CHAPTER 4—ROCKS AND MINERALS: DOCUMENTS THAT
RECORD EARTH’S HISTORY
CHAPTER OVERVIEW
This chapter offers a brief review of physical geology presenting the “documents” in which Earth’s
geologic history is preserved. These documents are minerals, rocks, and the fossils embedded in rocks.
The basics of Earth materials are presented to set the stage for understanding events that have shaped
our planet. A comprehensive review of common rock forming minerals, their occurrence, chemical
composition, and usefulness in the interpretation of ancient geologic environments is presented. A review
of the three rock families—igneous, sedimentary, and metamorphic—is given using the rock cycle where
geologic processes act continuously to change one rock type to another. Additional information on the
three rock families is offered explaining how each rock type is reflective of the specific environmental
conditions present during that rock’s formation.
LEARNING OBJECTIVES
By reading and completing information within this chapter, you should gain an understanding of the
following concepts:







Describe common minerals found in rocks and their properties.
Explain how the common rock-forming minerals are formed.
Explain the characteristics of silicate and non-silicate minerals and how they are grouped.
Discuss the conditions under which rock-forming minerals occur.
Describe the characteristics that differentiate igneous, sedimentary, and metamorphic
rocks.
Sketch and label the rock cycle, showing the alternative routes that can be taken.
Describe the conditions under which igneous, sedimentary, and metamorphic rocks are
formed and discuss how their formation reflects past environments.
CHAPTER OUTLINE
I.
Minerals as Documents of Earth History
II.
Minerals and Their Properties
III.
Common Minerals that Form Rocks
A. Silicate Minerals
1. Quartz
2. Feldspar
3. Mica
4. Hornblende
5. Augite
6. Olivine
7. Clay Minerals
B. Nonsilicate Minerals
1. Carbonates, Calcite, and Dolomite
2. Evaporites
IV.
Earth’s Three Great Rock Families and How They Formed
V.
Igneous Rocks: “Fire-Formed”
A. What Does the Texture of Igneous Rocks Tell Us About Their History?
B. What Does the Composition of Igneous Rocks Tell Us About Their History?
C. Representative Igneous Rocks
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
Page 1
D. Some Minerals Form Early, Others Later
E. Sequence of Mineral Crystallization
F. Volcanic Activity Has Produced Enormous Volumes of Rock
VI.
Sedimentary Rocks: Layered Pages of History
A. Where Sediments Come From
B. Classifying Sedimentary Rocks
1. Clastic Sedimentary Rocks
2. Carbonate Sedimentary Rocks
3. Other Sedimentary Rocks
C. What Do Sedimentary Rocks Tell Us About Earth’s History?
VII.
Metamorphic Rocks: Changed Without Melting
A. Contact Metamorphism and Regional Metamorphism
B. Kinds of Metamorphic Rocks
1. Foliated Metamorphic Rocks
2. Nonfoliated Metamorphic Rocks
C. What Do Metamorphic Rocks Tell Us About Earth’s History?
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
Page 2
KEY TERMS (pages given in parentheses)
andesite (63): A volcanic rock that in chemical composition is intermediate between basalt and granite.
anhydrite (57): A granular, white, anhydrous (water-lacking) calcium sulfate (CaSO4).
aragonite (57): Calcium carbonate (the same composition as calcite, but a polymorph of calcite)
augite (55): A dark-colored, ferromagnesian mineral which is an important member of the pyroxene
family of minerals (silicates of aluminum, calcium, magnesium, and iron). It is stumpy in shape, with good
cleavage developed along two planes that are nearly at right angles.
basalt (61): A fine-grained extrusive igneous rock composed of ferromagnesian minerals and tiny
rectangular grains of plagioclase feldspars.
