Weathering of feldspars

As in dissolution, a chemical and structural change can occur from hydrolysis as the ions replaced by or OH may be of a different size so that the crystal structure is stressed and weakened. An example of this is the weathering of feldspar or goethite by H ... 

Taken as a whole, a synthesis of all the above observations suggests that the weathering of feldspar proceeds as follows ... 

Quartz Quartz grains Chalcedony and quartz from the weathering of feldspar and mica Quartz —... 

Primary clay, known as kaolin, is found in the same place as the parent rock. Kaolin is formed by the weathering of feldspar. Kaolins are coarse in particle size and therefore nonplastic compared to most sedimentary clays. Kaolins are relatively free of mineral impurities such as iron. 

The application of chemical kinetics to weathering processes of soil minerals first appeared in the work of Wollast (1967). He concluded that the rate-limiting step for weathering of feldspars was diffusion (Chapter 7). This work touched off a lively debate that is still raging today about whether weathering of feldspars and ferromagnesian minerals is controlled by chemical reaction (CR) or diffusion. 

A balance for the remaining materials can be obtained by utilizing about 75% of the remaining silica in the chemical weathering of feldspars to kaolinite, and the rest in reaction of feldspars to montmorillonite ... 

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 ... 

In this chapter, the focus is on weathering of feldspars, aluminosilicate minerals, which are the most abundant mineral species in the earth crust (Banfield Hamers, 1997). Feldspars contain aluminium and silicon, which are arranged in a tetrahedral structure, with other cations in the voids of this structure. The common feldspars have compositions ranging between albite (NaAlSisOg) and K-feldspar (KAlSigOg) (alkali feldspars) and between albite and anorthite (CaAl2Si20g) (plagioclase feldspars). 

The acidic conditions resulting from the oxidation of hydrocarbons in near-surface soils and sediments promotes the diagenetic weathering of feldspar to clay and the conversion of smectite clay to kaolinite. The kaolinite thus formed remains chemically stable unless the enviromnent is changed (Schumacher, 1996). 

Fig. 4.15 Highly weathered granite, showing residual materials—mainly kalolinized feldspar and quartz. The granite has broken down principally due to the weathering of feldspars. Note the corestone of less weathered granite by the figure. Two Bridges Quarry, Dartmoor, UK. Photograph courtesy of J. Andrews.

Ceramic materials have been made since well before the dawn of recorded history. They are generally fashioned from clay or other natural earths at room temperature and then permanently hardened by heat. Silicate ceramics include objects made from clays, such as pottery, bricks, and table china. The three major ingredients of common pottery are clay (from weathering of feldspar as described previously), sand (silica), and feldspar (aluminosilicates). Clays mixed with water form a moldable paste because they consist of many tiny silicate sheets that can easily slide past one another. When the clay-water mixture is heated, the water is driven off, and new Si—O—Si bonds are formed so that the mass of platelets becomes permanently rigid. 

Equation 19.5 demonstrates why the presence of bicarbonate ions and molecular silicic acid in the streams of Taylor Valley is evidence that chemical weathering of feldspar and other silicate minerals is occurring in spite of the harsh climatic conditions. The only prerequisite to chemical weathering of minerals on the surface of the Earth as well as in the subsurface is the presence of liquid water. The work of Nezat et al. (2001) has demonstrated that silicic acid and bicarbonate ions are present in all meltwater streams in the watersheds of Lake Bonney, Lake Hoare, and Lake Fryxell. 

Nesbitt and Jambor (1998) have shown the fundamental role of mafic minerals in neutralization of the Waite-Amulet tailings. As in the weathering of feldspar, the weathering of felsic minerals leads to the formation of clay minerals. Muscovite, pyroxene and amphibole alter to chlorite. By decreasing pH, chlorite alters to sericite, kaolinite or Mg-montmorillonite. The products of biotite alteration are hydrobiotite, a regularly interstratified biotite-vermicuhte phase, vermiculite, and kaolinite (Acker and Bricker, 1992 Mahnstrom and Banwart, 1997). Direct conversion of biotite to kaolinite has also been described (Acker and Bricker, 1992). 

An important difference between the two subgroups is that kaolinites (and related minerals) are secondary minerals, known to form rather easily from weathering of feldspars, and are stable in ambient conditions (which explain its abundance). On the other hand, serpentines are not so stable, found in rocks as metamorphic alteration products, and usually have not been observed to form under low-temperature conditions, thus are more akin to primary minerals. 

Kaolin is the common name used for kaolinite (. v.), a white clay mineral of composition Al4[Si40io](OH)g derived from the weathering of feldspars. Correctly, kaolin consists of both crystalline and amorphous material, whereas kaolinite is completely crystalline. Particle size is normally extremely fine, though typically it appears as a highly laminated agglomerate of platelets (one author graphically describes it as a booklet ). Calcium, magnesium and potassium are also commonly associated with kaohn as trace elements. 

Clay minerals are classified into kaolinite, illite, smectite (montmorillonite), and palygorskite groups. The most important commercial clay minerals are kaolinite and montmorillonite. Kaolin or kaolinite, known as China clay , has the basic chemical formula Si2Al205(0H)4, and montmorillonite is AlSi205(0H) x H2O. Kaolinite was formed by various hydrothermal alterations or weathering of feldspars and other silicate minerals. 

Kaolin is a clay mineral. Clays are the primary ingredients in the making of traditional ceramic products. They are the layer silicates with a grain size of less than 2 p. Mechanical and chemical weathering of feldspars in igneous... 

The common mineral feldspar and the less abundant mineral olivine, selected as examples here, occur widely in many different rock types, and both are broken down during weathering by overall processes (physical and chemical disintegration and degradation). The typical reaction for the weathering of feldspar is shown in Equation 6.1 ... 

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