Encyclopedia of Global Resources part 30 doc

Uses of Corundum and Emery Corundum has limited use as crushed grit or powder for polishing and finishing optical lenses and metals and is used on paper, cloth, and abrasive wheels.. Cot

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dated to 5200 b.c.e By 3400 b.c.e., the fossil record

shows a marked change in corn, notably increased

cob and kernel size, indicating greater domestication

Fully domesticated corn (which could not survive

without human help) had replaced the wild and other

early types of corn by 700 c.e

Extensive attempts at hybridization began in the

late nineteenth century, but the increase in yield

was usually a disappointing 10 percent or so By 1920,

researchers had turned to inbreeding hybridization

programs In these, corn is self-fertilized, rather than

being allowed to cross-pollinate naturally Following a

complex sequence of crossing and testing different

varieties, the lines with the most desirable traits were

put into commercial use, and they often produced 25

to 30 percent gains in yield Although these early

hy-brids focused on increasing the yield, researchers

later began to look for insect-resistant and

disease-resistant qualities as well One of the hybridizers of the

1920’s was Henry A Wallace, founder of Pioneer Seed

Company (the world’s largest seed company) and

later U.S vice president under Franklin D Roosevelt

By the 1950’s, hybrid corn varieties were in

wide-spread use

Obtaining Corn

Corn processing takes place in one of three ways: wet

milling, dry milling, or fermentation In wet milling,

corn is soaked in a weak sulfurous acid solution,

ground to break apart the kernel, and then separated

The resulting by-products are found nearly

every-where Dry milling is a simpler process, involving the

separation of the hull from the endosperm (the food

storage organ, which is primarily starch in most corn)

and the germ (the plant embryo) by repeated

grind-ing and sievgrind-ing Fermentation of corn changes the

starch to sugar, which is then converted by yeast to

al-cohol The process eventually results in ethyl alcohol,

or ethanol (which is blended with gasoline to reduce

carbon monoxide emissions), acetone, and other

sub-stances

Uses of Corn

The types of corn still in use are dent, flint, flour, pop,

and sweet Dent corn, characterized by a “dent” in the

top of each kernel, is the most important commercial

variety Flint corn tends to be resistant to the rots and

blights known to attack other types; it is also more

tol-erant of low temperatures and therefore appears at

the geographical edge of corn’s range Flour corn is

known for its soft kernel, making it easier to grind into flour and thus popular for hand-grinding A mainstay

at American movie theaters and as a snack food, pop-corn will, with an optimum moisture content of about

13 percent, explode to as much as thirty times its origi-nal volume when heated Also popular in the United States and eaten fresh, sweet corn is so named be-cause, unlike other types, most of the sugars in the kernel are not converted to starch

Commercially, corn is used mostly for livestock feed and industrial processing It is high in energy and low in crude fiber but requires supplements to make a truly good feed Industrial processing creates

a great variety of products found in everyday life— underscoring the importance of corn to the world’s economy

The cornstarch from wet milling supplies corn syrup (it is sweeter than sugar and less expensive, and billions of dollars’ worth is produced for soft drink manufacturers each year), starches used in the textile industry, ingredients for certain candies, and sub-stances used in adhesives, to name a few Other by-products provide cooking oil; oil used in mayonnaise, margarine, and salad dressing; soap powders; and livestock feed Dry milling produces hominy, grits, meal, and flour, all of which are used for human con-sumption

Brian J Nichelson

Further Reading

Fussell, Betty The Story of Corn New York: Knopf, 1992 Mangelsdorf, Paul C Corn: Its Origin, Evolution, and Improvement Cambridge, Mass.: Belknap Press of

Harvard University Press, 1974

Pollan, Michael “Industrial Corn.” In The Omnivore’s Dilemma: A Natural History of Four Meals New York:

Penguin Press, 2006

Smith, C Wayne, Javier Betrán, and E C A Runge

Corn: Origin, History, Technology, and Production.

