Clay minerals crystalline

The development of apparatus and techniques, such as x-ray diffraction, contributed gready to research on clay minerals. Crystalline clay minerals are identified and classified (36) primarily on the basis of crystal stmcture and the amount and locations of charge (deficit or excess) with respect to the basic lattice. Amorphous (to x-ray) clay minerals are poody organized analogues of crystalline counterparts. 

Kaolinite, on the other hand, has no structural counterpart among the igneous minerals. It is also the most widespread of the crystalline clay mineral. The most likely mechanism for kaolinite formation is the complete breakdown of feldspar or mica particles and the precipitation of kaolinite from Al(OH)3 and Si(OH)4 from the soil solution or from amorphous, less stable intermediates. 

Soil Various natural clays Cu, Pb, Zn Spiked Heavy metal migration dominated by crystalline clay minerals lowest efficiency humic-aUophanic and allophonic soils Darmawan and Wada (2002)... 

Osorno and San Patricio soils are Andepts with a high organic matter content dominated by variable surface charge inorganic components (Table 1). On the other hand, Collipulli soil is an Ultisol with low organic matter content, dominated by crystalline clay minerals with little or no variable surface charge. 

Electron microscopy can also detect amorphous material in soils containing crystalline clay minerals. Fieldes and Williamson [1955] detected small quantities of amorphous matter in the electron micrographs of most of the soil clays they described. Similar features probably would be visible in soils such as those described by Mitchell and Farmer [1962] from Scotland. The electron microscope is a very useful tool for examining the fine detail of the clay fraction of soils, but care should be taken because of the changes that can take place with different treatments, as has been shown above. 

The presence of a large endothermic peak in the absence of montmorillonite-saponite minerals is usually indicative of the presence of noncrystalline aluminosilicate gels furthermore, such materials, unlike the crystalline clay minerals, are capable of rehydration after being heated to 600°C (Mitchell and Farmer [1962]). Mixtures of kaolinite and illite are difficult to interpret, since the principal peaks usually overlap, but attention to detail, especially in the low-temperature and high-temperature regions of the curve, can often give a clue as to the minerals present. 

In ceramics, plasticity is usually evaluated by means of the water of plasticity. Values for the common clay minerals are given in Table 1. Each clay mineral can be expected to show a range of values because particle size, exchangeable ion composition, and crystallinity of the clay mineral also exert an influence. Nonclay mineral components, soluble salts, organic compounds, and texture can also affect the water of plasticity. 

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. 

Since most trace elements in soils are at parts per million levels, a separate compound may be not formed. Most likely, trace amounts of these trace elements and their compounds are adsorbed on the surfaces of clay minerals and various crystalline and amorphous Fe/Mn/Al oxides and hydroxides. Curtin and Smillie (1983) reported that the solubilities of Mn2+ and Zn2+ in limed soils were not consistent with the solubilities of any... 

The clay minerals can now be discussed in terms of their relationship with the phyllosilicates (sheet silicates). It is important to keep clearly in mind here the difference between clay - the material which is dug out of the ground, and which may be a mixture of different clay minerals, together with various nonclay minerals (such as quartz, pyrite, etc), as well as unaltered rock fragments and incorporated organic material (Grim, 1968) - and the clay minerals themselves, which are crystalline compounds of specified stoichiometry and structure. At this stage, we are only considering the structure of the clay minerals. 

In view of the problems associated with the expanding 2 1 clays, the smectites and vermiculites, it seemed desirable to use a different clay mineral system, one in which the interactions of surface adsorbed water are more easily studied. An obvious candidate is the hydrated form of halloysite, but studies of this mineral have shown that halloysites also suffer from an equally intractable set of difficulties (JO.). These are principally the poor crystallinity, the necessity to maintain the clay in liquid water in order to prevent loss of the surface adsorbed (intercalated) water, and the highly variable morphology of the crystallites. It seemed to us preferable to start with a chemically pure, well-crystallized, and well-known clay mineral (kaolinite) and to increase the normally small surface area by inserting water molecules between the layers through chemical treatment. Thus, the water would be in contact with both surfaces of every clay layer in the crystallites resulting in an effective surface area for water adsorption of approximately 1000 tor g. The synthetic kaolinite hydrates that resulted from this work are nearly ideal materials for studies of water adsorbed on silicate surfaces. 

The pressure and temperature associated with long-term burial in the sediments eventually converts BSi into chert and quartz, both of which are crystalline. This conversion process involves the partial dissolution of BSi followed by its reprecipitation. Reprecipitation occurs within and upon BSi and probably on clay minerals... 

Quantitative determination of the major and minor minerals In geological materials Is commonly attempted by x-ray diffraction (XRD) techniques. Mineralogists use a variety of sophisticated and often tedious procedures to obtain semlquantltatlve estimates of the minerals In a solid sample. The mineralogist knows that XRD Intensities depend on the quantity of each mineral component In the sample even through expressions for conversion of signal Intensity to quantitative analysis often are unknown or difficult to determine. Serious difficulties caused by variables such as particle size, crystallinity, and orientation make quantification of many sample types Impractical. Because of a lack of suitable standards, these difficulties are particularly manifest for clay minerals. Nevertheless, XRD remains the most generally used method for quan-... 

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