woods 2009 - physical geology - laboratory manual 4th ed

7/30/2019 Woods 2009 - Physical Geology - Laboratory Manual 4th Ed

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PHYSICAL

GEOLOGYLABORATORY

MANUALFourth Edition

Karen M. Woods

Lamar University

Contributing Authors

Margaret S. Stevens

James B. Stevens

Roger W. Cooper

Donald E. Owen

James Westgate

Jim L. Jordan

Bennetta Schmidt

K E N D A L L / H U N T P U B L I S H I N G C O M P A N Y4 0 5 0 W e s t m a r k D r i v e D u b u q u e , I o w a 5 2 0 0 2


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Copyright 1994, 1997, 2001, 2006 by Kendall/Hunt Publishing Company

Revised Printing 2009

ISBN: 978-0-7575-6114-6

All rights reserved. No part of this publication may be reproduced,stored in a retrieval system, or transmitted, in any form or by anymeans, electronic, mechanical, photocopying, recording, or otherwise,without the prior written permission of Kendall/Hunt Publishing Company.

Printed in the United States of America

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Chapter 5 Streams, Rivers, and Landscapes 129

Water Cycle 129

Streams and Rivers General Terminology 129

Rivers and Erosion: Development of Landscapes 131

Stream Drainage Patterns 134

Chapter 6 Groundwater, Karst Topography, andSubsidence 139

Groundwater 139

Caves and Karst Topography 141

Karst Topography 141

Subsidence 143

Chapter 7 Shorelines 149

General Shoreline Features 149Sea-Level Changes: Eustatic, Local, and Regional 151

Emergent Shorelines, Causes and Characteristics 152

Submergent Shorelines, Causes and Characteristics 153

References 159

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Physical Geology is the first introductory course in the field of Geology. The faculty and staff

of Lamar University, Department of Earth and Space Sciences have collaborated to produce

a laboratory manual that is informative and easily understood. It has been customized to

present the concepts and ideas the faculty feel are most important in Physical Geology. It is

intended to supplement the main lecture course by exposing the student to conceptual

exercises and hands-on experience of the subjects introduced in lecture.


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INTRODUCTIONGeology deals with the physical and historical aspects of the Earth. Physical geology is the

study of the composition, behavior, and processes that affect the Earth's lithosphere. The

science of geology also provides the means to discover and utilize the Earth's natural re

sources (coal, gas, petroleum, minerals, etc.). Geologists also study the Earth and its

processes so that they can better understand and predict potentially dangerous geologicsituations (earthquakes, volcanic eruptions, floods, etc.), which results in the saving of

lives. Historical geology, the second introductory course, deals with geology as it relates to

the Earth's history.

This laboratory manual begins with the study of common Earth materials, minerals,

and rocks that make up the lithosphere, and proceeds to the kinds of forces and situations

that can alter (build up or tear down) the surface of the planet.

MINERALS

Minerals are the basic building blocks of nearly all Earth materials for most geological

purposes. A mineral is a naturally occurring, solid, inorganic combination (compound)of one or more elements, whose atoms are arranged in an orderly fashion (crystallinity),

and has an established chemical composition that can vary slightly within specific limits.

Minerals also have a set of physical properties (hardness, color, etc.) that distinguish them

from each other. "Inorganic" means that the compound was not the result of organicprocesses.

Natural compounds are not "pure" in the pharmaceutical sense, particularly if mod

ern analytical methods are used. Most chemical elements can be shown to consist of sev

eral "isotopes," atoms of different atomic weights that have a closely similar set ofchemical properties. Minerals as natural compounds are fairly complicated. They consistof one or more elements that consist of one or more isotopes, are not absolutely "pure"

compounds, and show some variation, even within materials called by the same mineral

name. The guideline geologists have agreed on to define a particular mineral is the nature

of the internal geometric arrangement (the crystallinity) of the atoms. This arrangementis usually called the crystal st ru cture (technically, the term "crystal structure" is

redundantthe word "crystal" by itself is sufficient). Materials such as glass and opal have

no particular geometric arrangement of their atoms, and are not true minerals because they

lack crystallinity. The term "mineraloid" is used for these materials, and some mineraloids

are simply called rocks (natural glass, obsidian, is a kind of volcanic rock).

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SUMMARY: a material must be/have the following characteristics to be classified as a

mineral:

1. be naturally occurring (not man-made).

2. be solid.

3. be inorganic (not compounds that can be produced only by living organisms).

4. have a geometric arrangement of its atomscrystallinity.

5. have a chemical composition that can vary only according to specific limits.

A substance that satisfies these requirements will have a characteristic set of physical

properties that can be used for identification.

Common Minerals

Many of the minerals studied in the laboratory (Table 1.1) are familiar to nongeologists.

Some elemental materials (sulfur, graphite, and diamond) are classified as minerals when

found in large, natural cohesive quantities. Quartz (Si02, silicon dioxide) is the most com

monly known mineral. Varieties of quartz include: rose quartz, milky quartz, chert (in

many different colors), flint, agate, rock crystal (clear), amethyst (purple), aventurine

(green), jasper (red), etc. Halite (NaCl, sodium chloride) is probably the most commonly

used mineral and is found in most spice cabinets as table salt. Minerals have many unexpected uses and a list of some of these uses is found at the end of this chapter.

Physical Propert ies of Minerals

All minerals have a set of distinctive physical properties that can be used to identify them.The goal of the student is to become familiar with geological terminology and apply the

terms to unknown mineral specimens in order to correctly identify them.

Students should note that the physical properties of each different mineral groupare not absolutes. Hardness is one property that can vary from sample to sample of the samemineral. The mineral magnetite has a hardness of 6, but it can actually range between 5.5

and 6.5. Therefore, some specimens of magnetite will easily scratch a glass plate (hardness

Ss 6) and some specimens may barely scratch glass or not scratch it at all. Color is another

property of minerals that can vary widely and thus should not be the only criterion usedfor identification of an unknown mineral specimen. Quartz comes in many different col

ors and is easily confused with other minerals of similar color. Amethyst purple quartz is

easily mistaken for purple fluorite, and vice versa. The student should not use any one

property alone to identify unknown minerals. A group of physical properties leads to amore accurate identification.

Crystal Form

Crystal form is the geometric arrangement of plane ("flat") surfaces on the outside of amineral that reflect the internal crystallinity of the mineral (Fig. 1.1a and Fig. 1.1b). Crys

tal faces develop only when the crystal has enough room to grow without interference. The

planar (flat) sides of a cube, for example, are called faces. A cube is a crystal form that hassix faces (flat sides) (Fig. 1.1a). Halite and fluorite often have cubic ciystal form, while gar

net and pyrite have more complicated crystal forms that are variations on the cube. Corun

dum, quartz, and calcite show different variations on the hexagonal (six-sided) ciystal form

(Fig. 1.1b). The hexagonal form of calcite (Fig 1.1b) is the most difficult of these to see,

but a calcite crystal will have one or two sharp points, and if one looks along the line between these two points, the visible outline is hexagonal. Minerals without an external crys

tal form are referred to as massive (chert, limonite, etc.).

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TABLE 1.1 Chemical Gro ups of Selected Minerals

Chemical Class Mineral/Mineraloid Chemical Composition

Natives Sulfur S (Sulfur)

Only one kind of element present, Graphite/diamond (not available) c (Carbon)"naturally pure"

Oxides Quartz (quartz crystal, milky,

rose, chert , smoky, agate, etc.) Si 02 (Silicon dioxide)

Oxides of Iron:

(A metal bonds directly with oxygen Oolitic Hematite Fe203 (Iron oxide)

as the nonmetal) Specular Hematite Fe 203 (Iron oxide)

Goethite FeO(OH) (Hydrous iron oxide)

Limonite (mineraloid) Fe20 3nH20 (Hydrous iron oxide)

Magnetite Fe304 (Iron oxide)

Corundum A1203 (Aluminum oxide)

Bauxite (mineraloid) Al 203nH20 (Hydrous Al oxide)

Sulphides Pyrite FeS2 (Iron sulfide)

(A metal bonds directly with sulfur Galena PbS (Lead sulfide)

as the nonmetal) Sphalerite ZnS (Zinc sulfide)

Sulfates Gypsum (Selenite, Satin spar, CaS04 2H 20 (Hydrous calcium(A metal bonds with the S0 4 Alabaster) sulfate)

complex ion acting as a nonmetal) Anhydrite CaS04 (Calcium sulfate)

Carbonates Calcite GaC03 (Calcium carbonate)

(A metal bonds with the C0 3 Dolomite MgCaC03 (Calcium-magnesium

complex ion acting as a nonmetal) carbonate)

Halides Halite NaCl (Sodium chloride)

(A metal bonds with a halogen [CI, Fluorite CaF2 (Calcium fluoride)

F, Br or I] as the nonmetal)

Silicates (A metal bonds with the Si0 4 complex ion as the nonmetal)

Nesosilicates (island silicates) Olivine

Garnet

(Fe, Mg)Si 04 (Iron magnesium

silicate)

Complex Ca, Mg, Fe, Al silicate

Inosilicates (chain silicates) Hornblende

Augite

Ca, Na, Fe, Mg, Al silicate

(Ca,Na)(Mg,Fe,Al)(Si,Al) 206

Phyllosilicates (sheet silicates) Muscovite

Biotite

Chlorite

Talc

Kaolinite

OH, K, Al silicate

(Hydrous potassium-aluminum

silicate)

OH, K, Mg, Fe, Al silicate

OH, Mg, Fe, Al silicate

OH, Mg silicate

OH, Al silicate

Tectosilicates (3-D silicates) Orthoclase

Plagioclase (Albite, Labradorite)

Quartz

K, Al silicate

Ca, Na, Al silicate

SiQ2

Cha pt er 1 Miner als 3


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Crystal Systems

Crystal systems are groups of crystals based on the symmetry of crystal faces. There are six

crystal systems and within these systems there are the thirty-two classes of minerals. Thesix crystal systems are cubic (isometric), hexagonal, tetragonal, orthorhombic, mono-

clinic, and triclinic (Fig. 1.1a and Fig. Lib).

