Minerals—The Building Blocks of Rocks - The Changing Earth: Exploring Geology and Evolution - page 76

◗ ◗ Figure 2 Muscovite is used in several kinds of makeup, as well

as glitter.

Figure 3 Muscovite in paint on automobiles and appliances gives
them their lustrous sheen.

joint compound and as an additive
to paint. It is an essential component
of joint compound because it makes
the compound smoother and easier to
work with and it prevents cracking. In
fact, wallboard compounds and paints
account for about 80% of all mica
used. In addition, ground dry mica is
used in plastics, roofi ng, rubber, and
welding rods.

When mica is ground up wet, how-
ever, it retains its sparkling shine and is
used in many cosmetics (

may be added to lipstick. The brilliant
sheen of some paints applied to auto-
mobiles ( Figure 2).

Mica body powder is brushed onto the
skin for an overall sheen. Fortunately,
it is chemically inert and poses no risk
when applied to the skin. Eye shadow,
eyeliner, lipstick, blush, and nail pol-
ish have mica added to give them a
resinous sheen, or mica-based powder

◗ Figure 3) and the changing

color depending on viewing angle
come from micas. Perhaps the state-
ment that mica is the most amazing stuff
on Earth is an exaggeration, but mica
certainly enhances the visual appeal of
many products.

◗

As magma or lava cools, minerals crystallize and grow,
thereby determining the mineral composition of various
igneous rocks such as basalt (dominated by ferromagnesian
silicates) and granite (dominated by nonferromagnesian
silicates) (see Chapter 4). Hot-water solutions derived from
magma commonly invade cracks and crevasses in adjacent
rocks, and from these solutions, a variety of minerals crystal-
lize, some of economic importance. Minerals also originate
when water in hot springs cools (see Chapter 13), and when
hot, mineral-rich water discharges onto the seafloor at hot
springs known as hydrothermal vents (see Chapter 9).
Dissolved materials in seawater, more rarely in lake
water, combine to form minerals such as halite (NaCl),
gypsum (CaSO 4 • 2H 2 O), and several others when the water
evaporates. Aragonite and/or calcite, both varieties of cal-
cium carbonate (CaCO 3 ), might also form from evaporat-
ing water, but most originate when organisms such as clams,
oysters, corals, and fl oating microorganisms use this com-
pound to construct their shells. A few plants and animals use
silicon dioxide (SiO 2 ) for their skeletons, which accumulate
as mineral matter on the seafloor when the organisms die
(see Chapter 9).

Some clay minerals form when chemical processes
compositionally and structurally alter other minerals (see
Chapter 6), and others originate when rocks are changed
during metamorphism (see Chapter 7). In fact, the agents
that cause metamorphism—heat, pressure, and chemi-
cally active fluids—are responsible for the origin of many
minerals. A few minerals even originate when gases such
as hydrogen sulfide (H 2 S) and sulfur dioxide (SO 2 ) react
at volcanic vents to produce sulfur.

NATURAL RESOURCES
AND RESERVES
Geologists at the U.S. Geological Survey defi ne a resource
as a concentration of naturally occurring solid, liquid, or
gaseous material in or on Earth's crust in such form and
amount that economic extraction of a commodity from the
concentration is currently or potentially feasible.
Natural resources are mostly concentrations of minerals,
rocks, or both, but liquid petroleum and natural gas are also
included. In fact, some of the resources we refer to are metal-
lic resources (copper, tin, iron ore, etc.), nonmetallic resources