Bowen’s Reaction Series (63): A series of minerals wherein any early-formed phase will react with the
melt later in the differentiation to yield a new mineral further in the series.
calcite (56): Calcium carbonate, the predominant mineral in the carbonate rock limestone. One of the
most common carbonate minerals; a polymorph of aragonite (see aragonite above).
carbonate clasts (71): An individual grain, fragment, or constituent of detrital sediment or sedimentary
rock produced by physical breakdown of a larger mass.
carbonate spar (71): As viewed microscopically, the clear, crystalline carbonate that has been deposited
in a carbonate rock as a cement between clasts or has developed by re-crystallization of clasts.
chert (71): A dense, hard sedimentary rock or mineral composed of sum-micro-crystalline quartz. Unless
colored by impurities, chert is white, as opposed to flint, which is dark or black.
chlorite (72): Any of a group of greenish, platy, hydrous monoclinic silicates of aluminum, ferrous iron,
and magnesium which are closely associated with and resemble the micas.
clastic sediment (69): Texture that characterizes a rock made up of fragmental grains such as sand, silt,
or parts of fossils. Conglomerates, sandstones, and siltstones are clastic rocks; the individual clastic
grains are termed clasts.
clay mineral (55): Silicates of hydrogen and aluminum with additions of magnesium, iron, and potassium.
Their basic structure is similar to that of mica, but because individual flakes are extremely small, their
mica-like form can only be seen with an electron microscope.
cleavage (51): The tendency of a mineral to break along certain directions in its crystal structure where
the bonds between planes of ions are weakest.
color (50): A property of a mineral or rock that helps discern it from others. Color often reflects chemical
compositions. Note variations in color exist in many minerals, rocks.
coal (72): A carbonaceous rock resulting from the accumulation of plant matter in a swampy environment
combined with alteration of that plant tissue by both biochemical and physical processes until it is
converted to a consolidated carbon-rich material. The stages are peat, lignite, sub-bituminous,
bituminous, and anthracite, it is sedimentary in nature until it reaches the sub-bituminous stage, at which
point it becomes metamorphic.
contact metamorphism (72): Alteration of rock immediately adjacent to an igneous intrusion.
Compositional and textural changes occurring are largely the result of high temperatures and the
emanation of chemically active vapors that accompany igneous intrusions.
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
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continuous series (of Bowen’s Reaction Series ) (64): The branch of Bowen’s Reaction Series that
comprise the plagioclase group of minerals in which reaction of early formed crystals with later liquids
takes place without abrupt phase changes.
crystal habit (52): The way a mineral grows, through the regular addition of ions to its surfaces from a
surrounding rock melt (magma), solution, or gas.
density (51): Is a mineral’s mass per unit volume (density = mass/volume).
diorite (63): A phaneritic, plutonic rock with granular texture composed largely of plagioclase feldspar
with smaller amounts of dark-colored minerals; used occasionally as ornamental and building stone. Also
know as black granite.
discontinuous series (of Bowen’s Reaction Series ) (64): The branch of Bowen’s Reaction Series that
includes the minerals olivine, pyroxene, amphibole, and biotite. Each change in the series representing
an abrupt phase change.
dolomite (57): A carbonate sedimentary rock that contains more than 50% of the mineral dolomite
CaMg(CO3)2.
evaporite (57): Sediments precipitated from a water solution as a result of the evaporation of that water.
Evaporite minerals include anhydrite gypsum (CaSO4) and halite (NaCl).
extrusive igneous rock (59): Igneous rocks formed from melts that have reached the Earth’s surface.
Examples are rocks formed from lava erupted from volcanoes or lava that has welled out of fissures.
feldspar (52): A group of silicate minerals that make up about 60% of the outer 9 miles (15 kilometers) of
the Earth’s crust.
ferromagnesian silicate mineral (=mafic silicate mineral) (55): Hornblende is designated a
ferromagnesian or mafic silicate mineral because of its content of iron and magnesium.
foliation (in metamorphic rocks) (73): A textural feature especially characteristic of metamorphic rocks
in which laminae develop by growth or realignment of minerals in parallel orientation. Examples includes,
slate, phyllite, schist, and gneiss.
fractional crystallization (66): The separation of components of a cooling magma by sequential
formation of particular mineral crystals at progressively lower temperatures.
gabbro (62): A group of dark-colored, intrusive, igneous rocks with granular texture, composed largely of
basic plagioclase and clinopyroxene.