The cubic (isometric) crystal system consists of three equal-length axes intersecting

at 90 angles from one another. The hexagonal crystal system consists of three equal-

length axes that intersect at 120 angles to one another and a fourth axis perpendicular the

first three axes. The tetragonal crystal system consists of two equal-length axes and a third

axes of a different length, all at 90 angles to one another. The orthorhombic crystal

system consists of three axes of different lengths that intersect at 90 angles to one another.The monoclinic crystal system consists of two unequal-length axes that intersect at 90

angles and a third that intersects obliquely. The triclinic crystal system consists of three

unequal-length axes that intersect obliquely. Crystal systems are studied in more detail in

the upper-level Mineralogy course.

Crystals "grow" as "invisible atoms" of a solution bond together in a given geometric

framework that is consistent with the atoms' electrical or size characteristics. As the geo

metric framework enlarges with continued "growth," that geometry becomes visible as

smooth surfaces that are called crystal faces. The smooth crystal faces give crystals of various minerals their characteristic shape and beauty.

Galena

Isometric (Cubic) Ciystal System Three equal-length axes

that intersect at 90" angles. Two of the axes intersect on the

same plane, and the third is perpendicular.

Typical Minerals

Pyrite

Malite

FluoriteGalena

Magnetite

Tetragonal Ciystal System Two equal-length axes and a

third, either longer or shorter, that intersect at 90" angles.Two of the axes intersect on the same plane, and the third is

perpendicular.

Typical Mineral

Zircon

Orthorhombic Ciystal System Three axes of different

lengths that intersect at 90" angles. Two of the axes intersecton the same plane, and the third is perpendicular.

Typical Minerals

Topaz

Staurolite

FIGURE 1.1a Crystal Systems, Crystal

Forms, and Typical Minerals.

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Monoclinic Crystal System Two unequal-length axes that in

tersect at 90 angles on the same plane, and a third that inter

sects obliquely.

Typical Minerals

Orthodase

Gypsum

\ I Plagioclasev Feldspar

Triclinic Crystal System Three unequal-length axes that in

tersect obliquely.

Typical Minerals

Plagioclase Feldspar

Corundum

Apatite

FIGURE 1.1b Crystal Systems, Crystal

Forms, and Typical Minerals.

Hexagonal Crystal System Three unequal-length axes that

intersect at 120 angles on the same plane, and a fourth that is

perpendicular to the other three.

Typical Minerals

Quartz

CorundumApatite

Calcite

Chapt er 1 Minerals 5

Calcite

Quartz


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Cleavage

Cleavage is the tendency of a mineral to break in a systematic (regular, ordered) way, alongplanes of weakness determined by the type and strength of the chemical bonds (see lecture

book) between the atoms that make up the mineral (Fig. 1.2a and Fig. 1.2b). The cleavages

(planes of weakness) represent layers between rows or sets of planar atoms where the

atomic bonds are weaker. Some minerals (micas and gypsum) have one direction of cleav

age (Fig. 1.2a) but most minerals have multiple cleavage directions. Not all specimens of a

given mineral will have readily identifiable cleavage planes, although it is a useful identifying

feature when present. Even when cleavage planes are not visible on a particular hand specimen, it does not mean that the mineral lacks cleavage. Look at other examples of the same

mineral. Some cleavage surfaces are microscopic and therefore invisible to the naked eye.

Since many minerals do not have cleavage or have microscopic cleavage (not visible to the

naked eye), you can use the presence of visible cleavage to eliminate those minerals that donot have cleavage. Some minerals always demonstrate cleavage, such as muscovite and

biotite, which have cleavage in one direction. Muscovite and biotite easily cleave (split) into

flat, flexible sheets.

Unfortunately, cleavage and crystal form are easily confused. They both result in flat

planes, but for different reasons. Some minerals have both crystal form and cleavage

(halite, fluorite, calcite, etc.), some only have cleavage (muscovite), and some only have

crystal form (quartz). Minerals with cleavage will break in the same direction or set of directions

every time and form flat planes or a stair-step pattern on the mineral face. A mineral with onlycrystal form will break in no particular direction and develop irregular (uneven) surfaces

when broken.

Fracture

Fracture is the nonsystematic and irregular way some minerals break. The fracture surfaceis rough or uneven, unlike cleavage planes, which are smooth and flat. Conchoidalfracture is a special kind of breakage that results in a curved parting surface. When a bulletpasses through glass, a curved or listric surface is produced (conchoidal fracture).

Conchoidal fracture is characteristic of homogenous materials that lack planes of weak

ness, thus the material is about equally strong in all directions (e.g., glass). Quartz

commonly shows conchoidal fracture.

NOTE: Some minerals display both fracture and cleavage. Albite, for example, has twodirections of cleavage (two flat sides) and two opposing sides with fracture (rough

sides).

Striations

Striations are very fine, parallel lines visible on the cleavage planes or crystal faces of

some minerals due to their crystal structure and growth patterns. Albite and labradorite,

both plagioclase feldspars, commonly exhibit striations on one cleavage plane. The

striations on plagioclase become increasingly obvious as the calcium content of the

feldspar increases. Striations may also be visible on the crystal faces of other minerals

such as pyrite, quartz, and garnet. Striations become more visible when the mineral is

slightly rotated back and forth in the light. As the mineral is turned, the striations reflect

the light.

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Cleavage: Cleavage is the tendency of certain minerals to split (cleave) along planes of

weakness, between layers of weak bonds that unite the atoms of which the mineral ismade, when the mineral is broken. Some minerals cleave in only one direction, others

have two, three, four, or even six directions of cleavage. Examples are shown below.

CAUTION: Beginning geology students often confuse the smooth cleavages surfaceswith the smooth crystal faces of minerals crystals, and thus often believe that

cleavage "chunks" are crystals. Crystal faces are produced when minerals "grow" as

invisible "atoms" of various elements within a solution and bond together in a given

geometric framework called crystallinity. The cleavage surfaces of cleavage "chunks"form when the mineral breaks.

One dimensionalcleavage sheet.

Biotite or Muscovite

Cleaved chunk

removed

Halite

FIGURE 1.2a Cleavage

Galena

One Direction of Cleavage Certain minerals, when bro

ken, break only along one plane.

Typical Minerals

BiotiteMuscovite

Chlorite

Talc

Selenite Gypsum

Two Directions of Cleavage Certain minerals, when bro

ken, break along two plane surfaces that intersect at a 90

angle to each other.

Typical Minerals

Orthoclase Feldspar

Plagioclase Feldspar

Three Directions of Cleavage Certain minerals, when

broken, break along three plane surfaces that intersect at a

90 angle to each other.

Typical Minerals

Galena

Halite

Cha pt er 1 Miner als 7


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Cleaved chunk

removed

Octahedral (8-sided)cleavage chunk,

Fluorite

Dodecahedral (12-sided)cleavage chunk,

Sphalerite

Three Directions of Cleavage Certain minerals when bro

ken, break along three planer surfaces that intersect obliquely

to each other.

Typical Minerals

Calcite

Four Directions of Cleavage Certain minerals, when broken,

break along four planar surfaces that intersect at different an

gles.

Typical Minerals

Fluorite

Six Directions of Cleavage Certain minerals, when broken,

break along six planar surfaces that intersect at different an

gles to each other.

Typical Minerals

Sphalerite

FIGURE 1.2b Cleavage.

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Tenacity

Tenacity is the resistance of a mineral to breakage. Some minerals are very hard to break,whereas others are easily broken. Terms used to describe tenacity include brittle, elastic,and malleable. Gold, a soft mineral, is malleable and easily deformed when hit. Diamond,

the hardest known mineral, is very brittle and will shatter when hit. Do not test the tenac

ity of mineral specimens unless instructed to do so.

TABLE 1.2 Mohs' Scale of HardnessBtssititsisisssijsiaswswm^!^ .,:,


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Color

Color is a function of how the surface of a mineral reflects or absorbs white light. It is one

of the least helpful physical properties of minerals because very few have a consistent color.

The mineral sulfur is an exceptionit is always bright yellowas is pyrite, which is a

brassy yellow. Both calcite and quartz are good examples of how color is varies within a

mineral. They can be green, yellow, red, brown, blue, clear, etc. There are three general

causes of color variation in minerals.

1. Impurities within the mineral change the color.2. The disturbance of the crystallinity of the mineral can cause variations in color.

3. The size of the mineral pieces can affect color. Thin pieces usually are lighter in color

than thicker pieces (one of the most common causes of color variation).

Although minerals can be grouped into groups of darker and lighter hues, do not

count on color alone to identify unknown minerals.