Hoboken, N.J.: John Wiley, 2004

Sprague, G F., and J W Dudley, eds Corn and Corn Im-provement 3d ed Madison, Wis.: American Society

of Agronomy, 1988

Wallace, Henry A., and William L Brown Corn and Its Early Fathers Rev ed Ames: Iowa State University

Press, 1988

Warman, Arturo Corn and Capitalism: How a Botanical Bastard Grew to Global Dominance Translated by

Nancy L Westrate Chapel Hill: University of North Carolina Press, 2003

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White, Pamela J., and Lawrence A Johnson, eds Corn:

Chemistry and Technology St Paul, Minn.: American

Association of Cereal Chemists, 2003

Web Sites

U.S Department of Agriculture, Economic

Research Service

Corn

http://www.ers.usda.gov/Briefing/Corn

U.S Department of Agriculture, Economic

Research Service

Feed Grains Database

http://www.ers.usda.gov/Data/FeedGrains

See also: Agricultural products; Agriculture

indus-try; Biofuels; Ethanol; Horticulture; Plant

domestica-tion and breeding

Corundum and emery

Category: Mineral and other nonliving resources

Where Found

Corundum occurs in a number of geological

environ-ments The most important of these are contact

meta-morphic zones, silica-poor igneous rocks, pegmatites,

and placers The principal producer of corundum is

South Africa, but commercial deposits also exist in

Canada, India, Madagascar, and Russia Minor

depos-its are found in North Carolina and Georgia The

fin-est rubies and sapphires have always been mined in

Asia: rubies from Burma, India, and Thailand;

sap-phires from Sri Lanka, India, and Thailand

Turkey is the world’s leading producer of emery,

with other significant deposits found on the Greek

is-land of Naxos and in the Ural Mountains of Russia In

the United States, the most important emery deposits

are around Peekskill, New York The United States

ex-ports no emery and imex-ports most of what it consumes

from Turkey and Greece

Primary Uses

Corundum and emery are used as abrasives In

addi-tion, the transparent, colored varieties of corundum,

ruby and sapphire, have long been prized as gems

be-cause of their rarity and beauty

Technical Definition Corundum, another name for aluminum oxide (Al2O3), is the second-hardest natural substance It oc-curs as an opaque material and as transparent gems Emery is a natural mixture of corundum and magne-tite

Description, Distribution, and Forms Corundum, or aluminum oxide, can be categorized

in two ways: as an abrasive and as a gem mineral Both uses result from corundum’s extreme hardness (nine

on the Mohs scale) Corundum as an abrasive has been largely replaced by alumina

Emery, named for Cape Emeri in Greece, is a natu-ral gray to black mixture of corundum and magnetite, usually with lesser amounts of spinel and hematite The hardness of emery ranges from seven to nine, and its usefulness as an abrasive increases with the co-rundum content Like coco-rundum, emery has largely been replaced, but in this case by synthetic materials

History The gem varieties of corundum, ruby and sapphire, have a long history of use Ruby attains its red color from the presence of chromic oxide Sapphires occur

in a variety of colors, but those most prized as gems are colored deep blue by the presence of iron and tita-nium oxides Beginning in the early twentieth cen-tury, both rubies and sapphires were synthesized Even the prized “star” varieties can be manufactured, and the synthetic gems are virtually indistinguishable from their natural counterparts

Obtaining Corundum and Emery Both corundum and emery are obtained through mining, the later of which has been mined in Greece for more than two thousand years However, most co-rundum and emery are now obtained synthetically

Uses of Corundum and Emery Corundum has limited use as crushed grit or powder for polishing and finishing optical lenses and metals and is used on paper, cloth, and abrasive wheels As co-rundum wears, small pieces constantly flake off to form fresh edges, enhancing its ability to cut In addi-tion to their value as gems, synthetic rubies are used in industrial and medical lasers

Emery finds some applications on coated abrasive sheets (emery cloth), as grains and flour for polishing glass and metal surfaces, on grinding wheels, and on

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nonskid pavements and stair treads Emery forms

principally by contact metamorphism in limestones

Donald J Thompson

Web Site

Corundum

http://www.minerals.net/mineral/oxides/

corundum/corundum.htm

See also: Abrasives; Gems; Metamorphic processes,

rocks, and mineral deposits; Mohs hardness scale;

