Homework 1.CLAY STRUCTURE AND IDENTIFICATION METHODS OF CLAY MINERALS.

Clay structure.Clay minerals are so tiny crystalline substances evolved primarily from chemical weathering of certain rock minerals, they are complex alumina silicates plus other metallic ions. All clay minerals are very small with colloidal sized crystals (D < 1 m). The individual crystal look like tiny plates or flakes these ones consist of many crystal sheets which have a repeating atomic structure. There are only two fundamental crystal sheets, the tetrahedral or silica and the octahedral or alumina sheets. Because of their small size and flat shape, they have very large specific surfaces. There is usually a negative electric charge on the crystal surfaces and electro chemical forces on these surfaces are therefore predominant in determining their engineering properties. In order to understand why these materials behave as they do, it will be necessary to examine their crystal structure in some detail.The tetrahedral sheet is basically a combination of silica tetrahedral units which consist of four oxygen atoms at the corners, surrounding a single silicon atom. Figure 1.a shows a single silica tetrahedron; fig. 1.b shows how the oxygen atoms are combined to form a sheet structure. A common schematic representation of the tetrahedral sheet is shown in fig. 1.c. A top view of the silica sheet which shows how the oxygen atoms at the base of each tetrahedron belong to two tetrahedrons and how adjacent silicon atoms are bonded is shown in fig. 1.d.

The octahedral sheet is basically a combination of octahedral units which consist of six oxygen or hydroxyls enclosing an aluminum, magnesium, iron or other atom. A single octahedron is shown in Figure 2.a. fig. 2.b shows the octahedrons combine to form a sheet structure. Fig. 2.c is a schematic representation of the octahedral sheet. A top view of the octahedral sheet showing how the different atoms are shared and bonded is in Fig. 2.d.According to the reticular structure clay minerals can be identify into three different and main groups, which are: kaolinite, montmorillonite and illiteKaolinite is the union of a reticule of cilice and another of aluminum. The union of this kind of reticule structure is so strong to dont allow the flow of waters molecules. Because of this reason the kaolinite is relatively stable in presence of water.

Fig. 1 Fig. 2Fig. 1 and 2. Silica and alumina sheets fundamental conformation. Sheets can layer in different ways, forming different types of clay minerals. Clay minerals tend to form flat, platelike, and middle shapes.

Montmorillonite is the undefined superposition of an alumina lamina between two silica laminas. The strength that makes this laminar union is a bit weak so water molecules can pass through of the laminas therefore the montmorillonites are unstable.Illite are the analogical case of the montmorillonites being their conformation a lamina of cilice between two of aluminum. These ones are a little more stables than montmorillonites but both are.

Universidad Autnoma de QuertaroGeothecnics IM. in CR. Guillermo Paz Cruz.

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Fig. 3. Schematic diagram of the structure of kaolinite (1953).

Fig. 4. Schematic diagram of the structure of montmorillonite (1953).

Fig. 5. Schematic diagram of the structure of illite (1953).

Identification methodsNowadays there are some accessible proceedings for the researcher to identify the constituent minerals of clays. The more popular methods are: the diffraction X-Ray method and the thermic balance of clays. The microscope is also useful to determinate important information like is the length and weight of the particles.Once heated each clay mineral becomes a specific material whit specific chemical characteristics therefore each specific material allow the identification of the clay minerals.Chemical analysis has also been used a plenty like method of identification of clay minerals.Not yet, the actual methods to identify minerals in clays are not completely right.Microscope allows to identify regular forms in some clay minerals with a regular and a specific shape but there are a lot of minerals that can be confused because of their common shape, which belong to almost all of the clay minerals.Thermic balance is doubly overall in clays formed with two or more types of minerals which is known that happen with almost all clays (are formed with mineralogical mixtures).Chemical analysis is useful but at all because says the integral conformation of clays and not the knowledge about how components of clay are distributed.X-Ray method diffraction seems to be the most complete method, specially the electronic diffraction method which has been more recent.