55
Important Nonsilicate Minerals
3.2 and 3.6, than nonferromagnesian
silicates. The most common dark silicate
minerals are olivine, the pyroxenes, the
amphiboles, dark mica (biotite), and garnet.
OLIVINE GROUP. Olivine is a family of
high-temperature silicate minerals that are
black to olive green in color and have a
glassy luster and a conchoidal fracture
(see Figure 2.24). Transparent olivine is
occasionally used as a gemstone called
peridot. Rather than developing large
crystals, olivine commonly forms small,
rounded crystals that give olivine-rich
rocks a granular appearance. Olivine and
related forms are thought to constitute up
to 50 percent of Earth’ s upper mantle.
PYROXENE GROUP. The pyroxenes are a
group of complex minerals that are important components in dark colored igneous
rocks. The most common member, augite,
is a black, opaque mineral with two
directions of cleavage that meet at nearly a
90-degree angle. Augite is one of the
dominant minerals in basalt, a common
igneous rock of the oceanic crust and
volcanic areas on the continents.
AMPHIBOLE GROUP. Hornblende is the
most common member of a chemically
complex group of minerals called
amphiboles (see Figure 2.24). Hornblende
is usually dark green to black in color, and
except for its cleavage angles, which are
about 60 degrees and 120 degrees, it is
very similar in appearance to augite. In a
rock, hornblende often forms elongated
crystals. This helps distinguish it from
pyroxene, which forms rather blocky
crystals. Hornblende is found in igneous
rocks, where it often makes up the dark
portion of an otherwise light-colored rock.
BIOTITE. Biotite is the dark, iron-rich
member of the mica family (see Figure
2.24). Like other micas, biotite possesses
a sheet structure that gives it excellent
cleavage in one direction. Biotite also has
a shiny black appearance that helps
distinguish it from the other dark
ferromagnesian minerals. Like hornblende,
biotite is a common constituent of igneous
rocks, including the rock granite.
GARNET. Garnet is similar to olivine in
that its structure is composed of individual
tetrahedra linked by metallic ions. Also like
olivine, garnet has a glassy luster, lacks
cleavage, and exhibits conchoidal fracture.
Although the colors of garnet are varied,
this mineral is most often brown to deep
red. Garnet readily forms equidimensional
crystals that are most commonly found in
metamorphic rocks. When garnets are
transparent, they may be used as
gemstones.
C O N C E P T C H E C K 2 . 8
What do ferromagnesian minerals have in
common? List three examples of ferromagnesian minerals.
What do muscovite and biotite have in
common? How do they differ?
Should color be used to distinguish
between orthoclase and plagioclase
feldspar? What is the most effective means
of distinguishing them?
Important Nonsilicate
Minerals
Matter and Minerals
Mineral Groups
Nonsilicate minerals are typically divided
into groups, based on the negatively
charged ion or complex ion that the
members have in common (TABLE 2.1). For
example, the oxides contain the negative
oxygen ion (
), which is bonded to one
or more kinds of positive ions. Thus, within
each mineral group, the basic structure and
type of bonding is similar. As a result, the
minerals in each group have similar
physical properties that are useful in
mineral identification.
Although the nonsilicates make up
only about 8 percent of Earth’ s crust,
some minerals, such as gypsum, calcite,
and halite, occur as constituents in sedimentary rocks in significant amounts.
Furthermore, many others are important
economically. Table 2.1 lists some of the
nonsilicate mineral groups and a few examples of each. A brief discussion of a few of
the more common nonsilicate minerals
follows.
O
2GEODe
ESSENTIALS
OF GEOLOGY
3
2
1
Some of the most common nonsilicate
minerals belong to one of three classes of
minerals—the carbonates (
), the sulfates (
), and the halides (
,
,
). The carbonate minerals are much
simpler structurally than the silicates. This
mineral group is composed of the carbonate ion (
) and one or more kinds of
positive ions. The two most common carbonate minerals are calcite, CaCO 3 (calcium
carbonate), and dolomite, CaMg(CO 3 ) 2
(calcium/magnesium carbonate). Because
these minerals are similar both physically
and chemically, they are difficult to distinguish from each other. Both have a vitreous
luster, a hardness between 3 and 4, and
nearly perfect rhombic cleavage. They can,
however, be distinguished by using dilute
hydrochloric acid. Calcite reacts vigorously
with this acid, whereas dolomite reacts
much more slowly. Calcite and dolomite
are usually found together as the primary
constituents in the sedimentary rocks limestone and dolostone. When calcite is the
dominant mineral, the rock is called
limestone, whereas dolostone results from a
predominance of dolomite. Limestone has
many uses, including as road aggregate, as
building stone, and as the main ingredient
in Portland cement.
Two other nonsilicate minerals frequently found in sedimentary rocks are
halite and gypsum. Both minerals are
commonly found in thick layers that are
the last vestiges of ancient seas that have
CO 3
2Br
1F
1Cl
1SO 4
2CO 3
2D I D Y O U K N O W ?
The material we commonly call motherof-pearl is a carbonate mineral called
aragonite. Mother-of-pearl is produced
by a large group of marine organisms
commonly called clams (mollusks).
D I D Y O U K N O W ?
The mineral pyrite is commonly known
as “fool’s gold,” because its goldenyellow color can lead to its being
mistaken for gold. The name pyrite is
derived from the Greek pyros (“fire”),
because it gives off sparks when
struck sharply.