Placer deposits

Cotton

Category: Plant and animal resources

Where Found

Cotton (genus Gossypium) is grown within the tropical

and subtropical regions of the world in areas that have

adequate amounts of sunshine and fertile soil In

gen-eral, areas that receive 600 to 1,200 millimeters of

rainfall annually are best suited for cotton production

because the plant requires a large amount of water in

order to grow well However, dryland cotton farming

occurs in areas with lower rainfall totals with the help

of irrigation

Within the United States, most of the cotton crop is

grown in Alabama, Arizona, Arkansas, California,

Georgia, Florida, Kansas, Louisiana, Mississippi,

Mis-souri, New Mexico, North Carolina, Oklahoma,

South Carolina, Tennessee, Texas, and Virginia

Cot-ton is also commercially produced in China, India,

Pa-kistan, UzbePa-kistan, Brazil, Australia, Egypt,

Argen-tina, and Turkey

Primary Uses

Cotton fibers are primarily used in the textile industry

for the manufacture of clothing Smaller amounts of

cotton are used to produce fishing nets, cotton paper,

tents, and gunpowder In some parts of the world,

cot-ton is still used to make mattresses Refined cotcot-ton-

cotton-seed oil is used as a vegetable oil in many foods, such

as baked goods Cottonseed hulls are often mixed in

with other plant materials to form a roughage ration

for cattle

Technical Definition Cotton is a plant in the mallow family, Malvaceae This botanical group is a large family containing a number

of plants important to horticulture, including the hi-biscus Cotton plants may grow to a height of 3 meters, but most commercial varieties have been bred to be shorter for easier harvesting The plant has leaves with three to seven lobes; the ovary of the cotton flower is a capsule or boll, which, when ripe, opens along the dark brown carpels to reveal the usually white inner fibers Longer fibers are known as staples, while shorter fibers are called linters When separated from one another by a process known as ginning, the fibers can be woven into cotton yarn and used for tex-tile manufacturing

Description, Distribution, and Forms

Four species of cotton—Gossypium hirsutum, G bar-badense, G arboreum, and G herbaceum—are commer-cially produced, with G hirsutum accounting for

about 90 percent of the world’s production Approxi-mately 8 percent of the world’s cotton is produced

from G barbadense, and the remaining 2 percent comes from G arboreum and G herbaceum G hirsutum,

up-land cotton, is native to Florida, the Caribbean, Mex-ico, and Central America and is the cotton with which

most Americans are familiar G barbadense is a plant of

tropical South America and is known commercially as

pima cotton Tree cotton, G arboreum, is native to

In-dia and Pakistan, while the last commercially

impor-tant species, G herbaceum, is found in the Arabian

Pen-insula and southern Africa and is known as levant cotton

In addition to the four commonly cultivated spe-cies of cotton, five noncommercial spespe-cies of this ge-nus are found in tropical and subtropical areas of the

world These include G australe and G sturtianum, both found in Australia; G darwinii, which grows in the Galápagos Islands; G thurberi, a plant of northern Mexico and Arizona; and G tomentosum, a Hawaiian

Is-land endemic

Cotton is one of the most pesticide-intensive of all cultivated crops Genetically modified cotton was de-veloped in the twentieth century in an attempt to alle-viate some of the cotton farmer’s dependence upon

pesticide use The bacterium Bacillus thuringiensis

pro-duces a natural pesticide that is toxic to a number of insects, most notably members of the insect orders Coleoptera (beetles) and Lepidoptera (butterflies and moths) By inserting within cotton tissues the

