Shale is a term that refers to sedimentary deposits that have been altered by compaction and, in some cases, the cementation of grains by deposition of other minerals such as sericite (a fine grained muscovite) . Shales are identical structurally and chemically to clays, although the water content of shales tends to be significantly lower. However, when they are mixed with water, shales develop plasticity similar to clays and can be used interchangeably . In fact, weathering of shales is one method for the formation of clays . Shales often contain high levels of iron, giving them a red color when fired . A typical shale composition is given in Table 7 .