Streak

Streak is the color of a mineral's powder (or the color of the mineral when the crystals arevery small). The streak is obtained by rubbing the mineral on an unglazed ceramic orporcelain plate. Gently shake or blow off as much as possible of the powdered mineral

formed in this way. The color of the powder that sticks to the streak plate is the actual streak. The

mineral hematite illustrates the importance of streak in mineral identification. Varieties of

hematite often have a visibly different color from one another (specular hematite is silveryand oolitic hematite is reddish brown), yet both have a red-brown streak.

Luster

Luster is the way that a mineral reflects light. It is described as either metallic (like fresh,

untarnished metal) or nonmetallic (pearly, waxy, greasy, vitreous [like glass], earthy,

rusty, etc.).

Reaction to Dilute Hydrochloric Acid

Some minerals will chemically react (fizz, give off H 20 and bubbles of C0 2) in the pres

ence of a dilute solution of hydrochloric acid (HO). This test is primarily used to identify

calcite (CaC03) and dolomite [CaMg(C03)2]. Calcite reacts strongly with cool, dilute HC1,

and most dolomites only react when powdered. Scratch dolomite with a nail to produce

enough powder to test its reaction with acid. Apply one to two drops of acid on the powder.

After the acid is applied and the result noted, wipe the excess acid off the mineral and/or streakplate with a paper towel.

CAUTION: All students are to wear safety goggles when using acid. Apply acid one

drop at a time to the specimen and wipe the acid off the specimen before putting it

back in its place.

Magnetism

Magnetism is the attraction of a magnet to the mineral. Minerals vary from nonmagnetic

(most minerals) to weakly magnetic (some hematite) to strongly magnetic (magnetite).



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Density

Density is mass per unit volume. Specific gravity is the ratio of the density of a given material to the density of an equal volume of water (at 4 C). Minerals that have a high specific gravity, such as galena, feel unusually heavy for their size, whereas those with low

specific gravity feel lightweight.

Diaphaneity

Diaphaneity refers to how and to what extent light is transmitted through a mineral. Athin section is a 0.03-mm slice of a mineral that is thin enough to allow light to pass

through it. Although diaphaneity is usually applied to thin sections, we will apply the

same terms to the hand samples seen in the laboratory. The diaphaneity for each mineral

is determined simply by looking at it.

1. Transparent: light passes easily through the mineral, thus images can be clearly seenthrough it. Clear quartz is an example.

2. Translucent: some light passes through the mineral but the light is diffused andabsorbed internally by the mineral, thus images cannot be seen clearly. Translu-

cency is, in part, a matter of the thickness and purity of the mineral. Hematite is

usually thought of as opaque, but extremely small, pure crystals are translucent.

Although pure quartz is clear and colorless, the presence of large numbers of verysmall bubbles (milky or vein quartz) can make it translucent. Disturbance of the

crystal by radiation from decaying radioactive elements can make quartz gray,brown, or black, and the crystal, particularly if thick, may be translucent, or nearly

opaque (see below).

3. Opaque: the mineral allows no light to pass, thus images cannot be seen through the

mineral. Opacity ("opaqueness") is, in part, a matter of the thickness and purity of

the crystal. Very pure minerals with metallic or submetallic luster (pyrite, magnetite)

are opaque even in very thin slices (thin sections). Luster and opacity are tied

together by the extreme ability of these minerals to bend light.

Double Refraction

Double refraction is the doubling of a single image seen through a transparent mineral.Minerals, except the cubic ones (such as fluorite, halite, and diamond), split light rays into

two parts that follow different paths as they pass through the crystal. Optical quality cal-

cite crystals are the best example of this because the two parts of the light follow very dif

ferent paths. To see double refraction, place an example of optical quality calcite on this

page and look at the words. Special microscopes and specially prepared specimens are

used in serious work with double refraction, but geologists frequently make use of this

property in hand specimen mineral identification.

Other Identifying Properties

There are other properties that help identify unknown minerals. Many minerals have a

strong smell, such as sulfur (like rotten eggs). A fresh streak of sphalerite smells stronglyof sulfur. The way minerals feel can also be used in conjunction with other properties. The

longer a person handles halite, the greasier it feels. Taste can also be used for identification

purposes. Halite (salt) tastes salty. DO NOT TASTE ANY MINERALS IN LAB.

Chapt er 1 Minerals 11


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IDENTIFICATION OF MINERAL UNKNOWNS

The identification of mineral unknowns is easier for the beginning geology student if a log

ical step-by-step procedure is followed.

Step one: Separate the minerals into like shades of color. See the "Mineral

Identification Key" (Fig. 1.3). Put all the white or light-colored minerals in one pile,

the dark-colored minerals in another pile, and the metallic minerals in a third pile.

Step two: Determine the relative hardness of each mineral. Place the light-colored

minerals that have a hardness of less than 5 V2 m t o a subpile and all the mineralsgreater than 5'/2 m t o another. Repeat this step with the dark-colored and metallic

minerals.

Step three: Separate the minerals into groups that have and do not have visible

cleavage.

Step four: Suggest a tentative identification of the mineral and then consider theother physical characteristics of the mineral to make a positive identification. Place

the minerals on the figures as you determine their identity, and your instructor will

verify your identification.

Use the "Guide to the Identification of White or Light Colored Minerals" and "Guide

to the Identification of Dark, Metallic or Green Unknowns" as study guides for review.

Mineral pictures can be found on the Earth & Space Sciences website

(http://ess.lamar.edu/). Click on People, Staff, Woods, Karen M., Teaching, Physi

cal Geology Lab, Minerals.

/

[Hardness >5J\

[Hardness


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Guide to the Identification of White or

Light Colored Unknown Minerals

ROSE

QUARTZ

ROCK CRYSTAL

QUARTZ

CHERT FLINT(Black)

MILKY

QUARTZ

ALETTE &*LABRADORITE

(Plagioclase

Feldspar)

ORTHOCLASE

FELDSPAR

CORUNDUM

GARNET J Dodecahedralcrystal

*CALCITE

DOLOMITE

Won't scratch glass}\Will scratch a penny

Color varies

" , : Y :'

''Cubic Cleavage,Feels slipperyLuster-glassy /Feels"Soapy'

HALITE

TALC

JASPER

(Ret,> SMOKY

QUARTZ

CHALCEDONY(Banded)

KA0L1NITE

MUSCOVITE

ALABASTER

GYPSUM

SATINSPAR

GYPSUM

SELENITEGYPSUM

SULFUR

How to Use: I. Determine the general hardness of the unknown mineral.2. Match the unknown mineral to the characteristics in the

outer circle that correspond with the hardness determined.

*May also be dark in color

Cha pte r 1 Miner als 13
http://will/http://will/


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Guide to the Identification of

Dark Colored

or Metallic Minerals

V May also be light colored


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MINERAL PROPERTY LIST

AugiteAugite is a pyroxene with two cleavage planes, one at 87 and the other at93. Augite is dark green to black, has a vitreous to dull luster, a specific gravity of 3

to 3.5, a hardness that ranges from 5 to 6, and lacks a streak. Other identifiableproperties include a hackly or splintery fracture opposite to the cleavage direction.

Crystal system: monoclinic. Chemical formula: (Ca,Na)(Mg,Fe,Al)(Si,Al)206(calcium, sodium, magnesium, iron, aluminum silicate).

BauxiteBauxite (a mineraloid) is brown, gray, white, or yellow, has a dull to earthyluster, no cleavage, a white to yellow-brown streak, and a hardness that ranges from

1 to 3. Bauxite usually occurs in compact masses of pisoliths (pea-sized concretions,

spheres coarser than ooliths). Fracture is uneven. Chemical formula: AlO(OH)

(hydrous aluminum oxide).

BiotiteBiotite is a black to dark brown mineral with a vitreous to pearly luster.Biotite has perfect cleavage in one direction, allowing it to be separated into thin

sheets. Biotite has a brown to dark green streak if the specimen is big enough, and ahardness of 2.5 to 3. Fracture is uneven perpendicular to cleavage direction. Crystal

system: monoclinic. Chemical formula: K(Mg,Fe)3(AlSi3O10)(OH)2, (hydrouspotassium, magnesium, iron, aluminum silicate).

CalciteCalcite is usually white to colorless, but may be yellow, green, blue, red,

black, etc. due to impurities. Calcite has perfect rhombohedral cleavage (see photo),hexagonal crystal form (if present), a white to gray streak, and a vitreous to earthy

luster. Hardness is 3 on the Mohs scale. Specific gravity is 2.71. Calcite is soluble indilute hydrochloric acid with a strong effervescence (fizz). Double refraction isvisible through colorless rhombs. Crystal system: hexagonal. Chemical formula:CaC03 (calcium carbonate).

ChloriteChlorite is a green to greenish-black mineral with a waxy to earthyluster. Chlorite has a perfect basal cleavage (not apparent in massive pieces),

and a pa le green to whit e streak. The specific gravity is 3 and hardness is

2 to 2.5. Chlorite feels slippery. Crystal system: monoclinic. Chemical formula:

(Mg,Fe)3(Si,Al)40K)(OH)2(Mg,Fe)3(OH)6 (magnesium, iron, aluminum silicate).

