Soil Science 100 ID____________________

Fall 2002 Lab___________________

Homework 2

Value 10 points Due Monday 7 October

Answer questions 1, 3 and 6 in the text (pages 41 and 42). In addition, answer the question below.

What are the texture class names for the following data? (1/2 point each)

Soil	Sand	Silt	Clay	Texture Class
ID	(%)
a	82	13	5	Loamy sand
b	15	30	55	Clay
c	38	45	17	Loam
d	22	66	12	Silt loam
e	41	26	33	Clay loam
f	57	30	13	Sandy loam

Text Questions:

1. (3 points) What is clay? Clay has three definitions in soil science. It is a particle size (<2mm). It is a texture class name and it is the name for a group of phyllosilicate minerals.

3. What is isomorphous substitution? (2 points)

During the formation of clay minerals, cations other than Si in the tetrahedral sheet and Al in the octabhedral sheet substitute for these atoms. Often, they are of lower charge but of similar size in the structure of clay minerals. This substitution does not change the morphology (shape) of the clay minerals (hence the substituted and unsubstituted forms are isomorphous), but the substitution leaves the clay mineral with a net negative charge that must be satisfied by exchangeable cations. The substitution also changes the chemical composition of the clay mineral producing stress in the structure.

6. How are kaolinite and montmorillonite different? The same? (2 points)

Kaolinite and Montmorillonite are both alumino-silicate clay minerals. They both have charge and high surface area compared with sand or silt, and they both occur as small (colloidal) crystals with layered structure. They differ in some important ways. First, kaolinite is a 1:1 mineral, meaning each layer consists of 1 aluminum octahedral sheet and 1 silica tetrahedral sheet, while Montmorillonite is a 2:1 mineral, each layer consisting of 2 silica tetrahedral sheets around an aluminum octahedral sheet. Isomorphous substitution is negligible in kaolinite but extensive in montmorillonite; thus kaolinite has much lower charge and cation exchange capacity than montmorillonite. Kaolinite particles tend to be larger than montmorillonite, which gives montmorillonite higher surface area per unit weight than kaolinite.