| Chemically, they are
multi-mineral rocks made of an aggregate of minerals and colloidal
substances. Essentially, mineral composition includes members of
mica group commonly sericite & muscovite, clay group commonly
paragonite, chlorite & kaonilite. The accessory minerals are
oxides, calcites & minor amounts of quartz and feldspar along
with subordinate amounts of ferro-magnesian minerals. Free silica is
normally poor. It is resistant to acids and staining. |