garnet (72): A family of aluminosilicates of iron and calcium that are particularly characteristic of
metamorphic rocks.
granite (61): A silica-rich, relatively light-colored, intrusive igneous rock composed primarily of potassium
feldspar, quartz (at least 25%), sodium plagioclase, hornblende, and mica.
granodiorite (62): Abundant quart-bearing, intrusive igneous rock, with plagioclase being the dominant
feldspar.
greenstone (75): A non-foliated, low-grade metamorphic rock, which is dark green in color due to the
presence of abundant chlorite, epidote, and biotite. It is derived from metamorphism of basalitic extrusive
igneous rocks. Great linear outcrops of greenstones are termed green stone belts and are through to
mark the locations of former volcanic island arcs. The texture is so fine that mineral components cannot
be seen without magnification (except for larger crystals).
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
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gypsum (57): A soft, hydrous, calcium sulfate. Referred to as an evaporate because it is often
precipitated from bodies of water that have been subjected to intense evaporation.
halite (57): A non-silicate mineral. Also known as rock salt. This mineral has a salty taste and cleaves to
form cubes. Referred to as an evaporate because it is often precipitated from bodies of water that have
been subjected to intense evaporation.
hardness (51): A mineral’s resistance to scratching by another substance of known hardness.
hornblende (54): A vitreous, black or very dark green mineral. Most common member of the larger family
of minerals called amphiboles. Designated as a ferromagnesian, or mafic mineral, because of its iron and
magnesium content. It contains crystals that are long and narrow and shows two good cleavage planes
parallel to the long axis which intersect each other at angles,
hornfels (75): A non-foliated metamorphic rock, which is very hard and fined grained. It is often studded
with small crystals of mica and garnet having no preferred orientation. May form from shale or other finegrained rocks that are intensely heated during contact metamorphism of intrusive igneous bodies.
igneous rock (58): A rock formed by the cooling and solidification of magma or lava. They constitute over
90% of the volume of the Earth’s crust.
intrusive igneous rock (59): Igneous rocks that have formed from magma that has penetrated into other
rocks and solidified before reaching the surface. Very large masses of such rocks are called plutons.
lava (59): The term used to describe molten material that has reached the surface of the Earth.
lithification (67): The process by which loose sediment is converted to coherent solid rock by any of
several processes: precipitation of a cementing material around individual grains, compaction, or
crystallization.
luster (51): The way a mineral shines in reflected light.
mafic silicate (55): Rocks (or lava) dominated by dark iron and magnesium silicates, as is the oceanic
crust.
magma (59): the term used to describe the mixture of molten silicates and gases while still beneath the
surface.
marble (74): A non-foliated metamorphic rock composed of calcite or dolomite, with a fine to coarsely
crystalline texture. Marble is derived from limestone or dolomite.
metamorphic index mineral (72): Metamorphic minerals that are known to form within a specific
temperature and pressure range.
metamorphic rock (58): Any rock that has been changed from previously existing rocks by the action of
heat, pressure, and associated chemical activity. The parent rock is subjected to high temperature and
pressure but does not melt.
metamorphism (72): The transformation of previously existing rocks into new types by the action of heat,
pressure, and chemical solutions. Metamorphism usually takes place at depth in the roots of mountain
chains or adjacent to large intrusive igneous bodies.
mica (54): A silicate mineral easily recognized by its perfect and conspicuous cleavage along one
directional plane. There are two chief varieties: the colorless or pale-colored muscovite mica, which is a
hydrous potassium aluminum silicate, and the dark-colored biotite mica, which also contains magnesium
and iron. Both are common rock-forming silicates.
