Botkin Environmental Science Earth as Living Planet 8th txtbk

524 CHAPTER 23 Materials Management
soluble elements, such as silica, calcium, and sodium, are
selectively removed by soil and biological processes.) If
sufficiently concentrated, residual aluminum oxide forms
an ore of aluminum known as bauxite. Important nickel
and cobalt deposits are also found in soils developed from
iron- and magnesium-rich igneous rocks.
23.4 Figuring Out How
Much Is Left
FIGURE 23.3 Diamond mine near Kimberley, South Africa. This
is the largest hand-dug excavation in the world.
Sedimentary processes related to the transport of sediments
by wind, water, and glaciers often concentrate materials
in amounts sufficient for extraction. As sediments
are transported, running water and wind help segregate
them by size, shape, and density. This sorting is useful
to people. The best sand or sand and gravel deposits for
construction, for example, are those in which the finer
materials have been removed by water or wind. Sand
dunes, beach deposits, and deposits in stream channels are
good examples. The sand and gravel industry amounts to
several billion dollars annually and, in terms of the total
volume of materials mined, is one of the largest nonfuel
mineral industries in the United States. 5
Rivers and streams that empty into oceans and lakes
carry tremendous quantities of dissolved material from the
weathering of rocks. Over geologic time, a shallow marine
basin may be isolated by tectonic activity that uplifts its
boundaries, or climate variations, such as the ice ages, may
produce large inland lakes with no outlets. As these basins
and lakes eventually dry up, the dissolved materials drop out
of solution and form a wide variety of compounds, minerals,
and rocks that have important commercial value. 10
Biological processes form some mineral deposits, such
as phosphates and iron ore deposits. The major iron ore
deposits exist in sedimentary rocks that were formed more
than 2 billion years ago. 10 Although the processes are not
fully understood, it appears that major deposits of iron
stopped forming when the atmospheric concentration of
oxygen reached its present level. 11
Organisms, too, form many kinds of minerals, such as
the calcium minerals in shells and bones. Some of these minerals
cannot be formed inorganically in the biosphere. Thirtyone
biologically produced minerals have been identified. 12
Weathering, the chemical and mechanical decomposition
of rock, concentrates some minerals in the soil, such as
native gold and oxides of aluminum and iron. (The more
Estimating how much is left of our valuable and nonrenewable
mineral resources will help us estimate how long
they are likely to last at our present rate of use and motivate
us to do everything we can to sustain them as long as
possible for future generations. We can begin by looking
at the classification of minerals as resources and reserves.
Mineral Resources and Reserves
Mineral resources are broadly defined as known concentrations
of elements, chemical compounds, minerals, or
rocks. Mineral reserves are concentrations that at the time
of evaluation can be legally and economically extracted as
a commodity that can be sold at a profit (Figure 23.4).
The main point here is that resources are not reserves.
An analogy from a student’s personal finances may help
clarify this point. A student’s reserves are liquid assets, such
as money in the bank, whereas the student’s resources include
the total income the student can expect to earn during
his or her lifetime. This distinction is often critical to
the student in school because resources that may become
available in the future cannot be used to pay this month’s
bills. 6 For planning purposes, it is important to continually
reassess all components of a total resource, considering
new technology, the probability of geologic discovery,
and shifts in economic and political conditions. 13
Availability and Use of Our
Mineral Resources
Earth’s mineral resources can be divided into broad categories
according to their use: elements for metal production
and technology, building materials, minerals for the
chemical industry, and minerals for agriculture. Metallic
minerals can be further classified by their abundance.
Abundant metals include iron, aluminum, chromium,
manganese, titanium, and magnesium. Scarce metals
include copper, lead, zinc, tin, gold, silver, platinum,
uranium, mercury, and molybdenum.
Some minerals, such as salt (sodium chloride), are
necessary for life. Primitive peoples traveled long distances