Minerals—The Building Blocks of Rocks - The Changing Earth: Exploring Geology and Evolution - page 79

too high, and what might be deemed a resource rather than
a reserve in the United States and Canada may be mined in a
developing country where labor costs are low. The commod-
ity in question is also important. Gold or diamonds in suf-
fi cient quantity can be mined profi tably just about anywhere,
whereas sand and gravel deposits must be close to their
market areas.
Obviously the market price is important in evaluating
any resource. From 1935 until 1968, the U.S. government
maintained the price of gold at $35 per troy ounce (1 troy
ounce = 31.1 g). When this restriction was removed, demand
determined the market price and gold prices rose, reaching
an all-time high of $843 per troy ounce in 1980. As a result,
many marginal deposits became reserves and a number of
abandoned mines were reopened.
The status of a resource is also affected by changes in tech-
nology. By the time of World War II (1939-1945), the richest
iron ore deposits of the Great Lakes region in the United States
and Canada had been mostly depleted. But the development
of a method for separating the iron from unusable rock and
shaping it into pellets ideal for use in blast furnaces made it
profi table to mine rocks that contained less iron.
Most people know that industrialized societies depend on
a variety of natural resources, but they know little about their
occurrence, methods of recovery, and economics. Geologists
are, of course, essential in fi nding and evaluating deposits; how-
ever, extraction involves engineers, chemists, miners, and many
people in support industries that supply mining equipment.
Ultimately, though, the decision about whether a deposit
should be mined or not is made by people trained in business
and economics. In short, extraction must yield a profi t.
In addition to resources such as petroleum, gold, and ores
of iron and copper, some quite common minerals are also es-
sential. For example, pure quartz
sand is used to manufacture
glass and optical instruments, as
well as sandpaper and steel al-
loys. Clay minerals are needed
to make ceramics and paper;
feldspars are used for porcelain,
ceramics, enamel, and glass; and
phosphate-bearing rock is used
for fertilizers. Micas are used in
a variety of products, including
lipstick, glitter, and eye shadow,
as well as the lustrous paints on
appliances and automobiles (see
Geo-Focus on pages 76 and 77).
Access to resources is es-
sential for industrialization and
the high standard of living en-
joyed in many countries. The
United States and Canada are
resource-rich nations; however,
many resources are nonrenew-
able , which means that there
is a limited supply and they
cannot be replenished by natu-

TABLE 3.3

Important Rock-Forming Minerals

Mineral Primary Occurrence

Ferromagnesian silicates
Olivine Igneous and metamorphic

rocks

Pyroxene group
Augite most common

Igneous and metamorphic

rocks

Amphibole group
Hornblende most common

Igneous and metamorphic

rocks

Biotite All rock types

Nonferromagnesian silicates
Quartz All rock types

Potassium feldspar group
Orthoclase, microcline All rock types

Plagioclase feldspar group All rock types

Muscovite All rock types

Clay mineral group Soils, sedimentary rocks,

and some metamorphic

rocks

Carbonates
Calcite

Sedimentary rocks

Dolomite Sedimentary rocks

Sulfates
Anhydrite

Sedimentary rocks

Gypsum Sedimentary rocks

Halides
Halite Sedimentary rocks

Plagioclase
feldspar

Biotite

Quartz

Potassium
feldspar

◗ Figure 3.19 Minerals in Granite The igneous rock granite (see Chapter 4) is made up of mostly
three minerals—quartz, potassium feldspar, and plagioclase feldspar—but it may also contain small
amounts of biotite, muscovite, and hornblende.