CorundumCorundum varies in color (brown, blue, red, etc.), has an adamantine

to vitreous luster, a hardness of 9 on the Mohs scale, and a specific gravity of 4.Corundum is found in massive deposits as emery and as hexagonal crystals (see

photo) with striations on basal faces and has conchoidal fracture. Gem-quality

corundum is commonly known as sapphire and ruby. Crystal system: hexagonal.

Chemical formula: Al203 (aluminum oxide).

DolomiteDolomite varies from colorless to white, gray, brown, and pink.

Dolomite has perfect rhombohedral cleavage, hexagonal crystal form, and a dull to

vitreous to pearly luster. Cleavage and crystal form are not evident in massive pieces.

Specific gravity is 2.85, hardness is 3.5 to 4, and dolomite has a white streak. In

powdered form, dolomite effervesces in cold, dilute hydrochloric acid. Crystal

system: hexagonal. Chemical formula: CaMg(C03)2 (calcium, magnesium

carbonate).

FluoriteFluorite has perfect octahedral cleavage, cubic crystal form, andconchoidal fracture. Fluorite is colorless and transparent when pure but may be

blue, green, pink, purple, yellow, or black. Fluorite has a vitreous luster, specific

gravity of 3.18, hardness of 4, and a white streak. Crystal system: isometric (cubic).

Chemical formula: CaF2 (calcium fluoride).

GalenaGalena has a perfect cubic cleavage and cubic or octahedral crystal form.

Galena is lead gray, has a gray streak, metallic luster, and a hardness of 2.5. Galena

has a high specific gravity (7.57). Crystal system: isometric (cubic). Chemical

formula: PbS (lead sulfide).

Chapter 1 Mine rals 15


23/186

GarnetGarnet has a splintery or conchoidal fracture, no cleavage, and a resinousto vitreous luster. Color varies with composition but is commonly dark red to

reddish brown or yellow. Garnet forms dodecahedral crystals in some metamorphic

rocks and is also found in coarse granular masses. Garnet has a specific gravity of 3.5to 4.3, and a hardness of 6.5 to 7.5. Crystal system: isometric (cubic). Chemicalformula: Fe, Mg, Mn, Ca, Al silicate (complex iron, magnesium, manganese,

calcium, aluminum silicate).

GoethiteGoethite is a variety of iron oxide. Goethite has a prismatic ciystal form

and cleaves parallel with the prisms. Goethite is yellow or yellowish-brown to silverybrown in color, has a brownish-yellow streak, a specific gravity of 4.37, and a

hardness that ranges from 5 to 5.5. Massive goethite has an adamantine to dull

luster. Goethite is also found with rounded (reniform) masses that have a metallic

luster. Crystal system: orthorhombic. Chemical formula: FeO(OH) (hydrous iron

oxide). Pronounced "guhr-thite."

GraphiteGraphite has perfect cleavage in one direction, although the mineral isusually found as foliated masses. Graphite is dark gray to black in color, has a gray to

black streak, a metallic luster, a specific gravity of 2.23 (low), and a hardness of 1 to 2.

Graphite feels "greasy." Crystal system: hexagonal. Chemical formula: C (carbon).

GypsumGypsum is translucent and generally white, but may be tinted to variouscolors. Gypsum has a white streak, pearly to vitreous luster, cleavage a conchoidal,

irregular, or fibrous fracture, a specific gravity of 2.32, and a hardness of 2 on theMohs scale. Crystal system: monoclinic. Chemical formula: CaSCy2H 20 (hydrous

calcium sulfate). Three varieties are distinctive.

Alabaster gypsumAlabaster is the fine-grained, massive variety of gypsum.

Alabaster, also called rock gypsum, is generally white, but may be slightly tinted

with other colors. It has a pearly luster and cleavage is not apparent. Chemical

formula: See above.

Selenite gypsumSelenite gypsum has perfect cleavage in one direction and a

conchoidal fracture. Selenite is colorless to white, transparent to translucent, and

has a vitreous luster. Chemical formula: See above.

Satin spar gypsumSatin spar gypsum is fibrous, colorless to white, and has a

silky luster. Cleavage is not apparent in this variety. Chemical formula: See above.

HaliteHalite has perfect cubic cleavage and cubic crystal form (see photo). Halite

is colorless to white but impurities can give it a yellow, red, blue, or purple tint.

Halite is transparent to translucent, has a vitreous luster, a specific gravity of 2.16,

and a hardness of 2.5. Halite feels greasy and tastes salty (tasting of laboratory

specimens is not recommended). Crystal system: isometric (cubic). Chemical

formula: NaCl (sodium chloride).

HematiteHematite is steel gray, to black, to red, to reddish brown. Hematite has a

red to red-brown streak, a specific gravity of 5.26, a hardness that ranges from 5.5 to

6.5, an irregular fracture, and a metallic or a dull luster. Crystal system: hexagonal.

Chemical formula: Fe203 (iron oxide). Oolitic and specular are two important

varieties.

Oolitic hematiteOolitic hematite is composed of small spheres (ooliths) of

hematite. Oolitic hematite is red to brownish red, has a red streak, and an earthy

luster. See hematite above for other properties. Chemical formula: See above.

Specular hematiteSpecular hematite has a platy (glitter-like) appearance and

may be slightly to strongly magnetic. Specular hematite is steel gray or "silvery"

with a metallic luster, and has a red streak. See hematite above for other

properties. Chemical formula: See above.



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HornblendeHornblende is dark green to black, has a vitreous luster, a specificgravity of 3 to 3.5, a white to gray streak, and a hardness of 5 to 6. Hornblende is an

amphibole with two cleavage angles (56 and 124) and an uneven fracture

opposite of the cleavage directions. Crystal system: monoclinic. Chemical formula:

Ca, Na, Mg, Fe, Al silicate (calcium, sodium, magnesium, iron, aluminum silicate).

KaoliniteKaolinite has perfect cleavage (not apparent in massive pieces). Kaolinite iswhite, has a dull to earthy luster, a white streak, a specific gravity of 2.6, and a hardness

of 2. Kaolinite looks and feels like chalk, a kind of limestone, but does not react withhydrochloric acid. Kaolinite fractures irregularly. Crystal system: tridinic. Chemical

formula: Al4Si4O10(OH)8 (hydrous aluminum silicate).

LimoniteLimonite, a variety of iron oxide, is dark brown to brownish yellow, has ayellow to brown streak, an earthy to dull luster, a specific gravity of 2.9 to 4.3, and a

hardness of 4 to 5.5. Limonite fractures irregularly. Chemical formula: FeO(OH)

(hydrous iron oxide).

MagnetiteMagnetite is a black mineral with a gray to black streak, a specificgravity of 5, a hardness of 5.5 to 6, a dull luster, is strongly magnetic, and fractures

irregularly. Crystal system: isometric (cubic). Chemical formula: Fe 304 (iron oxide).

MuscoviteMuscovite is colorless to brown, gray, or green. Muscovite has a vitreous

to silky to pearly luster, perfect cleavage in one direction allowing it to be separatedinto thin flexible sheets, a white streak (if sample is thick enough), a specific gravity

of 2.8, and a hardness of 2 to 2.5. Fracture is uneven perpendicular to the cleavage

direction. Crystal system: monoclinic. Chemical formula: KAl2(AlSi3)O]0(OH)2(hydrous potassium, aluminum silicate).

OlivineOlivine is an olive-green to light gray mineral with a vitreous luster,conchoidal fracture, a specific gravity of 3, and a hardness of 6.5 to 7. Cleavage, when

visible, is poor. Crystal system: orthorhombic. Chemical formula: (Mg,Fe)2Si04(magnesium, iron silicate).

Orthoclase FeldsparOrthoclase feldspar is white to pink, has a vitreous luster, aspecific gravity of 2.57, and hardness of 6 on the Mohs scale. Orthoclase has two

directions of cleavage at 90 angles and an uneven fracture opposite the cleavagedirections. Crystal system: monoclinic. Chemical formula: KAlSi308 (potassium,

aluminum silicate).

Plagioclase FeldsparPlagioclase feldspar includes a group of feldspars that occupy

gradational positions within a single series (see Bowen's Reaction Series, Chapter 2).

Plagioclases are white to gray, to dark gray, have a vitreous luster, a specific gravity of

2.62 to 2.76, and a hardness of 6 on the Mohs scale. The minerals in this group have

cleavage planes at or almost at 90 angles and striations may be noticeable on some

cleavage planes. Crystal system: triclinic. Chemical formula: (Ca,Na)(Al,Si)AlSi 208(calcium, and/or calcium-sodium, and/or sodium, aluminum silicate). Albite and

labradorite are low- and medium-temperature varieties.

AlbiteAlbite is a low-temperature, light-colored, plagioclase feldspar with twodirections of cleavage. Fracture is uneven perpendicular to the cleavage direction.

Striations may be present. Chemical formula: NaAlSi 306 (sodium aluminum

silicate).

LabradoriteLabradorite is gray-blue, medium-temperature, plagioclase feldspar

with two directions of cleavage, and two opposing sides with uneven fracture.

Some samples exhibit a flash ("play") of different colors on cleavage surfaces.

Striations may be present. Chemical formula: (Ca,Na)AlSi3Os (calcium-sodium

aluminum silicate).

Chapter 1 Mineral s 17


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PyritePyrite is a brassy-yellow mineral with a greenish to brownish-black streak,

has a metallic luster, a specific gravity of 5.02 (high), and a hardness of 6 to 6.5.