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
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micrite (71): An exceptionally fine-grained carbonate mud (limestone).
mineral (50): A naturally occurring, solid, inorganic substance having a specific chemical composition
and a specific crystal structure.
nonsilicate mineral (56): Those minerals that do not contain the silicon-oxygen structures that
characterize silicate minerals.
nonfoliated metamorphic rocks (74): A metamorphic texture in which there is no discernable preferred
orientation of mineral grains. Examples are marble or quartzite.
obsidian (61): A jet-black volcanic glass, usually of rhyolitic composition, formed by rapid cooling of
viscous lava; generally forms the upper parts of lava flows.
olivine (56): A glassy looking iron and magnesium silicate, often with an olive green color. Common
mineral in dark, mafic rocks. The gem variety is called peridot.
oöid (71): Found in back-reef facies along with carbonate sands and muds.
oölite (71): Limestones composed largely of small, round or ovate calcium carbonate bodies called oöids.
partial melting (66): The general process by which a rock subjected to high temperature and pressure is
partly melted and the liquid component is moved to another location. Partial melting results from the
variations in melting points of different minerals in the original rock mass.
phyllite (74): Has a very fine texture, although some grains of mica, chlorite, garnet, or quartz may be
visible. Surfaces often develop a wrinkled aspect and are more lustrous than slate. Phyllite is intermediate
in degree of metamorphism between slate and schist. Parent rocks are commonly shale or slate.
pluton (59): A body of igneous rock that has formed beneath the surface of the earth by consolidation
from magma.
plutonic igneous rock (59): Igneous rock that cools and crystallizes deep underground.
porphyritic texture (61): Texture shown due to a two-stage cooling of an igneous rock. The large
crystals (phenocrysts) were formed slowly at depth and were then swept upward and incorporated in the
lava as it hardened at the surface (more rapid cooling).
pumice (61): A rock froth, formed by the extreme puffing up of liquid lava by expanding gases liberated
from solution in the lava prior to and during solidification. Also known as foam; pumice stone; pumicite;
volcanic foam.
quartz (52): Most important of all the silicate minerals. In appearance, quartz is a glassy, colorless, gray,
or white mineral. It is relatively hard and will scratch glass.
quartzite (75): A non-foliated metamorphic rock which is fine-grained and often sugary-textured. It is
composed of inter-grown quartz and is very hard. Derived from quartz sandstone and may vary in color.
regional metamorphism (73): A type of metamorphic rock alteration that is really extensive and occurs
under the conditions of great confining pressures and heat accompanying deep burial and mountain
building.
rhyolite (61): A light-colored, aphanitic volcanic rock composed largely of alkali feldspar and free silica
with minor amounts of mafic minerals; the extrusive equivalent of granite.
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
Page 6
schist (74): Consists of platy or needle-like minerals sufficiently large to be visible to the unaided eye.
Minerals tend to be segregated into distinct layers. Schists are named according to the most conspicuous
mineral present. The parent rock is usually shale, although some are derived from fine-grained volcanic
rocks.
sedimentary rock (58): A rock that has formed as a result of the consolidation (lithification) of
accumulations of sediment.
sillimanite (72): A brown, pale-green, or white neosilicate mineral with vitreous luster crystallizing in the
orthorhombic system; commonly occurs in slender crystals, often in fibrous aggregates; hardness is 6-7
on Mohs scale, and specific gravity is 3.23.
slate (74): Foliated metamorphic rock with the foliation being microscopic and caused by the parallel
alignment of minute flakes of silicates. Planes of foliation are smooth and the rock may split along these
planes of “slaty cleavage.” Derived from the regional metamorphism of shale.
strata (67): A feature of sedimentary rocks in which the rocks occur in beds or layers called strata.
streak (of mineral) (51): The color of a mineral when it is ground to powder, or rubbed against a streak
plate.
texture (of rock) (58): The size, shape, and arrangement of constituent minerals in a rock.
tuff (61): Volcanic ash that has become consolidated into rock.
vesicles (61): A small cavity or cavities formed by the entrapment of gas bubbles or steam during the
solidification of a rock.
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
Page 7
MULTIPLE-CHOICE QUESTIONS
1.
The rock cycle shows us that
a. metamorphic rocks can only be derived from igneous rocks.
b. metamorphic rocks are derived from magma.
c. any rock type can be derived from any other rock type.
d. igneous rocks are composed of quartz and gabbro.
2.
