Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 123

suitable for the construction of aeroplanes and spacecraft, not to mention golf clubs,
the casings of Macintosh computers and the masts of racing yachts. The major use
of titanium is not as a metal, however, but as an oxide: TiO 2 is the pigment that
imparts a brilliant white colour to a wide variety of paints, papers, plastics and
other materials. The rare-earth elements, which are finding increasing uses in
superconductors, ceramics, batteries, magnets, phosphors in TV screens, and
catalysers in petroleum refineries, are present in monazite, a minor component in
beach sands.
Heavy mineral deposits in relatively young beach sands are the only significant
source of zirconium, in addition to being a major source of titanium. The latter
metal is also mined in magmatic deposits of ilmenite (FeO.TiO 2 ) in anorthosites in
Canada and Norway and as anatase (TiO 2 ) in residual deposits overlying alkaline
intrusions in Brazil.
The principal ore minerals in heavy-mineral beach sands are ilmenite (FeO.
TiO 2 ), leucoxene (approximately TiO 2 , an alteration product of ilmenite), rutile
(TiO 2 ), zircon (ZrSiO 4 ) and monazite (a phosphate of Th and the rare earth
elements). The concentrations of ore minerals are highly variable. The richest
deposits contain up to 50% of ilmenite, 5-20% of both rutile and zircon and
1-3% of monazite; more normal ores contain only a few percent of combined
heavy minerals. However, since the deposits are located at the surface in the form of
unconsolidated sands, mining costs are much lower than those of normal deposits in
which the hard solid rock must first be extracted from deep mines then crushed
before the ore minerals can be extracted.
All the heavy minerals in beach-sand deposits occur as accessory phases that are
present in small quantities ( 1%) in normal felsic magmatic and metamorphic

rocks. Grains of these minerals are released when the host rock is broken down by
subaerial weathering and these grains, which are stable in the low-temperature
fluvial environment, are then transported by streams and rivers to the shorelines of
lakes or oceans where the action of waves, wind, tides or long-shore currents
winnows out the lighter quartz and feldspar, leaving the sands enriched in the
dense Ti and Zr minerals. For the process to be effective, a large reservoir of
the source rock must be subjected to long periods of chemical weathering. This is
the situation in old Precambrian cratons which are rich in granitic or gneissic rocks
that contain relatively high concentrations of Zr and Ti minerals, and which are
eroded down to a peneplane that is subject to protracted and intense weathering,
particularly when located in equatorial regions. When similar rocks are exposed in
younger mountain ranges in temporal or cold climates, mechanical weathering
dominates and the heavy minerals are not so effectively separated from other
components. For these reasons, the largest deposits are located around old stable
continents in equatorial regions, namely along the coastlines of Australia, South
Africa and India. These countries are therefore the major producers of Zr and Ti.
Smaller or less rich deposits are located in southern USA, West Africa, Malaysia
and China.

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