Primary clay or residual clay, as they are sometimes called are those clays which have been formed on the site of their parent rocks and have not been transported from its origin. In typical deposits of primary clay, much unaltered rock remains and the clay is formed in irregular pockets. Normally it contains large and small grains of clay.
Primary clay, when they have been cleared of rock fragments, tend to be relatively pure and free from contamination with non-clay minerals. The reason for this is that most primary clays originate from beds of more or less pure feldspar, a rock which is relatively easily broken down in geologic time by the action of water alone. Another important reason is that since the clay is not carried by streams, there is much less chance for admixtures from other localities to alter its composition. Hence we give value for Primary clays for its purity, whiteness, freedom from objectionable mineral and other contaminations.
To be continued….
