**2.2 Other minerals found in clay**

In addition to the phyllosilicates mentioned, the soil clay fraction may also contain minor amounts of oxides, hydroxides, and hydroxy-oxides (sesquioxides) of Si, Al, and Fe, as well as some weakly crystalline aluminosilicates. Quartz (owing to its excellent resistance to weathering) and opal (a weakly crystalline variant of quartz that precipitates from Si-supersaturated fluids or is of volcanic or biogenic origin) are two common Si-oxide minerals found in the clay fraction. Gibbsite (Al(OH)3) is the most prevalent Al-hydroxide representative in severely worn soil clay fractions, whereas goethite (FeOOH) is the most common Fe-mineral in clay fractions. Allophane and imogolite, two weakly crystalline aluminosilicate clay minerals, are commonly found in clay fractions of volcanic soils. The primary varieties of commercial clays include ball clay, common clay and shale, and fire clay, in addition to bentonite (and Fuller's earth) and kaolin. Ball clay is mostly kaolinite, with traces of illite, chlorite, smectite minerals, quartz, and organic matter.