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B thuringiensis gene that codes for this specific

toxic-ity, geneticists were able to develop cotton varieties

that were resistant to some of the important pests,

such as boll weevils and bollworms In recent years,

some of this cotton has been found to be no longer

re-sistant to pests

A small percentage of commercially grown cotton

is produced with organic methods No insecticides

are used on organically grown cotton, and crop

rota-tion is a technique used in an attempt to keep the soil

fertile and to discourage pests

History

Cotton has been cultivated by a number of cultures

for at least six thousand years The ancient peoples of

India, China, Egypt, and Mexico all grew and made

use of cotton in weaving textiles The fiber has been

extensively traded throughout both the Old and New

Worlds for the past two thousand years During the

first century c.e., traders from the Middle East

brought fabrics such as calico and muslin to markets

in southern Europe Great Britain’s famous East India

Company brought cotton cloth from India during the

seventeenth century Raw cotton was imported from

the American colonies in the 1700’s, and this import

spurred a need for the development of machinery

that could process and spin the cotton Advances such

as the spinning jenny, developed in 1764, and Sir

Richard Arkwright’s spinning frame, developed in

1769, enabled Britain to produce cotton yarn and

cloth with increased speed and efficiency American

Eli Whitney’s well-known 1793 invention of the cotton

gin allowed cotton seeds to be easily stripped from the

fibers

During the American Civil War, Britain could not

obtain cotton from the United States and so

bar-gained with Egypt for its supply After the war,

how-ever, Britain turned again to buying its cotton from

the United States, and the resulting loss of trade was a

severe blow to the Egyptian economy Cotton

contin-ued to be a staple crop for the southern United States

throughout the 1800’s and 1900’s and remains a

pri-mary export crop for the country

Obtaining Cotton

In traditional cotton farming, cotton fields are

cleared of old plants from the previous growing

sea-son and thoroughly plowed into rows The farmer

may clear fields in the winter or wait until early spring

before planting Cotton seeds are planted

mechani-cally in the spring, when the soil is warm enough for seeds to germinate Germination occurs in five to ten days if adequate soil moisture is available; a full stand

of cotton is generally present in eleven days if germi-nation is successful Within five to seven weeks

“squares” (cotton flower buds) open to produce a creamy yellow flower that self-pollinates within three days As the flower matures it changes color from light yellow to pink to darker red before falling off the plant to reveal the tiny “boll.” Approximately forty-five to eighty days after the bolls form, they split along the carpels of the fruit to reveal white fibers A boll may contain as many as 500,000 of these fibers, which are called staples Staple length varies among the dif-ferent cotton species, with upland cotton having sta-ple lengths of 0.81 inch to 1.25 inches and pima cot-ton having lengths of 1.31 inches to 1.5 inches

If the cotton is to be mechanically picked, it must

A woman in India brings a bundle of organic cotton to the town cen-ter to be ginned (AP/Wide World Photos)

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first be defoliated, so that leaves will not be picked

along with the cotton bolls After completing the

de-foliation, cotton pickers can drive through the fields

and pick the cotton as long as it is dry Moisture, from

either dew or rain, damages the cotton fibers once the

bolls have opened, so farmers hope for dry weather

during harvesting

Picked cotton is formed into bales weighing 218

ki-lograms each; thirteen to fifteen bales may then be

formed into modules and transported to the cotton

gin The ginning process fluffs the cotton and cleans

it of dirt, plant trash, and seeds Cleaned cotton is

compressed again into bales, which are inspected; if

cleared for sale, the bales are stored in a

temperature-and moisture-controlled warehouse until being moved

to a processing facility

Worldwide, 31.3 million hectares of cotton were

planted in 2008, with 112.9 million 218-kilogram bales

produced China leads the world in cotton

produc-tion, with 25.3 million bales produced in 2007 India,

the United States, Pakistan, and Brazil complete the

list of the top five cotton-producing countries

Uses of Cotton

Cotton’s primary use is in the manufacture of textiles

Although there are many different types of cotton

fab-ric, some of the best known include terrycloth, a soft

fabric used to make bath cloths, towels, and robes;

denim, used in jean manufacture, which can be dyed

a variety of colors but usually is dyed blue; chambray,

a soft, blue cloth from which work shirts are made;

