Granite is made up of quartz, mica and feldspar. As quartz is resistant to chemical weathering, it may be eroded only as mineral grains of quartz. Feldspars and micas are susceptible to chemical weathering and break down to form clay minerals.
The main group of clay minerals are kaolinite, illite and montmorillonite. The layers in kaolinite are held together by fairly weak bonds, whereas there is strong bonding in illite and montmorillonite because of the presence of positively charged metal ions; potassium in the case of illite, and calcium and sodium in the case of montmorillonite.
Generally, potassium feldspar breaks down to form kaolinite; micas weather to give illite, and ferromagnesian minerals break down to form montmorillonite. This chart shows that precipitation is also an important factor:
|Source (PDF): Clay mineral formation and transformation in rocks and soils. Eberl. Phil. Trans. R. Soc. London 1984|
Alternating dry and wet climates can be inferred by the types of clays formed:
The major types of clay
Kaolinite, smectite (montmorillonite) and Illite:
The sum of the many hydrogen bonds between micelles results in the micelles being very strongly bonded together and nearly impossible to separate. This bonding of the layers together results in kaolinite being a nonexpanding clay mineral. Since each micelle is constructed of a layer of silicon tetrahedral units and a layer of octahedral units, kaolinite is called a 1:1 clay mineral.
Kaolinite is formed by weathering or hydrothermal alteration of aluminosilicate minerals. Thus, rocks rich in feldspar commonly weather to kaolinite. In order to form, ions like Na, K, Ca, Mg, and Fe must first be leached away by the weathering or alteration process. This leaching is favored by acidic conditions (low pH). Granitic rocks, because they are rich in feldspar, are a common source for kaolinite.
2:1 Smectite (AKA Bentonite) and Vermiculite
Smectites have more Mg2+, Fe2+, or Mn2+ substituted for Al, giving their sheets a permanent negative layer charge, which in turn leads to high CEC values. Montmorillonite is a type of smectite. Montmorillinite is the main constituent of Bentonite, derived by weathering of volcanic ash. Smectites have a high shrink/swell capacity because water molecules can intercalate between the clay sheets, greatly expanding their total volume.
Illite clays are non-expanding. Illites are formed from weathering of K and Al-rich rocks under high pH conditions. Thus, they form by alteration of minerals like muscovite and feldspar. Illite clays are the main constituent of ancient mudrocks and shales.
Vermiculite weathers from mica, which often forms at the contact between felsic and mafic rocks. It swells intermediate amounts.
Minerology maps of the U.S (PDF)
example: calcium carbonate map:
New Mexico shrink/swell clays:
|Source: Swelling clays map of the conterminous United States. From a good website.|