Pyrite has cubic or octahedral crystals and striations may be seen on some crystal

faces. Crystal system: isometric (cubic). Chemical formula: FeS2 (iron sulfide).

QuartzQuartz is colorless to white but is often tinted. Quartz has a vitreous luster,conchoidal fracture, a specific gravity of 2.65, and a hardness of 7 on the Mohs scale.

Crystal system: hexagonal. Chemical formula: Si0 2 (silicon dioxide). Quartz has

many varieties.

AmethystAmethyst is the purple-tinted hexagonal crystal variety of quartz. See

quartz (above) for other properties and chemical formula.

Chalcedony/AgateChalcedony is a milky colored cryptocrystalline variety of

quartz. Chalcedony is frequently banded, and more transparent varieties with

darker mineral inclusions ("growths") are usually called agate. Chalcedony/agate

has a waxy to vitreous luster, and an obvious conchoidal fracture. See quartz

(above) for other properties and chemical formula.

Chert/FlintChert/flint is an opaque, cryptocrystalline, and darker variety ofquartz. Chert is generally lighter in color than flint. The dark gray to black varietyis usually called flint. Chert/Flint has waxy to vitreous luster, and an obvious

conchoidal fracture. See quartz (above) for other properties and chemical

formula.

JasperJasper is a red to reddish-brown cryptocrystalline quartz with an obvious

conchoidal fracture. See quartz (above) for other properties and chemical formula.

Milky quartzMilky quartz is the translucent to white, crystalline variety of

quartz with microscopic conchoidal fracture. See quartz (above) for other

properties and chemical formula.

Rock crystalQuartz crystals are bipyramidal hexagonal, and usually show

striations. See quartz (above) for other properties and chemical formula.

Rose quartzRose quartz is the pink-tinted crystalline variety of quartz. See

quartz (above) for other properties and chemical formula.Smoky quartzSmokey quartz is the smoky-yellow, to brown, to black variety of

crystalline quartz. See quartz (above) for other properties and chemical formula.

SphaleriteSphalerite is brown, yellow or black, has a brown to yellow streak

(strong sulfur smell), a resinous to submetallic luster, a specific gravity of 4, and a

hardness of 3.5 to 4. Sphalerite has a perfect dodecahedral cleavage. Crystal system:

isometric (cubic). Chemical formula: ZnS (zinc sulfide).

SulfurSulfur is usually bright yellow but may vary with impurities to green, gray,or red. Sulfur has a white to pale yellow streak, a resinous to greasy luster, no

cleavage, a conchoidal to uneven fracture, a specific gravity of 2, and a hardness of

1.5 to 2.5. Sulfur has a "rotten egg" odor. Crystal system: orthorhombic. Chemical

formula: S (sulfur).TalcTalc is white, brownish, gray, or greenish-white, has a white streak, a pearly todull luster, a specific gravity of 2.7 to 2.8, and a hardness of 1 on the Mohs scale of

hardness. Talc has perfect basal cleavage (not apparent in massive specimens), and a

smooth or soapy feel. Crystal system: monoclinic. Chemical formula: Mg^Si4Oi()(OH)2(hydrous magnesium silicate).



26/186

MINERAL USES

AugiteMost augite is only of interest to mineral collectors. Clear varieties are

occasionally used as gemstones. Name derivation: from Greek augities, meaning

"brightness" or "luster."

BauxiteBauxite is a mineraloid, not a true mineral. It is important as analuminum ore, the source material for aluminum as metal. Bauxite forms by the

concentration of hydrated aluminum oxides in the soils of humid tropical regions.

Bauxite is a heterogeneous mixture of the minerals gibbsite [AlO(OH)3], boehmite,and diaspore [both AlO(OH)]. Hematite and/or limonite may be present in smallamounts. Name derivation: for occurrence near Baux, France.

BiotiteBiotite has no economic use but is of interest to collectors. Namederivation: for French physicist, J. B. Biot.

CalciteCalcite has many uses: lime (Ca oxide) is a fertilizer, the raw material fromwhich Portland cement (for making concrete) is made, and is used as a buildingstone (limestone and marble). Name derivation: from Latin calx, meaning "burnt

lime."

ChloriteChlorite has no commercial value, but is a natural green pigment often

found in marbles, etc. Name derivation: from Greek chloros, meaning "green."

CorundumBecause of its great hardness (9), corundum is used as an abrasive

("black" sandpaper), or for emery wheels for the grinding of metal. Rubies (if red)

and sapphires (if blue, pink or yellow) are transparent varieties. Name derivation:

kauruntakIndian (Hindu) name for corundum.

DolomiteBecause dolomite contains magnesium, it is a source of this element formagnesium-deficient diets. It is also used as a building stone or as road gravel.

Name derivation: after French scientist D. de Dolomieu.

FluoriteFluorite is a source of fluorine, used to fluoridate drinking water or added

to toothpaste to increase the hardness of dental enamel; is used in the manufacture of

hydrofluoric acid (the only acid that will dissolve glass); as a flux in steel making, etc.

Name derivation: Latin fluere, meaning "to flow." Refers to the ease at which fluorite

melts when heated, compared to other minerals.

GalenaGalena is a source of lead as metal when refined, is used in glass making(leaded crystal), and is used in radiation-shielding material. Name derivation: Latingalenaoriginal name for lead ore.

GarnetGarnet is slightly harder than quartz and thus is a good abrasive ("red"sandpaper). It is used as a sandblasting medium and as a grit and powder for optical

grinding and polishing. When transparent and without internal fractures, garnet is

also a semiprecious gem. Name derivation: Latin granatus, meaning "like a grain."

GoethiteGoethite is an ore of iron. Name derivation: after J. W. Goethe, a German

poet and scientist.

GraphiteGraphite is the "lead" in pencils, a dry lubricant, and is used in the steelindustry. Name derivation: Greekgraphein, meaning "to write."

GypsumWhen the H 20 is driven off by heat, gypsum becomes anhydrite, and

when ground to a powder, it becomes plaster of Paris. Gypsum is used in themanufacture of sheet rock, plaster, plaster casts, as a fertilizer, etc. The alabaster

variety is used to make statuary, and satin spar is used as ornamental decoration.

Name derivation: Arabic jibs, meaning "plaster."

HaliteUsed as table salt, a food preservative, for tanning leather, and as a source of

sodium and chlorine, etc. Name derivation: Greek halos, meaning "salt."

HematiteHematite is an ore of iron, the material from which, through the

smelting process, iron is extracted as pure metal. Hematite ores can run up to about

70 percent (by weight) iron. It is also used as a red pigment in paint. Name

derivation: Greek haimatos, meaning "blood" for the "blood" red streak color.

Chapter 1 Mineral s 19


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HornblendeHornblende has no commercial value, but is of interest to collectors.

Name derivation: from German horn and blenden, meaning "horn" and "blind" in

reference to its luster and lack of value.

KaoliniteKaolinite is pure china clay and is used for clay for ceramics, filler in

paper, rubber, candy, medicines, etc. Name derivation: Chinese name Kao-ling,

meaning "high ridge," refers to the area in China where it was first obtained for

export.

LimoniteLimonite is a hydrous, powdery variety of hematite that comes in many

shades of yellow, orange, red, and brown. Limonite is the primary pigment in manysuch colored paints. It is also a natural pigment responsible for soil color. A darker

brown limonite rock formed in the red soils of East Texas (Jefferson City) sometimes

is used as iron ore.

MagnetiteMagnetite is the most superior iron ore because of its high iron content.Name derivation: for Magnesia, an area near Macedonia, near Greece, where it was

originally found.

MuscoviteBecause muscovite is a transparent heat-resistant mineral, it is used as

the "windows" in high temperature ovens. It is also used as an electrical insulator,

and was earlier used as decorative "snow" for Christmas ornaments. Name

derivation: from the Muscovy area in Russia where it was used as window glass and

from Latin micare, meaning "to shine."

OlivineOlivine, because it is heat resistant, is used as "brick" liners for high

temperature ovens or furnaces. It is, when transparent, the gem peridot. Name

derivation: from its olive-green color.

OrthoclaseWhen ground to a powder and mixed with water, orthoclase forms

the coating on ceramics that, when fired in a kiln, turns to glaze, glass. Name

derivation: Greek orlhos, meaning "right angle," and klasis, meaning "to break."

Plagioclase FeldsparLabradorite is used as an ornamental stone when it displays

labradoresence (play of colors). Albite, when opalesent, is cut and polished and

known as the gem moonstone. Name derivation: Greek plagio, meaning oblique

(cleavage angle).

PyritePyrite, because of its high sulfur content, is used in the manufacture of

sulfuric acid. Name derivation: Greek word pyx, meaning fire.

QuartzVarieties include citrine (yellow), rose (pink), amethyst (purple), smoky

(brown-black), milky (white), chalcedony-agate (banded), jasper (red), chert (light

gray), flint (dark, dull color), rock crystal (crystal form), etc. Quartz crystals are often

used as semiprecious gems or for display in mineral collections. Agate, if partially

transparent or translucent, is often polished and used as a semiprecious gem, etc. Chert

and flint are the raw material from which stone tools were once made. Pure quartz sand

is used to make glass. Name derivation: German quartz.