and corduroy and twill, from which heavier, sturdier

items of clothing are made Cotton yarn is used in

quilt making Egyptian cotton is often used to

pro-duce bedsheets and pillowcases

After cotton seeds are removed from raw cotton

during the ginning process, cottonseed oil can be

re-fined and used as a vegetable oil in cooking It is also

used in shortening and salad dressing and is a

com-mon component of baked goods such as crackers and

cookies Cottonseed meal and cottonseed hulls are

fed to ruminant livestock such as cattle and goats, and

the meal can be fed to fish and poultry Nonruminant

mammals are unable to eat cottonseed products

be-cause of a toxic chemical, gossypol, which will sicken

and possibly kill these animals

Strong fishnets and tents can be made from cotton

fibers When exposed to nitric acid, cotton can be

used to form “guncotton” or “smokeless powder,” a

type of explosive that is safer to use than black powder

Cotton fibers have been used for many years in the production of paper and as binding for books Cotton paper is stronger than wood-pulp-based paper and re-tains ink better Therefore, it is often used to produce paper money and archival copies of important books and documents

Lenela Glass-Godwin

Further Reading

Hake, S Johnson, T A Kerby, and K D Hake Cotton Production Manual Oakland: University of

Califor-nia, Division of Agriculture and Natural Resources, 1996

Smith, C Wayne Crop Production: Evolution, History, and Technology New York: John Wiley and Sons,

1995

Smith, C Wayne, and Joe Tom Cothren Cotton: Ori-gin, History, Technology, and Production New York:

John Wiley and Sons, 1999

Tripp, Robert Burnet Biotechnology and Agricultural Development: Transgenic Cotton, Rural Institutions, and Resource-Poor Farmers New York: Routledge,

2009

Web Sites National Cotton Council of America http://www.cotton.org/

Sustainable Cotton Project http://www.sustainablecotton.org/

See also: Agricultural products; Agriculture indus-try; Agronomy; American Forest and Paper Associa-tion; Botany; Farmland; Flax; Hemp; IrrigaAssocia-tion; Paper; Paper, alternative sources of; Plant fibers; Renewable and nonrenewable resources; Textiles and fabrics

Council of Energy Resource Tribes

Category: Organizations, agencies, and programs Date: Established 1975

The Council of Energy Resource Tribes (CERT) seeks fair payment for resources pumped or mined on Ameri-can Indian reservation land and advises tribes re-garding conservation, lease arrangements, royalties, and economic development.

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The Council of Energy Resource Tribes was founded

by a group of tribal leaders seeking to monitor and

re-ceive appropriate payment for energy resources on

American Indian land Historically, tribes had been

underpaid, sometimes scandalously, for mineral

re-sources obtained on their lands The leasing policies

of the U.S Bureau of Indian Affairs (BIA)

engen-dered considerable controversy and resentment; the

BIA frequently allowed corporations to obtain oil,

coal, and other resources from American Indian land

for prices well under market value Moreover, leasing

royalties sometimes were underpaid or went unpaid

altogether

Estimates indicate that energy resources contained

on American Indian land account for 10 percent of

the U.S total One of the founders of CERT, Peter

MacDonald, a Navajo who was CERT’s first elected

chair, referred to these resources as wealth “so vast it

has not yet been measured.” CERT set out to

inven-tory the resources of the tribes of the West and found

that they controlled one-third of U.S coal and

ura-nium resources and large supplies of petroleum and

natural gas CERT began to demand higher royalties

for coal, oil, and uranium mined on American Indian

lands and worked to integrate various aspects of

reser-vation energy development

Impact on Resource Use

The founders of CERT had noted the activities of

the Organization of Petroleum Exporting Countries

(OPEC) as an influential international energy

re-source organization, and they hoped to achieve

simi-lar influence over tribal resources as they entered

the domestic market CERT helps tribes negotiate

contracts regarding resources found on reservation

lands It provides on-site technical assistance and

ad-vice in the areas of conservation, resource

manage-ment, and economic development CERT was founded

by leaders from twenty-five tribes; by the end of the

first decade of the twenty-first century, it had more

than sixty tribal members The organization’s

head-quarters are in Denver, Colorado

Vincent M D Lopez

Web Site

Council of Energy Resource Tribes

http://www.certredearth.com/

See also: Coal; Oil and natural gas distribution; Oil embargo and energy crises of 1973 and 1979; Oil in-dustry; Organization of Petroleum Exporting Coun-tries; Uranium

Cropland See Farmland

Crystals

Category: Mineral and other nonliving resources

Crystals are composed of regularly repeating three-dimensional patterns of atoms or ions; a crystal is therefore a highly ordered structure Crystals have a number of electronic and scientific applications, in-cluding uses in optics and in radio transmitters (piezo-electric quartz crystals) Well-formed crystals are also prized by collectors, and crystals of gem minerals are cut into jewelry.

