Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 132

Groote Eylandt in Australia and the rhodochrosite ores of the Molango district of
Mexico.
Phosphorites, which are mined to be used as fertilisers, form on shallow conti-
nent shelves either through direct precipitation from seawater or by diagenetic
replacement of limestones. Biological processes are important in controlling the
buildup of dissolved phosphorus in seawater and its subsequent precipitation.
The evaporation of brines in estuaries or lakes produce chemical precipitates that
are mined for normal salt (NaCl), sylvite (KCl, a source of potassium in fertilisers),
gypsum (CaSO 4 .2H 2 O, used in construction) and anhydrite (used in cement).
Sodium and potassium nitrates find a remarkable list of uses: as a fertilizer, a
solid rocket propellant, a rust inhibitor and in gunpowder; in food preservation, in
glass and pottery enamels; in the manufacture of cigarettes to maintain an even burn
of the tobacco, and in toothpastes for sensitive teeth. The world's largest natural
deposits of sodium nitrate are in the Atacama Desert of Chile, one of the driest
regions of the world. The mining of these generated great wealth for Chile for over a
century, until the 1940s when the German chemist Fritz Haber developed a process
to produce ammonia from atmospheric nitrogen.
5.4 Laterites
Laterites are soils that develop through prolonged and intensive rock weathering in
hot humid climates. Most laterites are relatively rich in iron and have no economic
value, but when they form on granites or clay-rich shales, they form bauxite, the ore
of aluminium, and when they develop on ultramafic rocks, they may form lateritic
nickel deposits.
5.4.1 Bauxite
Table 5.5 is a list of the world's largest producers of aluminium metal. Included in
the list are the large industrial countries like China and the USA and several
countries with large reserves of bauxite, like Australia, India and Brazil. But also
included in the list are small countries with no bauxite at all. Iceland, for example, is
a small, sparsely populated volcanic island in the middle of the frigid North Atlantic
- what is it doing in the list?
The reason is simple: to extract aluminiummetal from the oxides and hydroxides
that are the ore minerals in bauxite requires a lot of electricity, a commodity that
Iceland has in excess. So bauxite or its refined product alumina (Al 2 O 3 ) is shipped
halfway around the world from Australia, refined in Iceland into aluminium metal
which is then shipped to markets in Europe and America. Does this make any
economic sense? And what is the impact on the environment of these “bauxite
miles”?