SphaleriteSphalerite is zinc ore, the material that, when refined, gives us zinc as

metal. A thin coating of zinc on iron or steel offers considerable protection from

oxidation (rusting). Originally the zinc was applied by electrolysis, which gave rise

to the name "galvanized iron," but it is cheaper to dip the material in a bath of

molten zinc. Zinc is used to galvanize corrugated iron roofing, iron buckets, or nails,

etc. Name derivation: Greeksphaleros, meaning "treacherous."

SulfurSulfur has many uses. It is used in the manufacture of sulfuric acid. Also,

when added to rubber (vulcanized rubber) it makes the rubber able to withstand high

temperatures as for tires, rubber hoses, etc. It is also used in the production of sulfa

drugs. Name derivation: From sulphur, meaning "brimstone."

TalcTalc, when ground to a powder and scented, is used as body powder (talcum,

baby powder), and as an ingredient in paint, paper, etc. Name derivation: Arabic

word talq, meaning "pure."

20 Physical Geol ogy Laboratory Manual


28/186

EXERCISE 1: IDENTIFICATIONOF MINERAL UNKNOWNS AND THEIR

PROPERTIES

Identify the mineral specimens supplied using the properties discussed in the lecture. You will be supplied with the

following materials:

Glass plate Paper towels Steel nail Penny

Streak plates Magnet Dilute hydrochloric acid

SAFETY INFORMATION

The identification of minerals utilizes materials that may cause minor injury if used improperly. The following

instructions are intended to familiarize the student with proper laboratory procedures.

Glass Plate

The purpose of the glass plate is to determine whether or not a mineral is harder than the glass plate (>5.5) or softer

than the glass plate (


29/186

^~^-^^^ Mineral

P r o p e r t i e s ^ ^ l a

Chemical Formula

Hardness Range

Exact if on Mohs' Scale

Luster? if Metallic

Describe if nonmetallic

Streak (color)

Diaphanaeity

Transparent,

Translucent or Opaque

Magnetism? if Magnetic

X if Nonmagnetic

Crystal FormDescribe if Visible

X if Not Visible

Specific Gravity

? if Heavy,

N if Normal, X if Light

Cleavage

# of Planes if Visible,

X if None VisibleFracture

Yes, No,? if Conchoidal

Reaction to HCI Acid

Describe Reaction

Color of Mineral

Striations

? if Present, X ifNot

List one Use


30/186

I >U-' w *&> iL

^""---^^^ Mineral

Properties '""^me

Chemical Formula

Hardness RangeExact if on Mohs' Scale

Luster? if Metallic


Streak (color)

DiaphanaeityTransparent,



X if Nonmagnetic


X if Not Visible

Specific Gravity? ifHeavy,

N if Normal, X if LightCleavage


X if None VisibleFractureYes, No,

? if Conchoidal

Reaction to HC1 Acid

Describe Reaction

Color of Mineral

Striations

? if Present, X if Not

List one Use


31/186

^ - - ^ ^ ^ Mi neral

Properties ~-~^[ame

Chemical Formula

Hardness Range


Luster

? if MetallicDescribe if nonmetallic

Streak (color)

Diaphanaeity

Transparent,


Magnetism? ifMagnetic

X if Nonmagnetic


X if Not Visible

Specific Gravity? if Heavy,


Cleavage





Describe Reaction

Color of Mineral

Striations


List one Use

^^^Jr


32/186

in I Imi I III I'II t w i n " I i i - - ^"''-"- '' ' . .1 .. . . H ,1

vet -w ^isL ' ,,

"* ^ ^ ^ Mineral

Properties"""""-" Njime

Chemical Formula

Hardness Range


Luster? if Metallic


Streak (color)

Diaphanaeity

Transparent,



X if Nonmagnetic


X if Not Visible



Cleavage



Yes, No,

? if Conchoidal


Describe Reaction

Color of Mineral

Striations


List one Use


33/186

"^^^.^^ Mineral

Properties^^^^-^3^

Chemical Formula

Hardness Range


Luster


Streak (color)

Diaphanaeity

Transparent,


Magnetism? ifMagnetic

X if Nonmagnetic


X if Not Visible



Cleavage



? if Conchoidal


Describe Reaction

Color of Mineral

Striations


List one Use

- v j NL^' -~^IJ L-*'


34/186

||_ s|_^ ~^Iarne

Chemical Formula

Hardness Range


Luster


Streak (color)

Diaphanaeity

Transparent,



X if Nonmagnetic


X if Not Visible



Cleavage



Yes, No,

? ifConchoidal


Describe Reaction

Color of Mineral

Striations

? ifPresent, X ifNot

List one Use


35/186

~ ^ ^ ^ ^ ^ Mineral

Properties^^^^-^l36

Chemical Formula

Hardness Range


Luster


Streak (color)

Diaphanaeity

Transparent,



X if Nonmagnetic


X if Not Visible



Cleavage





Describe Reaction

Color of Mineral

Striations


List one Use

[

r


36/186

JLJ J l.._ > s_r*

^~-~^^^^ Mineral

Properties """-N ame

Chemical Formula

Hardness RangeExact if on Mohs' Scale

Luster? if Metallic


Streak (color)

DiaphanaeityTransparent,

Translucent or OpaqueMagnetism

? if MagneticX if Nonmagnetic


X if Not Visible


N if Normal, X if LightCleavage



? if Conchoidal


Describe Reaction

Color of Mineral

Striations

?ifPresent,XifNot

List one Use


37/186


38/186

Rocks

A rock is a natural aggregate (combination) of one or more minerals, mineraloids, glass,and/or organic material. There are three families of rocks distinguished from one anotherby the processes involved in their formation. The three rock families are:

Igneousoriginating from a molten silicate melt.

Sedimentaryoriginating from the deposi tion of the by-products of weathering.

Metamorphicdevelop via the change in form or chemical composition of

preexisting rocks and minerals by new conditions of temperature, pressure, and/orthe addition of hot chemical fluids.

Igneous, sedimentary, and metamorphic rocks are described and identified on thebasis of their composition and texture. Composition, in general, refers to the chemicalmakeup, the particular elements that are present in the rock. Texture, in general, refers tothe size, arrangement, and shape ("morphology") of the constituent minerals or materials

in the rock. There are different sets of textural terms for each rock family that often denotethe same or closely similar conditions.

IGNEOUS ROCKS

Igneous rocks are the solids produced by the cooling and crystallization of molten silicatematerial initially formed beneath the Earth's surface. Crystallization occurs when coolingallows for the growth of mineral crystal grains. The cooling rate and space available determine the size of the crystals that form. Large crystals form when magma, molten silicatematerial below ground, is insulated by the surrounding country rock (rock that has beenintruded by the magma), and therefore cools very slowly. When magma solidifies underground, it forms intrusive (plutonic) igneous rocks. The shape and position of emplacementdifferentiate plutonic igneous rock bodies. A dike is a pluton that cuts across pre-existingrock (strata). (Principle of Cross-Cutting Relationships: a rock body must already exist in order for

it to be cut by another). A sill is a two-dimensional pluton that is placed parallel to andbetween layers (strata) of existing rock. Batholiths are very large, three-dimensional plutons,usually the result of multiple intrusions of magma, hundreds of miles in length and width,which cool and crystallize very slowly beneath the Earth's surface. A laccolith is a smallerthree-dimensional pluton with a convex roof and a flat floor.

Volcanic (extrusive) igneous rocks form on or above the surface of the Earth by the

cooling of lava (molten silicate flows on the surface), or by the deposit ion of violently

ejected pyroclastic (pyro = fire, dast = fragment) material such as volcanic ash. Lava cools

31


39/186

faster than magma because it is exposed to environments that allow for the rapid dissipa

tion of heat and therefore prevent the formation of large crystals. In general, most extru

sive igneous rocks develop crystals that are too small to be seen without the aid of a

microscope. There are different types of basaltic lava. Aa is blocky, sharp-edged lava that

moves very slowly and pahoehoe is ropy, "smooth" lava. Volcanic glass (obsidian) forms

when lava is cooled too rapidly for crystals to develop.

Bowen's Reaction Series

Igneous rocks, with few exceptions, are made of silicate minerals. An understanding of

igneous rock formation can be gained by considering Bowen's Reaction Series.

Bowen's Reaction Series (Figure 2.1) is the result of experiments conducted by N. L.

Bowen and first published in 1928. Bowen 's Reac tio n Series is an organization of the

silicate minerals according to the conditions required to crystallize them, as the tem

perature of a melt lowers. Bowen discovered that in addition to the availability of

needed chemical elements, temperature and pressure determine when and where given

silicate minerals form. He observed that some minerals form as a continuous series

belonging to a single silicate family (tectosilicates) but with progressive change (substitution) of chemical composition, whereas others form as a disco nti nuou s series of

different silicate crystal families as their crystal structures readjust. The discontinuous

series of readjustments proceeds from what could be thought of as 0 (zero) dimen

sional arrangements (highest temperatures and pressures) through 1-D, 2-D, to3-D arrangements (low temperature/pressure) if nil of the necessary elements to build a

particular mineral are available.

The continuous series involves the plagioclase feldspar group. These minerals have a

three-dimensional covalently bonded structure that includes metal ions. The structure is

Matiy textbooks showBowen's Reaction Series

in fliis orientation

#

^

9

#

Bowen's Reaction Series

Quartz

MuscoviteSheet Silicate

Microcline

Orthoclase

3-D Silicates a

.c4?

