Granite
(Rock)
Quartz
(Mineral)
Hornblende
(Mineral)
Feldspar
(Mineral)
39
Minerals: Building Blocks of Rocks
3. Orderly crystalline structure. Minerals are
crystalline substances, which means their atoms are
arranged in an orderly, repetitive manner (FIGURE 2.2).
This orderly packing of atoms is reflected in the
regularly shaped objects called crystals. Some naturally
occurring solids, such as volcanic glass (obsidian), lack
a repetitive atomic structure and are not considered
minerals.
4. Generally inorganic. Inorganic crystalline solids,
such as ordinary table salt (halite), that are found
naturally in the ground are considered minerals.
(Organic compounds, on the other hand, are generally
not. Sugar, a crystalline solid like salt but which comes
from sugarcane or sugar beets, is a common example
of such an organic compound.) Many marine animals
secrete inorganic compounds, such as calcium carbonate (calcite), in the form of shells and coral reefs. If
these materials are buried and become part of the rock
record, they are considered minerals by geologists.
5. Can be represented by a chemical formula. Most
minerals are chemical compounds having compositions that can be expressed by a chemical formula. For
example, the common mineral quartz has the formula
SiO 2 , which indicates that quartz consists of silicon
(Si) and oxygen (O) atoms in a ratio of one-to-two.
This proportion of silicon to oxygen is true for any
sample of pure quartz, regardless of its origin. However, the compositions of some
minerals vary within specific, well-defined limits. This occurs because certain elements
can substitute for others of similar size without changing the mineral’ s internal structure. An example is the mineral olivine in which either the element magnesium (Mg)
or the element iron (Fe) may occupy the same site in the crystal structure. Therefore,
olivine’ s formula, (Mg, Fe) 2 SiO 4 , expresses variability in the relative amounts of magnesium and iron. However, the ratio of magnesium plus iron (
) to silicon (Si)
and oxygen (O) remains fixed at 2:1:4.
In contrast to minerals, rocks are more loosely defined. Simply, a
rock is any solid mass of mineral, or mineral-like, matter that occurs
naturally as part of our planet. Most rocks, like the common rock
granite shown in FIGURE 2.3, occur as aggregates of several different
minerals. The term aggregate implies that the minerals are joined in
such a way that their individual properties are retained. Note that the
mineral constituents of granite can be easily identified. However,
some rocks are composed almost entirely of one mineral.
A common example is the sedimentary rock limestone, which
consists of impure masses of the mineral calcite.
Mg + Fe
FIGURE 2.3 Most rocks
are aggregates of two or
more minerals. Shown
here is a hand sample of
the igneous rock granite
and three of its major
constituent minerals.
(Photos by E. J. Tarbuck)
D I D Y O U K N O W ?
Archaeological finds show that more
than 2000 years ago the Romans used
lead for pipes to transport water within
their buildings. In fact, Roman smelting
of lead and copper ores between
500 BC and AD 300 caused a small but
significant rise in atmospheric pollution,
as recorded in Greenland ice cores.
Some rocks are composed of nonmineral matter. These include the volcanic
rocks obsidian and pumice, which are noncrystalline glassy substances, and coal,
which consists of solid organic debris.
Although this chapter deals primarily
with the nature of minerals, keep in mind
that most rocks are simply aggregates of
A. Sodium and chlorine ions.
B. Basic building
block of the
mineral halite.
C. Collection of basic building
blocks (crystal).
D. Intergrown crystals of the mineral halite.
Na
+
Cl
–
FIGURE 2.2 This diagram illustrates the orderly arrangement of sodium and chloride ions
in the mineral halite. The arrangement of atoms into basic building blocks having a cubic
shape results in regularly shaped cubic crystals. (Photo by Dennis Tasa)
(Rock)
Quartz
(Mineral)
Hornblende
(Mineral)
Feldspar
(Mineral)
39
Minerals: Building Blocks of Rocks
3. Orderly crystalline structure. Minerals are
crystalline substances, which means their atoms are
arranged in an orderly, repetitive manner (FIGURE 2.2).
This orderly packing of atoms is reflected in the
regularly shaped objects called crystals. Some naturally
occurring solids, such as volcanic glass (obsidian), lack
a repetitive atomic structure and are not considered
minerals.
4. Generally inorganic. Inorganic crystalline solids,
such as ordinary table salt (halite), that are found
naturally in the ground are considered minerals.
(Organic compounds, on the other hand, are generally
not. Sugar, a crystalline solid like salt but which comes
from sugarcane or sugar beets, is a common example
of such an organic compound.) Many marine animals
secrete inorganic compounds, such as calcium carbonate (calcite), in the form of shells and coral reefs. If
these materials are buried and become part of the rock
record, they are considered minerals by geologists.
5. Can be represented by a chemical formula. Most
minerals are chemical compounds having compositions that can be expressed by a chemical formula. For
example, the common mineral quartz has the formula
SiO 2 , which indicates that quartz consists of silicon
(Si) and oxygen (O) atoms in a ratio of one-to-two.
This proportion of silicon to oxygen is true for any
sample of pure quartz, regardless of its origin. However, the compositions of some
minerals vary within specific, well-defined limits. This occurs because certain elements
can substitute for others of similar size without changing the mineral’ s internal structure. An example is the mineral olivine in which either the element magnesium (Mg)
or the element iron (Fe) may occupy the same site in the crystal structure. Therefore,
olivine’ s formula, (Mg, Fe) 2 SiO 4 , expresses variability in the relative amounts of magnesium and iron. However, the ratio of magnesium plus iron (
) to silicon (Si)
and oxygen (O) remains fixed at 2:1:4.
In contrast to minerals, rocks are more loosely defined. Simply, a
rock is any solid mass of mineral, or mineral-like, matter that occurs
naturally as part of our planet. Most rocks, like the common rock
granite shown in FIGURE 2.3, occur as aggregates of several different
minerals. The term aggregate implies that the minerals are joined in
such a way that their individual properties are retained. Note that the
mineral constituents of granite can be easily identified. However,
some rocks are composed almost entirely of one mineral.
A common example is the sedimentary rock limestone, which
consists of impure masses of the mineral calcite.
Mg + Fe
FIGURE 2.3 Most rocks
are aggregates of two or
more minerals. Shown
here is a hand sample of
the igneous rock granite
and three of its major
constituent minerals.
(Photos by E. J. Tarbuck)
D I D Y O U K N O W ?
Archaeological finds show that more
than 2000 years ago the Romans used
lead for pipes to transport water within
their buildings. In fact, Roman smelting
of lead and copper ores between
500 BC and AD 300 caused a small but
significant rise in atmospheric pollution,
as recorded in Greenland ice cores.
Some rocks are composed of nonmineral matter. These include the volcanic
rocks obsidian and pumice, which are noncrystalline glassy substances, and coal,
which consists of solid organic debris.
Although this chapter deals primarily
with the nature of minerals, keep in mind
that most rocks are simply aggregates of
A. Sodium and chlorine ions.
B. Basic building
block of the
mineral halite.
C. Collection of basic building
blocks (crystal).
D. Intergrown crystals of the mineral halite.
Na
+
Cl
–
FIGURE 2.2 This diagram illustrates the orderly arrangement of sodium and chloride ions
in the mineral halite. The arrangement of atoms into basic building blocks having a cubic
shape results in regularly shaped cubic crystals. (Photo by Dennis Tasa)