1. Describe the principal bonding mechanisms that are most important to all soil
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Question
1. Describe the principal bonding mechanisms that are most important to all soil mineral structures. Are there other mechanisms that impart stability to specific types of minerals? If so, what are they and what are the minerals involved? (Give specific examples.) 2. Employ Pauling?s bond valence principle to show that the residual charge on an O2- ion bound to Si4+ in tetrahedral coordination can be satisfied by two Al3+ atoms in octahedral coordination or by three Mg2+ atoms in octahedral coordination. Discuss briefly how this relates to the existence of dioctahedral and trioctahedral sheets in layer silicates.Explanation / Answer
1.Soil consists of mainly fine grained particles especially clay and silt. Clays are phyllosilicates that consists of tightly bonded sheets of silicate tetrahedra or t sheets which are internally linked together with shared corner oxygen anions. These tetrahedral sheets are attached with octahedral sheets or o sheets. These octahedral sheets are composed of hydroxyl anions arranged around cations eg calcium, aluminium and magnesium.
The simplest type of arrangement is found in clay mineral kaolinite mainly consists of aluminium cations. It is composed of one tetrahedral sheet and one octahedral sheet in each repeating layer of 1:1 arrangement. These layers are tightly bonded together with only hydrogen cations between them so that their structure can be expressed as t-o-H+-t-o-H+. In this structure there is no room for water, hydroxyl or larger cations between the layers.
The other important type of clay mineral is ilite which is mostly consists of potassium cations which give the structure strong ionic bonding and does not expand easily and water and hydroxyl cannot penetrate easily in the structure. Smectite is another important clay mineral in which small amount of Mg2+ substitute for Al3+ in the octahedral sheets and interlayers cations like Na+,K+,Ca2+.These clays are very expandable and absorb water.
2. According to Pauling's bond valence principal the strength of an electrostatic bond is defined as the valence charge of an ion (Z) divided by its coordination number (C.N). Therefore bondstrength or electrostatic valence=Z/C.N. This electrostatic valence is a measure of the strength of any of the bonds that reach the coordinating ion from its nearest neighbour.
eg. Since Si4+ is in tetrahedral coordination with oxygen anions which means Si4+ has valency charge of 4 and also its coordination number is 4 with the neighbouring oxygen anion which implies their bondstrength will be 4/4=1. This charge is satisfied when Al3+ whose valence charge is 3 with octahedral coordination number or C.N=6 with oxygen ions implies that bond strength for Al3+ will be 3/6=1/2. Since there are 2 Al3+ ions which means 2*1/2=1. Therefore bonds are satisfied. For Mg2+ the valence charge is 2 with octahedral coordination number i.e C.N.=6. For Mg2+ the bondstrength will be 2/6=1/3. Since there are 3 Mg2+ ions which means 3*1/3=1. Therefore we can see that for tetrahedral coordination of Si4+ with oxygen anion can be satisfied by 2 Al3+ or 2Mg2+ ions in octahedral coordination with oxygen anions.
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