A textural term used to describe an igneous rock with large crystals called phenocrysts in a matrix
of more finely crystalline rock is
a. phenocryptic.
c. porphyritic.
b. polymorphic.
d. dolomitic.
3.
Which of the following is a foliated metamorphic rock?
a. schist
c. gabbro
b. shale
d. marble
4.
A carbonate mineral that occurs in a different crystal form and is less common than either calcite or
dolomite is
a. aragonite.
c. calcite.
b. gypsum.
d. silica.
5.
An example of a sedimentary evaporite is
a. calcite.
c. limestone.
b. dolomite.
d. gypsum.
6.
Rocks that cool from a melt are
a. metamorphic rocks.
b. igneous rocks.
c. sedimentary rocks.
d. hard rocks.
e. on the rocks.
7.
Rocks that have recrystallized under high temperatures and pressures are
a. metamorphic rocks.
d. granite.
b. igneous rocks.
e. basalt.
c. sedimentary rocks.
8.
The size, shape, and arrangement of mineral grains in a rock constitutes its
a. permeable origin.
c. cement.
b. porosity.
d. texture.
9.
A clastic rock composed of water-worn, rounded particles larger than 2 mm in diameter is
a. mudstone.
c. breccia.
b. siltstone.
d. conglomerate.
10.
The overwhelmingly predominant constituent of an igneous rock is
a. silicon.
c. aluminum.
b. oxygen.
d. calcium.
11.
Which of the following is one of the eight most common elements in the earth’s crust?
a. nitrogen
d. thorium
b. potassium
e. hydrogen
c. uranium
12.
The fine extrusive equivalent of granite is
a. diorite.
d. basalt.
b. rhyolite.
e. gabbro.
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
Page 8
c.
andesite.
13.
The ferromagnesium silicate mineral that occurs commonly in igneous and metamorphic rock is
a. amphibole.
d. sylvite.
b. pyrite.
e. goethite.
c. sphalerite.
14.
An igneous rock that is an extrusive is
a. rhyolite.
b. gabbro.
c. granite.
d. conglomerate.
e. schist.
15.
Because of its iron and magnesium content which one of the following is considered a
ferromagnesium (mafic) mineral?
a. orthoclase
c. hornblende
b. plagioclase
d. obsidian
16.
The elements that comprise 60% of earth’s crust and are silicates are called
a. dolomites.
d. feldspars.
b. carbonates.
e. quartz.
c. sulfates.
17.
The most common extrusive rock composed of fine mineral crystals is
a. basalt.
d. diorite.
b. gabbro.
e. conglomerate.
c. rhyolite.
18.
Diorite is a relatively coarse-grained intrusive igneous rock composed mainly of plagioclase
feldspar. Its fine grained equivalent is
a. basalt.
c. pumice.
b. andesite.
d. rhyolite.
19.
A biochemical rock made from microscopic nonsiliceous plant remains is
a. coal.
d. rock salt.
b. chalk.
e. siltstone.
c. chert.
20.
Silicates of hydrogen and aluminum with additions of magnesium, iron and potassium are called
a. nonsilicate minerals.
c. mica.
b. olivine.
d. clay minerals.
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
Page 9
FILL IN THE BLANK
1.
A naturally occurring element or compound formed by inorganic processes that has a definite
chemical composition or range of compositions as well as distinctive properties that reflect its
internal atomic structure is
.
2.
The size, shape, and arrangement of constituent minerals in a rock is called
.
3.
Rocks that have cooled from a molten state are
.
4.
Halite and the various gypsum minerals are sometimes referred to as
because they are often precipitated from bodies of water that have been subjected to intense
evaporation.
5.
Rocks that have been changed from previously existing rocks by the action of heat, pressure, and
associated chemical activity are
.
6.
Orthoclase and plagioclase are members of this major group of silicate minerals called
__________.
7.
The minerals that are silicates of hydrogen and aluminum with additions of magnesium, iron, and
potassium are called
.
8.
A carbonate mineral that occurs more rarely than either calcite or dolomite is
9.
A silica-rich, relatively light-colored intrusive rock composed primarily of potassium feldspar, quartz,
sodium plagioclase, hornblende, and mica is
.
