The soil that is often called "dirt" is not as dirty as many people think. If we remove the 5 to 10% of the soil material, composed of organic matter and amorphous Fe, Al and Si hydrous oxides, the remaining 90 to 95% is a mixture of crystalline or subcrystalline minerals. We are able to see and identify with the naked eye, the large crystalline particles of the sand fraction. For the smaller mineral particles of the silt and clay fractions microscopic or other more sensitive methods (x=ray diffraction, thermal) are needed for their identification and quantification. While little attention is often given to these crystalline minerals as to their relation to soil productivity, their different morphological and physicochemical properties play a very important role, in controlling soil behavior. Sometimes the presence of even very small amounts of certain minerals in the soil is more critical than the presence of large amounts of others because of unique characteristics affecting their reactivity. For example. the mineral smectite (montmorillonite) with its high cation exchange capacity, surface area, and shrink-swell potential is a determining factor in ion exchange reactions, sorption processes and mechanical strength of soils. On the other hand quartz, the dominant mineral in many soils. is relatively non-reactive and is largely considered as a dilutant of other more reactive mineral components in soils. Other soil minerals such as mica and kaolinite are intermediate in reactivity between those of smectite and quartz.
Karathanasis, Anastasios D., "Mineralogy and Soil Productivity" (1985). Agronomy Notes. 75.