Terje Solbakk and Astri Fritsen (photo).
High humidity and temperatures can cause chemical decomposition of extruded volcanic rock, and the products of such processes include clay minerals such as kaolinite. Derived from feldspar, this mineral can accumulate in large quantities – either on site or through longdistance transport – if the decay continues over enough time.
As a deposition product, it is known as kaolin or china clay. A kaolin mine at Newbridge in Dorset, UK, is pictured above. The dark layers are lignite, which can be used as fuel, and the kaolin here is accordingly impure.
The clay has many applications, including sanitary porcelain – where it is the main ingredient – and coated paper. It is also used in toothpaste and medicines. Norway has a few deposits of kaolin precipitated hydrothermally at such locations as Seljord south of Oslo and the Jøssing Fjord near Stavanger.