Environmental

No sooner has a rock formed than it becomes vulnerable to attack by weathering. The word

‘weathering’ is slightly misleading. We associate it with wind, water, freezing, and thawing. These

are important agents of weathering, but they are not the only ones. Weathering can be chemical as

well as physical and it often begins below ground, completely isolated from the weather.

Beneath the surface, natural pores and fissures in rocks are penetrated by air, containing oxygen and

carbon dioxide, and by water into which a wide variety of compounds have dissolved to make an

acid solution. Depending on their chemical composition, rock minerals may dissolve or be affected

by oxidation, hydration, or hydrolysis (HOLMES, 1965, pp. 393–400). Oxidation is a reaction in

which atoms bond with oxygen or lose electrons (and other atoms gain them, and are said to be

‘reduced’). Hydration is the bonding of water to another molecule to produce a hydrated compound;

for example, the mineral gypsum (CaSO4

.2H2

O) results from the hydration of anhy-drite (CaSO4

).

Hydrolysis (lysis, from the Greek lusis, ‘loosening’) is a reaction in which some parts of a molecule

react with hydrogen ions and other parts with hydroxyl (OH) ions, both derived from water, and this

splits the molecule into two or more parts.

The result of chemical weathering can be seen in the limestone pavements found in several parts of

England, Wales, and Ireland.8

South Devon, England, is famous for its red sandstones, well exposed

in the coastal cliffs of the Torbay area. These date from the Devonian Period, some 400 million years 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.

Iron oxidizes readily and this form of weathering has produced hematite (Fe2O3) , 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 (SiO2). 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, Al2O32SiO2.2H2O, 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.

to steam, boron, fluorine, and vaporized tin