Background Crystals are solids that naturally display smooth pla-nar exterior surfaces called “faces,” which form dur-ing the growth of the solid These faces collectively produce a regular geometric form that mimics the or-derly internal atomic arrangement of the elements present in the solid Some scientists use the term

“crystal” to refer to any solid having an ordered inter-nal atomic structure regardless of whether the solid displays faces However, most scientists use the word

“crystalline” for such solids when no faces are present Many solids display a cleavage, a flat planar surface formed when the solid is broken; cleavage fragments are sometimes mistaken for crystals

Crystals are described and classified according to the symmetrical relationship existing between the faces The fundamental way of describing a crystal

is to list the “forms” that it displays Scientists recog-nize a total of forty-eight different forms, many desig-nated by common geometric terms such as cube, octahedron, tetrahedron, pyramid, and prism Most crystals display multiple forms For example, quartz crystals display one prism and at least two sets of pyra-mids Considering every possible symmetrical arrange-ment of faces, every crystal can be placed into one

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of thirty-two groupings called crystal classes These

classes are further grouped into six crystal systems

based on similar symmetry characteristics The names

of the six systems, from most to least symmetrical,

are isometric, hexagonal, tetragonal, orthorhombic,

monoclinic, and triclinic

Where Crystals Are Formed

Large crystals can develop when the faces growing

in a melt, solution, or gas are unimpeded by other

surrounding solids This situation commonly occurs

where open cracks and cavities exist in rock and the

liquid or vapor from which the crystal is growing has

free access to the open space The largest crystals are

found in igneous pegmatites The Etta pegmatite in

the Black Hills of South Dakota contained a 12-meter

crystal weighing more than 18 metric tons The

larg-est known crystal was a single feldspar from a

pegma-tite in Karelia, Russia, that weighed several thousand

metric tons Crystals are also found along fault planes,

in hot springs areas, around vents for volcanic gases,

and in cavities within igneous and sedimentary rocks

where underground water is circulating Another

mechanism for the growth of crystals occurs during

the process of metamorphism Preexisting rocks that are subjected to elevated temperatures and pressures within the Earth can recrystallize while still solid During this metamorphism some of the new minerals that form have a strong surface energy and will de-velop faces even while in contact with other growing minerals

The growth conditions discussed above are so com-mon within the Earth that crystals can be found in al-most every state in the United States and every coun-try in the world It is impossible to specify all the important occurrences of large, well-formed crystals Some of the more notable classic localities in the United States include quartz in Hot Springs, Arkan-sas, and Herkimer County, New York; galena in the tristate district of Missouri, Kansas, and Oklahoma; zinc-bearing minerals in Franklin, New Jersey; garnets

at Gore Mountain, New York; iron oxides in the upper peninsula of Michigan; and fluorite and celestite at Clay Center, Ohio

Uses of Crystals Particularly well-formed crystals are highly prized by collectors and museums Most crystals, however, are

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more valuable for their chemistry or as crystalline

sol-ids Many crystals are crushed during the processing

of ore minerals It was a common practice for miners

to save the larger, better-formed crystals from the

crushing mill because they were worth more as

speci-mens for collectors than they were worth as ore

mate-rial Most crystals of gem minerals are cut and faceted

to make jewelry A large diamond crystal, for

exam-ple, is worth more as a well-faceted gemstone than as a

crystal specimen

There are a growing number of technological uses

of “crystalline solids” where the systematic internal

arrangement of atoms can produce a variety of

de-sirable physical phenomena useful in the fields of

electronics and optics As an example, very pure

untwinned quartz is called “optical grade crystal” even

though it lacks faces Quartz crystal is cut, ground, and

made into lenses and prisms for optical instruments

and is also used in radio oscillators, timing devices,

and pressure gauges in the electronics industry

Crystal Defects and Growth Rates

Crystal defects occur naturally as crystals are formed;

