Neoformed minerals, weathering

Figure 13. Simulation of the genesis of a roll-type uranium deposit and of the weathering of a pyrite-rich sandstone. Twenty boxes model at logi = —1 with Vx = 10, In the upper part we present an instant view of the alteration profile with neoformed minerals. Numbers in brackets correspond to the molar percentage of the neoformed minerals vs. total neoformed minerals in each box. In the lower part we present the major physicochemical parameters PE, pH, and [O2] (mol/1) in the percolating water in the above boxes. The scale is the same in the upper part and the lower part. IPW = initial percolating water.

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. 

The major commercial deposits of vermieulite are neoformed by weathering or hydrothermal alteration of biotite of large bodies of basic and ultrabasic rocks. Other mafic minerals present in those roeks, like phlogopite, chlorite, pyroxene ete. also participate in the reaetion. 

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) ... 

Smectites also form by precipitation of dissolved ions from alkaline solution, a process referred to as neoformation because none of the structure is inherited from weathering parent minerals. Neoformation of minerals in soils will be discussed in more detail later in this chapter. 

The particular secondary minerals formed by neoformation are controlled by the leaching intensity of the local soil environment, and to what extent the environment is confining, allowing the more soluble weathering products (silica, base cations) to accumulate. The temperature and moisture conditions also have an effect on the specific secondary minerals that form. To understand in general terms the complex balance of factors that dictates which secondary minerals tend to form, it is first necessary to appreciate the chemical factors that limit solubility of two weathering products—alumina and silica—the major constituents of clay minerals. 

A flow diagram of the weathering sequences that can be expected for the most important rock-forming minerals is depicted in Figure 6.9. This diagram incorporates the processes of alteration, neoformation by addition (silica added to Fe and A1 oxides in confined environments), and neoformation by subtraction (silica removed... 

The last-mentioned factor influencing weathering rates of minerals in soils (see list above), efficiency of removal of soluble weathering products, warrants further comment. In confined soil environments, dissolved silica and base cations accumulate from weathering reactions. So, for example, the neoformation of kaolinite from smectite ... 

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. 

Mineral constituents of soils are inherited from parent material and have been exposed to weathering and pedogenic processes. Processes of degradation and neoformation of minerals - as well as interactions with organic compounds - greatly affect trace... 

Although detrital micas may generally prevail in soils, pedogenic formation of micas from nonmica minerals and their weathering products has frequently been suggested in the literature. Neoformation depends on the kind of parent material and pedochemical environment and can best be demonstrated in soils derived from parent material containing no mica. 

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