Biotite / \

Sheet Silicate

A , b

t e

Amphlbole (Hornblende)Double Chain Silicate

/ '

rQj

Ohgoclase

Andesinc

rt-

(Near SurfaceConditions)

Low TemperatureLow Pressure

\

Pyroxene (Augite)Single Chain Silicate

Olivine yIsland Silicate

Labrador! te

x High TemperatureBytowmle High Pressure

> ,; ., Deeper or DeepAnorthUe ... \

Underground

FI GU RE 2.1 Bowen's Reaction Series.

32 Physical Geology Labor ator y Manua l


40/186

continuously modified as ions are exchanged with the magma during cooling.

Calcium-rich (Ca) plagioclase crystals(anorthite, CaAl2Si208) first begin to form

when the magma has cooled to 1400 to

1200C. As cooling continues (1200 to1000C), the crystals exchange Ca and alu

minum (Al) ions for sodium (Na) and sili

con (Si) ions from the magma, to formcrystals that are more sodium and silicon-

rich. Calcium-rich plagioclase crystals alsoform directly from the magma at this tem

perature range. If the temperature of themagma continues to decrease very slowly

so that equilibrium is approximately main

tained, plagioclase feldspars will continueto exchange ions in this manner until the

magma solidifies. If there is sufficientsodium, Ca plagioclases disappear com

pletely, but in many magmas all of the Na

and Al becomes bonded early and is lost

from the system. Thus this process whichcan proceed successively from anorthite(Ca-rich), to bytownite, labradorite, ande-

sine, oligoclase, and albite (Na-rich)in

practice produces a variety of different minerals, depending on the original composi

tion of the magma and the rate of cooling.Silicate minerals of the discontinuous

series have a variety of different structuresof increasing complication that appearand disappear successively and pre

dictably, as conditions (mainly temperature) in magmas change. The following

discussion is primarily concerned with decreasing temperature, but the effects of pressure are generally similar. Olivine(1400-1200C) is the first mineral (stable silicate or structure) to appear. The olivine crys

tal consists of individual tetrahedra (plural of "tetrahedron;" four oxygen and a muchsmaller silicon hidden in the center; Fig. 2.2a) tied together by bivalent iron [Fe++] and mag

nesium [Mg++] ions in a three-dimensional network. Olivine crystals become unstable

when the melt cools to about 1200 to 1000C, the temperature range in which pyroxene becomes stable. Augite is an example of a common mineral in the pyroxene family. Olivine

crystals suspended in the magma react to form the more complex single chain (pyroxene,augite, Fig. 2.2b) silicate structure. Amphibole (another family of silicate minerals, of which

hornblende is a common example) becomes stable at approximately 1000 to 800C. Again

the earlier-formed (pyroxene, augite) crystals react with the melt and form double chain(Fig. 2.2c) amphibole (hornblende) crystal structures. If sufficient magma and silica (Si0 2)

are still available, the hornblende will react with it and will begin to change to biotite, asheet silicate (Fig. 2.2d). Orthoclase and microcline (both three-dimensional covalently

bonded structures with metal ions), muscovite (sheet structure), and quartz (three-dimen

sional structure) will form last if enough magma is left.

FIGURE 2.2 Silicate Structures.

Igneous Rocks: Composition

The composition of igneous rocks can be determined, in a general way, in hand specimensby the relative abundance and color intensity (pale versus dark or strong color) of the min

erals that make up the rock (Figure 2.3).

Chapter 2 Rock s 33


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FIGURE 2.3 Common Igneous Rocks (mineralogy and composition).

1. Felsic (sialic) igneous rocks are composed mainly of potassic and sodic feldspars(light-colored minerals) that formed under low-temperature and low-pressureconditions (Bowen's Reaction Series). Felsic rocks include syenite, trachyte, granite,rhyolite, granodiorite, dacite, and some obsidian.

2. Intermediate igneous rocks have subequal amounts of light and dark minerals.Intermediate rocks include andesite (named for the Andes Mountains), and diorite.

3. Mafic igneous rocks have a large percentage of darker and strongly colored mineralsrich in ferromagnesian components and calcic plagioclase feldspars. These are

minerals that form under high-temperature and high-pressure conditions (Bowens

Reaction Series). Mafic rocks include basalt and gabbro, and some obsidian.

4. Ultramafic igneous rocks often contain 70 to 90 percent olivine, other dark and

strongly colored ferromagnesian minerals, the most calcic plagioclases, and very

minor, if any, percentages of silica. These minerals form under very high-temperature

and high-pressure conditions (Bowen's Reaction Series). Ultramafic rocks include

peridotite and komatiite. Ultramafic rocks are not common at or near the Earth's

surface, but form in the asthenosphere and mantle.

Igneous Rock Texture

The texture of igneous rocks refers to the physical appearance ("visual feel") and arrangement of minerals within the rock. Texture may include the absence of crystals in a rock, the

presence and/or relative size of the crystals that make up the rock, any contrast in crystalsizes within a given rock specimen, the arrangement of minerals in a rock, and/or the pres

ence of bubbles (vesicles) in the rock.

1. Glassy texture is applied to igneous rocks that have cooled so rapidly that crystals

didn't have time to develop and grow. Igneous rocks that have a combination of

glass and visible crystals are referred to as vitrophyres. Vitrophyres often form dur

ing rapid intrusion when magma comes in contact with much cooler, surrounding,



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country rock. A chilled (rapidly cooled) margin is a thin zone of rapidly cooledigneous rock that forms a rind on the pluton, and can be aphanitic, glassy, orvitrophyric.

2. Aphanitic is the textural term used to describe igneous rocks that have crystals thatare approximately uniform and small in size. "Small" means that the crystals are not

distinguishable by the unaided eye ( 1 millimeter) ; both microcrystalline (crystalsvisible only under the microscope) and cryptocrystalline (crystals too small to beclearly distinguished with the ordinary microscope) textures are included.

3. Phaneritic is the textural term used to describe igneous rocks that have crystals large

enough to be seen without magnification ( ~1 mm) and less than 1 inch (2.54 cm,

medium size). The rock is "megascopically crystalline."

4. Pegmatitic is the textural term applied to igneous rocks in which the crystals arelarge or very large. "Large" means very coarsely crystalline; crystals in a pegmatitemay be a few centimeters (1 inch = 2.54 cm) or several meters (100 cm, or =39and 1/3 in/m) in length.

5. Porphyritic texture is the term used when the crystals in a rock fall into two distinct

size groups (small versus large). When a rock has this combination of crystal sizes,

porphyritic is usually appended to the rock name. Porphyritic basalt is an example.

A vitrophyre is a special kind of porphyritic rock. The smaller aphanitic crystals andglass, if present, form what is called the matrix or groundmass matrix of the rock.The matrix, if aphanitic or partly glassy, contains minerals that formed at low

temperatures and pressures. The larger crystals are called phenocrysts. Phenocrystsare commonly early-formed, slow-growing minerals that crystallize at higher

temperatures and pressures. As a result, phenocrysts are more likely to have betterformed crystal faces (be euhedral) than crystals that form later. Crystals that formlate, quartz in a granite for instance, do not develop crystal faces (are anhedral).The rock is probably volcanic or part of a very shallow intrusion (plumbing of avolcano) if the groundmass is aphanitic.

6. Vesicular texture describes volcanic igneous rock with bubbles (holes). The bubblesform when pressure is released during eruption and volatile components of a

magma exsolve (come out of solution). Water (H 20) and carbon dioxide (C0 2) arethe two most abundant volatile components.

Bubbles are most commonly found in volcanic rocks, but sometimes occur in theuppermost parts of dikes that were part of the plumbing for an eruptive center. Expansion

of the gasses formed the bubbles, and the expansion helps to cool the magma/lava.Bubbles range in size from very small (small fractions of a mm) to more than a meter,

although very large vesicles are uncommon. The rate of cooling and the viscosity ofthe magma/lava control the size ofthe bubbles. Most of the terms arising trom vesiculartextures are associated with abundance of vesicles as well as size. Rocks with widelyspaced and clearly visible vesicles are referred to as vesicular. Vesicular basalt is acommon volcanic igneous rock.

Rocks with closely spaced bubbles that are on the order of 1 to 2 mm in diameter or

larger may be referred to as scoria. Scoriaceous basalt identifies the tops of basalt flows.Contact with oxygen in the air oxidizes iron in the glass to produce tiny crystals of

hematite, and a reddish color in many scorias. Scoria has a very low density for a rock,but usually does not float on water. Most scoriaceous rocks are mafic in composition.

Rocks with microscopic bubbles (less than a mm, usually) may be referred to as

pumice. Most pumiceous rocks are felsic. Pumice often has a density so low that the rockwill float on water. Scoria and pumice both have vesicular texture, yet they are bothforms of obsidian, volcanic glass. The release of gases creates a frothy or vesicular texturein the obsidian.

Vesicles later filled with solid material (secondary minerals) are called amygdules.Amygdaloidal basalt is basalt with filled vesicles. Common vesicle-filling minerals include quartz crystals, chalcedony, agate, and calcite. Large chalcedony or agate-filled

Chapter 2 Rocks 35


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amygdules can be handsomely colored and have some value to collectors. Large, partially

hollow amygdules are sometimes referred to as "geodes," though technically geodes form

in sedimentary rocks.

