil forever. As conditions change it too may weather and a new mineral may form that is more in equilibrium with the new conditions. For example, it is common in young soils for the concentrations of cations such as K, Ca, or Mg in the soil solution to be high, but as primary minerals are weathered and disappear, cation concentrations will decrease. With a decrease in solution cations, a layer-silicate such as vermiculite will no longer be stable and can weather. In its place. 

Contributions made by physical weathering are much greater in steep terrains (i.e., more primary minerals remain), and overall weathering rates are higher. 

The most stable minerals are often physically eroded before they have a chance to chemically decompose. Minerals that decompose contribute to the dissolved load in rivers, and their solid chemical-weathering products contribute to the secondary minerals in the solid load. The secondary minerals and the more stable primary minerals are the most important constituents of clastic sedimentary rocks. Consequently, the secondary minerals of one cycle of erosion are... 

When soil thickness is at the stable value (F), erosion is transport limited. Chemical weathering is also transport limited. This is, however, not because of reaction kinetics instead this limitation is primarily controlled by physical factors, most probably, restricted access of water to the primary minerals. 

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

Weathering of snbsurface solid phases occnrs as a result of their direct interaction with liqnid phases, which may also in tnm be affected by the gaseous environment. Examples of weathering processes inclnde reactions that convert primary minerals snch as qnartz and clays into metal oxides and metal hydroxides. 

A possible chemical weathering process of two primary minerals, muscovite and biotite, and their varions mineral products is presented in Fig. 2.1. 

Most of the heavy metal contaminants associated with mining or agricultural practices are released from less common mineral phases, some of which are primary (i.e., formed initially by an igneous, metamorphic, or sedimentary process) and some of which are secondary (i.e., formed by chemical alteration of the primary minerals). Table 7.4 lists some of the more common contaminant ions and the mineral phases with which they are associated in economic mineral deposits or weathered zones associated with these deposits. 

Aluminum is present in many primary minerals. The weathering of these primary minerals over time results in the deposition of sedimentary clay minerals, such as the aluminosilicates kaolinite and montmorillonite. The weathering of soil results in the more rapid release of silicon, and aluminum precipitates as hydrated aluminum oxides such as gibbsite and boehmite, which are constituents of bauxites and laterites (Bodek et al. 1988). Aluminum is found in the soil complexed with other electron rich species such as fluoride, sulfate, and phosphate. 

Generally for sedimentary rocks substitution in silicate lattices of primary minerals is of lesser importance. As the primary minerals decompose during weathering the alkaline earths and alkali metals tend to remain in solution and some of the metallic micronutrients pass into the lattices of the secondary or clay minerals. Others become adsorbed onto clay sized particles, are incorporated into humified organic matter by complexation or separate as precipitates following changes in redox potential. 

Gooding, J. L. (1978) Chemical weathering on Mars thermodynamic stabilities of primary minerals (and their alteration products) from mafic igneous rocks. 

Figure 6.2 Primary minerals containing Fe (e.g., pyroxenes and amphiboles in basaltic volcanic rocks) are weathered, releasing Fe into solution, which is then precipitated and accumulated in oxides as illustrated when examining the composition and percentage of clay and free oxides in Hawaiian soils. (Modified from Sherman, 1952.)...

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