Important Nonsilicate Minerals
3.2 and 3.6, than nonferromagnesian
silicates. The most common dark silicate
minerals are olivine, the pyroxenes, the
amphiboles, dark mica (biotite), and garnet.
OLIVINE GROUP. Olivine is a family of
high-temperature silicate minerals that are
black to olive green in color and have a
glassy luster and a conchoidal fracture
(see Figure 2.24). Transparent olivine is
occasionally used as a gemstone called
peridot. Rather than developing large
crystals, olivine commonly forms small,
rounded crystals that give olivine-rich
rocks a granular appearance. Olivine and
related forms are thought to constitute up
to 50 percent of Earth’ s upper mantle.
PYROXENE GROUP. The pyroxenes are a
group of complex minerals that are important components in dark colored igneous
rocks. The most common member, augite,
is a black, opaque mineral with two
directions of cleavage that meet at nearly a
90-degree angle. Augite is one of the
dominant minerals in basalt, a common
igneous rock of the oceanic crust and
volcanic areas on the continents.
AMPHIBOLE GROUP. Hornblende is the
most common member of a chemically
complex group of minerals called
amphiboles (see Figure 2.24). Hornblende
is usually dark green to black in color, and
except for its cleavage angles, which are
about 60 degrees and 120 degrees, it is
very similar in appearance to augite. In a
rock, hornblende often forms elongated
crystals. This helps distinguish it from
pyroxene, which forms rather blocky
crystals. Hornblende is found in igneous
rocks, where it often makes up the dark
portion of an otherwise light-colored rock.
BIOTITE. Biotite is the dark, iron-rich
member of the mica family (see Figure
2.24). Like other micas, biotite possesses
a sheet structure that gives it excellent
cleavage in one direction. Biotite also has
a shiny black appearance that helps
distinguish it from the other dark
ferromagnesian minerals. Like hornblende,
biotite is a common constituent of igneous
rocks, including the rock granite.
GARNET. Garnet is similar to olivine in
that its structure is composed of individual
tetrahedra linked by metallic ions. Also like
olivine, garnet has a glassy luster, lacks
cleavage, and exhibits conchoidal fracture.
Although the colors of garnet are varied,
this mineral is most often brown to deep
red. Garnet readily forms equidimensional
crystals that are most commonly found in
metamorphic rocks. When garnets are
transparent, they may be used as
gemstones.
C O N C E P T C H E C K 2 . 8
What do ferromagnesian minerals have in
common? List three examples of ferromagnesian minerals.
What do muscovite and biotite have in
common? How do they differ?
Should color be used to distinguish
between orthoclase and plagioclase
feldspar? What is the most effective means
of distinguishing them?
Important Nonsilicate
Minerals
Matter and Minerals
Mineral Groups
Nonsilicate minerals are typically divided
into groups, based on the negatively
charged ion or complex ion that the
members have in common (TABLE 2.1). For
example, the oxides contain the negative
oxygen ion (
), which is bonded to one
or more kinds of positive ions. Thus, within
each mineral group, the basic structure and
type of bonding is similar. As a result, the
minerals in each group have similar
physical properties that are useful in
mineral identification.
Although the nonsilicates make up
only about 8 percent of Earth’ s crust,
some minerals, such as gypsum, calcite,
and halite, occur as constituents in sedimentary rocks in significant amounts.
Furthermore, many others are important
economically. Table 2.1 lists some of the
nonsilicate mineral groups and a few examples of each. A brief discussion of a few of
the more common nonsilicate minerals
follows.
O
2GEODe
ESSENTIALS
OF GEOLOGY
3
2
1
Some of the most common nonsilicate
minerals belong to one of three classes of
minerals—the carbonates (
), the sulfates (
), and the halides (
,
,
). The carbonate minerals are much
simpler structurally than the silicates. This
mineral group is composed of the carbonate ion (
) and one or more kinds of
positive ions. The two most common carbonate minerals are calcite, CaCO 3 (calcium
carbonate), and dolomite, CaMg(CO 3 ) 2
(calcium/magnesium carbonate). Because
these minerals are similar both physically
and chemically, they are difficult to distinguish from each other. Both have a vitreous
luster, a hardness between 3 and 4, and
nearly perfect rhombic cleavage. They can,
however, be distinguished by using dilute
hydrochloric acid. Calcite reacts vigorously
with this acid, whereas dolomite reacts
much more slowly. Calcite and dolomite
are usually found together as the primary
constituents in the sedimentary rocks limestone and dolostone. When calcite is the
dominant mineral, the rock is called
limestone, whereas dolostone results from a
predominance of dolomite. Limestone has
many uses, including as road aggregate, as
building stone, and as the main ingredient
in Portland cement.
Two other nonsilicate minerals frequently found in sedimentary rocks are
halite and gypsum. Both minerals are
commonly found in thick layers that are
the last vestiges of ancient seas that have
CO 3
2Br
1F
1Cl
1SO 4
2CO 3
2D I D Y O U K N O W ?
The material we commonly call motherof-pearl is a carbonate mineral called
aragonite. Mother-of-pearl is produced
by a large group of marine organisms
commonly called clams (mollusks).
D I D Y O U K N O W ?
The mineral pyrite is commonly known
as “fool’s gold,” because its goldenyellow color can lead to its being
mistaken for gold. The name pyrite is
derived from the Greek pyros (“fire”),
because it gives off sparks when
struck sharply.