10.
The general process by which a rock subjected to high temperature and pressure is partly melted
and the liquid component is moved to another location is
.
11.
Loose sediment is converted to coherent solid rock by any of several processes: precipitation of a
cementing material around individual grains, compaction, or crystallization. These processes
constitute
.
12.
Alterations of rock immediately adjacent to igneous intrusions constitute this type of metamorphism.
.
The texture of a rock that is composed of grains and broken fragments (clasts) of pre-existing
minerals, rocks, and fossils is called
.
13.
.
14.
The term reserved for clastic rocks composed of fragments that are angular but similar in size to
those of conglomerates is
.
15.
A soft, porous variety of limestone that is composed largely of extremely minute calcareous skeletal
elements is called
.
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
Page 10
TRUE/FALSE
1.
Lava is the term used to describe a mixture of molten silicates and gases while it is still
beneath the surface.
2.
Regional metamorphism is a type of rock alteration that is really extensive and occurs under
the conditions of great confining pressures and heat accompanying deep burial and mountain
building.
3.
The common foliated metamorphic rocks are gneiss and marble.
4.
Most metamorphic rocks exhibit a layering called foliation which results from the parallel
alignment of mineral grains.
5.
Slate is a nonfoliated metamorphic rock that is composed of calcite or dolomite and therefore
relatively soft.
6.
All sedimentary rocks are classified according to their composition and texture.
7.
The most abundant limestones are of marine origin and have formed as a result of
precipitation of calcite or aragonite by organisms that have died and settled to the sea floor.
8.
Evaporites are chemically precipitated rocks that are formed as a result of evaporation of
saline water bodies.
9.
The span of time through which the earth has evolved is termed geologic time.
10.
Sedimentary rocks are formed by the accumulation and consolidation of the products of
weathering derived from older rock masses, as well as by chemical precipitation and the
accumulation of organic debris.
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
Page 11
ANSWER KEY
Multiple Choice
1. c
2. c
3. a
4. a
5. d
6. b
7. a
8. d
9. d
10. a
11. b
12. b
13. a
14. a
15. c
16. d
17. a
18. b
19. a
20. d
Fill Ins
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
mineral
texture
igneous
evaporites
metamorphic
feldspar
clay minerals
aragonite
granite
partial melting
lithification
contact
clastic
breccia
calk
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
True/False
1. F
2. T
3. F
4. T
5. F
6. T
7. T
8. T
9. T
10. T
Page 12
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
RESPONSES TO QUESTIONS ACCOMPANYING SELECTED FIGURES
FIGURE 4–13 (p. 55) Iron and magnesium are generally abundant elements in dark-colored (dark gray,
black, or green) igneous rocks.
FIGURE 4–22 (p. 60) Porphyritic texture results when a magma is initially cooling very slowly so that lare
crystals (phenocrysts) have ample time to grow in the melt, and then rapidly cooled (as in a volcanic
eruption) so that the surrounding melt solidifies as a finely crystalline mass. As indicated in the legend,
the phenocrysts are likely to be orthoclase.
FIGURE 4–25 (p. 62) According to the chart,‘midrange basalt’ would be composed of about 55 percent
calcium plagioclase, 30 percent pyroxene, and 15 percent olivine.
FIGURE 4–27 (p. 64) Amphibole and biotite are likely to form when pyroxene crystals react with liquid
magma. Plagioclase in granite is mostly likely to be of sodium rich variety.
FIGURE 4–33 (p. 68) Clay is the most abundant insoluble product resulting from the weathering of
feldspars. Quartz is the most stable and least likely to experience dissolution during chemical weathering.
FIGURE 4–35 (p. 69) Conglomerate is composed of rounded fragments greater than 2 mm in diameter
cemented in a fine-grained matrix of finer particles. In a breccia, the fragments are of the same size but
are angular.
FIGURE 4–47 (p. 76) Quartzite has the same hardness as quartz and will scratch plate glass. In addition,
quartzite will not effervesce on the application of dilute hydrochloric acid (as does marble).
Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
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