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Questions3AltAnswered - Study Questions Exam 3 1. Name the...

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Study Questions Exam 3 1. Name the four general types of soil colloids. Layer aluminosilicates, amorphous aluminosilicates, Al and Fe oxides, and organic colloids (humus). 2. What are the three general types of layer silicate clay minerals? Based on numbers of Si tetrahedral sheets and Al (or Mg) octahedral sheets, 1:1, 2:1 (2 Si tetrahedral sheets, 1 octahedral sheet in between) and 2:1:1 (same as 2:1 but with an additional, different type octahedral sheet). 3. What are the two basic building blocks (units of atoms in certain geometric configurations with respect to a central atom) of silicate clay minerals. The Si tetrahedron and the Al (or Mg) octahedron. 4. How many silicon tetrahedral sheets and aluminum octahedral sheets are there in a 1:1 aluminosilicate clay? In a 2:1? In a 2:1:1? See # 2 above. 5. Why don't 1:1 minerals expand when these are exposed to water? The layer aluminisilicate minerals tend to occur stacked together. Consequently, the Si tetrahedral sheet of one crystal faces the octahedral sheet of its neighbor. In close proximity, therefore, are surface O atoms on the tetrahedral sheet and surface –OH groups of the octahedral sheet. The H of the –OH groups is H-bonded to the O, linking the neighboring crystals and preventing entry of water in the space (interlayer) in between neighboring crystals, hence, preventing expansion. 6. Of these general types of layer silicate minerals, only certain 2:1 minerals expand. Name the three general types of 2:1 minerals. Which is expanding? Which is limited-expanding? Which is non-expanding? Smectite is expanding. Negative charge in the crystal lattice is mostly in the central octahedral sheet and while there is certainly attraction for interlayer cations by both of the neighboring crystals in the stacked configuration, the mutual attraction is not so strong as to prevent water from entering the interlayer region and expansion to occur.
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In contrast, there is extensive negative charge (due to isomorphic substitution) in the tetrahedral sheets of illite and K balances this negative charge. The ionic radius of K is such that it can fit into partial voids in the surfaces of the tetrahedral sheets of adjacent illite crystals, bringing the – charges of adjacent crystals and + charges of K in close proximity, leading to strong mutual attraction and preventing expansion due to entry of water into interlayer spaces. Vermiculite is intermediate in extent of isomorphic substitution and location of negative charge, leading to intermediate behavior. It is considered a limited-expanding mineral. 7. Explain why illite does not expand. See # 6 above. 8. Are silicate clay minerals primary or secondary? Arrange 2:1 minerals, Al and Fe oxides and 1:1 minerals in their natural weathering sequence. They are secondary, being chemically derived from primary minerals or formed from the
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This note was uploaded on 01/08/2012 for the course AGRO 2051 taught by Professor Gaston during the Fall '08 term at LSU.

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Questions3AltAnswered - Study Questions Exam 3 1. Name the...

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