they are also sometimes introduced artificially, as they

have useful electrical, mechanical, and optical

quali-ties A growing crystal typically requires the proper

placement of trillions of atoms per hour About one

atom in every one hundred thousand is misplaced to

form a defect These defects can be point disorders,

or they can geometrically be combined to form line,

plane, or three-dimensional disorders The Schottky

defect is a point disorder in which an atom is missing

from the spot it should occupy, leaving a hole in the

pattern The Schottky defect results when a second

layer of atoms is quickly deposited before all the

posi-tions can be filled in the first layer The Frenkel defect

occurs when an atom is out of its proper position and

can be found nearby, inappropriately stuck between

other atoms The impurity defect is yet another point

disorder, occurring when an atom of a foreign

ele-ment (an impurity) either substitutes for the normal

atom or is stuffed between the proper atoms of the

structure

Coloration can be caused by various point defects

When an electron is captured by the hole of a Frenkel

defect it will absorb energy from passing light and

be-come what is known as a “color center.” An

abun-dance of Frenkel color centers in fluorite will give the

crystal a purple color An impurity defect can be

ac-companied by a shift in electrons, also causing a color

center Smoky quartz is caused by color centers result-ing from impurity defects The electron shifts are ei-ther induced by low levels of radiation in the Earth over geological time or by artificial exposure to an in-tense X-ray or gamma-ray beam for a few minutes A significant number of the smoky quartz crystals on the market began as natural colorless quartz that has been irradiated

Line disorders are linear defects and are com-monly called “dislocations” because they create an off-set within the crystal The most common is an edge dislocation resulting when an entire plane of atoms is pinched out and terminated as adjacent planes on ei-ther side begin to bond directly togeei-ther When crys-tals are stressed they will often deform by slipping along linear disorders

Crystals can also become deformed or malformed because of variations in the growth rates of different faces or different parts of the crystal When the chem-istry of the growing solution begins to lack the atoms needed by the crystal, then the faces can stop growing while the edges where faces meet will continue to grow

In extreme instances the resulting malformed crystal has a skeletal look, showing a network of edges with-out any faces, yet all the symmetrical forms are still evi-dent, allowing proper classification of the crystal

Twinning During formation, a solid may produce a symmetrical intergrowth of two or more crystals When the inter-growth is crystallographically controlled, the result-ing composite is called a twinned crystal The individ-ual crystals within the twinned aggregate are related

to one another by a different symmetrical element— one that is not seen in any of the individual parts This often results in a symmetrical, exotically shaped ag-gregate that does not appear to belong to any single crystal class Crystals displaying exceptional twins can

be more valuable for their twinning than as mineral specimens

History of Crystals Crystals have a history that reaches back into the realm of legends and myths An important early work that combined legend with the first sound science was

the thirty-seven-volume Historia Naturalis, written by

Pliny the Elder in the first century Pliny described many real as well as nonexistent crystals, which he stated were formed by such exotic processes as “the light of the moon” or “the purge from the sea.”

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Nicolaus Steno established the first law of

crystallog-raphy in 1669, known as the law of constancy of

inter-facial angles The law holds that for all crystals of a

given mineral the angles measured between similar

faces are always exactly the same This law allows for

the positive identification of deformed or malformed

crystals simply by measuring the angles between

exist-ing faces In 1781, René-Just Haüy was the first to

rec-ognize that a crystal is composed of a large number of

smaller particles arranged in a regular geometric

or-der such that it fills space without gaps This was a

re-markable advance, considering that it preceded the

concept of the atom in chemistry by more than twenty

years In 1830, based on graphical and mathematical

considerations, Johann Hessel predicted the

exis-tence of thirty-two classes of symmetry corresponding

to modern crystal classes In the 1920’s, two

crystallog-raphers, C H Hermann and Charles-Victor

Mau-guin, developed the notation that is used to designate

the symmetrical arrangement of faces found on any

crystal

Dion C Stewart

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