28 / Basics of Environmental Science
ago, when what is now Devon was a hot, arid desert. The desert sand contained some iron, which
was oxidized to its insoluble red oxide, giving the sandstone its present colour.
Limestone pavementA distinctive feature, sometimes covering a large area, that occurs in many
parts of the world. It forms when horizontal limestone beds are exposed by the
erosion of any material that may once have covered them and joints within
them are penetrated by rain water carrying dissolved CO 2. This weak carbonic
acid (CO 2 + H 2 O → H+ + (HCO 3 )- ) reacts with calcium carbonate to produce
calcium bicarbonate, which is soluble in water and is carried away. This widens
the joints to form deep crevices (called ‘grikes’ in England) separated by raised
‘clints’. Small amounts of soil accumulating in the sheltered grikes provide a
habitat for lime-loving plants, making limestone pavements valuable botani-
cally. At a deeper level, the grikes may join to form caves. Particular areas of
limestone pavement are protected in Britain by Limestone Pavement Orders
issued under the Wildlife and Countryside Act 1981, mainly to prevent the
stone being taken to build garden rockeries and for other ornamental uses.Iron oxidizes readily and this form of weathering has produced hematite (Fe 2 O 3 ) , one of the most
important iron ore minerals, some of which occurs in banded ironstone formations, 2–3 billion years
old, composed of alternating bands of hematite and chert (SiO 2 ). Iron and other metals can also be
concentrated by hydrothermal, or metasomatic, processes. Near mid-ocean ridges, where new basalt
is being erupted on to the sea bed, iron, manganese, and some other metals tend to separate from the
molten rock and are then oxidized and precipitated, where particles grow to form nodules, sometimes
called ‘manganese nodules’ because this is often the most abundant metal in them. Vast fields of
nodules, containing zinc, lead, copper, nickel, cobalt, silver, gold, and other metals as well as
manganese and iron, have been found on the floor of all the oceans (KEMPE, 1981). A few years ago
serious consideration was given to the possibility of dredging for them, but at present metals can be
obtained more cheaply by conventional mining on land.
Hydrothermal weathering, in which hot solutions rise from beneath and react with the rocks they
encounter, produces a range of commercially valuable minerals, perhaps the best known of which is
kaolin, or ‘china clay’. This material was first discovered in China in 500 BC and was used to make
fine porcelain, hence the names ‘china clay’ and ‘kaolin’, from kao ling, meaning ‘high ridge’, the
type of landscape in which it occurred. Today it is still used in white ceramics, but most is used as a
filler and whitener, especially in paper. The paper in this book contains it. Kaolin deposits
(www.wbb.co.uk/)Welcome.htm) occur in several countries, but the most extensively mined ones are
in Cornwall and Devon, Britain.
Kaolin is a hydrated aluminium silicate, Al 2 O 3 2SiO 2 .2H 2 O, obtained from the mineral kaolinite. The
British deposits occur in association with the granite batholiths and bosses intruded during the
Hercynian orogeny. Granites consist of quartz crystals, mica, and feldspars. Feldspars are variable in
composition. All are aluminium silicates, those associated with the kaolinite deposits being plagioclase
feldspars, relatively rich in sodium. As the intruded granite was cooling, it was successively exposed
to steam, boron, fluorine, and vaporized tin. The feldspar reacted with these, converting it into kaolinite
(the process is known as kaolinization), a substance consisting of minute white hexagonal plates