Clay is an impure form of aluminum silicate. Clay is formed by the slow "breaking up" or decomposition of certain parts of rocks called the feldspars (silicates of aluminum, sodium, or potassium). The decomposition causes the feldspar to form an insoluble silicate and a soluble silicate (sodium or potassium silicate). The soluble part is washed away and the insoluble portion which remains - particles of mica, quartz, carbonate of lime and magnesium, and iron - is called clay. The greater part of the clay is pure aluminum silicate (H4Al2Si2O9).