Arrangements of Crystals and Bubbles

The arrangement of crystals and bubbles is also an important aspect of texture in igneous

rocks. For the purposes of this discussion, an "arrangement" of textural elements is a sit

uation where the occurrence or orientation of the feature is not random. Arrangements area product of local variations in the chemical and physical conditions within a magma or

lava and gravity. Understanding of such nonrandom organizations of textural features, and

particularly specific kinds of crystals (minerals) is a major area of study in igneous petrologyand geochemistry.

The beginning of crystallization of any mineral variety requires just the right balance

of ion availability, temperature, and pressure. A simple way for all of these to vary at once

is for the magma to be in motion, flowing. Shearing stresses induced by flow can align ex

isting acicular (needle-shaped) crystals producing an arrangement called flow lineation.Surfaces of low pressure sub-parallel to the boundaries of flow develop when flow expands

(cross-sectional area of the flow increases). These can localize precipitation of sheet-like

mineral masses, flow foliation. Early-formed crystals have densities greater than that of themagma, and collect near the bottom of magma chambers under the influence of gravity.

The natural form of bubbles and vesicles is spherical, or, when many bubbles areclosely packed (scoria), compact. Under the influence ofgravity, bubbles rise and expandto collect at the top of a lava flow, if the viscosity is low. Flow will deform vesicles into ellipsoids all aligned in the direction of flow. Basaltic lava can move fast enough to achievevery complicated ( turbulen t) flow, somewhat like that of water in a brook. In this situa

tion, vesicles can take complicated shapes.

Identification of Igneous Unknowns

Use the "Key to the Identification of Some Common Igneous Rocks" as you identify your

igneous rock unknowns, then use the study guide for review.

Igneous rock pictures can be found on the Earth & Space Sciences website(http://ess.lamar.edu/) Click on People, Staff, Woods, Karen M., Teaching, Physical

Geology Lab, Igneous Rocks

36 Physical Geol ogy Laborato iy Manual
http://ess.lamar.edu/http://ess.lamar.edu/


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i Mill I I III ll III U I

Key to the Identification of Igneous Rocks

Felsic (Sialic) Composition, Light Colored MineralsTEXTURE

Phaneritic

Aphanitic

Glassy

Vesicular

Porphyritic

If with substantial quartz

If with small bubbles, finely puffed

If with phenocrysts, primary angular crystals.

If with blue quartz crystals

NAME

- GRANITE

RHYOLITE

"*~ OBSIDIAN

* PUMICE

PORPHYRITIC

*" RHYOLITE

>- LLANITE

Plutonic

(Intrusive)

-; Volcanic

(Extrusive)

Dual Origin

Intermediate Composition, Subequal amounts of Light & Dark Colored MineralsPhaneritic DIORITE Plutonic

(Intrusive)D . ^ T o .+. . t . . __ +- PORPHYRITIC DIORITE \. _ , _ . .

Porphyritic If with phenocrysts, primary angular crystals "^~~~--~-~w Dual On gin

Aphanitic

Mafic Composition, Dark Colored MineralsPhaneritic

PORPHYRITIC DIORITE **

PORPHYRITIC ANDESITE -

*- ANDESITE

GABBRO

Aphanitic BASALT

Glassy

Porphyritic

\

If with large bubbles, coarsely puffed

If vesicles filled with secondary

mineral such as quartz or calcite

If with medium sized bubbles

- VESICULAR BASALT.

_^ AMYGDALOIDALBASALT

SCORIA

BASALTIC OBSIDIAN-

Volcanic

(Extrusive)

Plutonic

(Intrusive)

Tf with phenocrysts, primary angular crystals PORPHOR1TIC BASALT

UJ^ r

Ultramafic Composition, 70-90% OlivinePhaneritic PERIDOTITE

Volcanic

(Extrusive)

Dual Origin

Plutonic

(Intrusive)


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CO

Guide to the Identification

of Common Igneous Rocks

PUMICE

^ v ^


46/186

IGNEOUS ROCK WORKSHEET

Igneous rocks are classified on the basis of and

Define the following terms completely

Igneous rock

Felsic (sialic)

Examples: 1. 2..

Intermediate

Examples: 1.

Mafic

Examples: 1.

Ultramafic

Examples: 1.

List the Igneous Rock Textures

1. 2. 3.

4. 5. 6.

Vocabulary

1. Magma

2. Lava

3. Plutonic (intrusive),

4. Volcanic (extrusive)

5. Sill

6. Dike

7. Batholith

8. Laccolith

Chapt er 2 Rocks 39


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48/186

EXERCISE 2.1: IGNEOUSROCK IDENTIFICATION

For each rock, list the composition, texture, percentage of each mineral in the rock, and environment of formation,

and provide a brief description.

o /

P* / 0>

/ '/ w

/ o/ PLI

.2'c /3

oex

so

u

0)

orSx!

2 mm) includes pebbles, cobbles, and boulders (conglomer

ates and breccias).

2. Medium-grained (1/16-2 mm), sand-sized material (sandstone).3. Fine-grained (1/256-1/16 mm) silt-sized material (siltstone).

4. Very fine-grained (< 1/256 mm), clay-sized grains (claystone or shale).

The grains that make up silicate clastic sedimentary rocks have an angular, sub-

rounded, or rounded shape. Angular shaped dasts indicate that the dasts either did nottravel far from the source area to the basin of deposition or that the clasts are hard and more

resistant to erosion. The size, shape, and process of deposition determine the arrangement

(orientation) of the grains as they are deposited. Roundness is a partial indicator of the

distance sediment is transported by running water before deposition. A variable, however,

Chapt er 2 Rocks 43


51/186

BOULDER

CONGLOMERATE

Ferrugeneous - To contain substantial amountsof hematite-limonite (iron oxides); a coloring that

maked fine-grained sedimentary rocks

(usually shale or sandstone) yellow, orange, or red.

CARBONACEOUS

SHALE

SHALE

SILTSTON

Carbonaceous - To containsubstantial amounts of organic debris,

carbonaceous material; a coloring that

maked fine-grained sedimentary rocks

(usually shale or limestone) grey or black

FIGURE 2.4 Clastic Sedimentary Rocks.

MEDIUM

CONGLOMERATE

MEDIUM BRECCIA

FINE

CONGLOMERATE

CLASTIC

SEDIMENTS,

TERRIGENOUS

ORIGINCOARSE

SANDSTONE

S3UIJ

MEDIUM

SANDSTONE

FINE

SANDSTONE


52/186

that influences roundness is the hardness and tenacity of the material being abraded.

A coarse sedimentary rock with rounded (even slightly rounded) clasts is called conglomerate, but if the clasts remain angular, the rock is called breccia. Breccias can be composedof either igneous or sedimentary rock fragments.

Evaporites (Crystalline and Cryptocrystalline Texture)

The evaporites, the second major group of sedimentary rocks, originate in different and

often complex ways. Simple examples of this mode of origin include the chemical precipitation of some dolostone, gypsum, halite, and calcite as mineral-bearing water evaporates

on the bottom of the basin of deposition. Most evaporites have a crystalline texturebecause the atoms of the rock are arranged in an orderly fashion. Chert, a more complex

example (Fig. 2.5), has cryptocrystalline (hidden crystal) texture.

Biogenic (Bioclastic Texture)

A third major way that sediments (sedimentary rocks) can form is through biologic

processes. They have a biogenic mode of origin. A bioclastic rock is a rock composed ofbits and pieces of biogenic material such as shell and plant material. Examples include peat(actually a sediment), lignite, and bituminous coal (plant accumulations, coals), andvarious kinds of limestone. Lignite and bituminous coal are composed of compacted bits

and pieces of plant matter and have a bioclastic; coarse-grained texture. Most limestoneis made up of the accumulation of microscopic or macroscopic skeletons of calcareous

marine organisms. When alive, these organisms extracted Ca+ and (C03)" ions from thewater, and used them to form the mineral aragonite (CaC03) in the construction of their

hard parts. When the organisms die, their skeletons (shells) settle to the bottom of thedepositional basin and later become compacted or recemented into micrite, chalk, coquina,or fossiliferous limestone (Fig. 2.6). The same size classifications exist for biogenic rocksas with terrigenous rocks (coarse, medium, fine, and very fine). Therefore, the textures become

bioclastic: coarse, bioclastic: medium, bioclastic: fine, and bioclastic: very fine.

Sedimentary Structures

The deposition of sediments results in features, sedimentary structures, that are useful inthe interpretation of the rock's environment of origin. Horizontal layering is a large-scaleprimary structure that can be seen in cross sections (such as in the walls of a canyon) ofany material deposited under the influence of gravity. Within given layers, sedimentary

rocks often have thinner layers inclined to the overall horizontal layering. This is called

cross-bedding.

Cross-bedded deposits can be marine (deposited in the ocean), aeolian (deposited bywind action), or fluvial (deposited by stream action) in origin. Small-scale cross-beddingis made by the movement of sediment ripple marks (>1 to a few inches high). A ripplemark is a ridge-and-trough set that is formed by the action of wind or water. These can either be symmetrical (nonbreaking waves) or asymmetrical (breaking waves, currents) andare developed in aeolian, marine, fluvial or lacustrine (lake) environments. If a ripplemark is asymmetrical,

woods 2009 - physical geology - laboratory manual 4th ed

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