The Properties Of Minerals

Minerals have specific properties determined by their chemistry and crystal structure. Certain properties are characteristic of certain minerals, and one can identify minerals by learning these properties. The most common properties are crystal form, color, hardness, luster, cleavage, specific gravity, and taste.

When a mineral grows freely, it forms a characteristic geometric solid bounded by geometrically arranged plane surfaces (this is the crystal form). This symmetry is an external expression of the symmetric internal arrangement of atoms, such as in repeating tetrahedron arrays. Individual crystals of the same mineral may look somewhat different because the relative sizes of individual faces may vary, but the angles between faces are constant and diagnostic for each mineral.

Every mineral has a characteristic crystal form. some minerals have such distinctive forms that they can be readily identified without measuring angles between crystal faces. For instance, pyrite is recognized as interlocking growth of cubes, whereas asbestos forms long silky fibers. These distinctive characteristics are known as growth habit.

Cleavage is the tendency of a mineral to break in preferred directions along bright reflective planar surfaces. External structure deters the planar surface along which cleavage occurs; cleavage occurs along planes where the bands between the atoms are relatively weak.

Luster is the quality and intensity of light reflected from a mineral. Typical lusters include metallic (like a polished metal), vitreous (like a polished glass),

MOH'S HARDNESS SCALE

10

Diamond

9

Corundum (ruby, sapphire)

8

Topaz

7

Quartz

6

Potassium feldspar (pocketknife, glass)

5

Apatite (teeth, bones)

4

Fluoride

3

Calcite (copper penny)

2

Gypsum ( ngernail)

1

Talc

resinous (like resin), pearly (like a pearl), and greasy (oily).

Color is not reliable for identification of minerals, since it is typically determined by ionic substitution. For instance, sapphire and rubies are both varieties of the mineral corundum, but with different types of ionic substitution. However, the color of the streak a mineral leaves on a porcelain plate is often diagnostic for opaque minerals with metallic lusters.

The density of a mineral is a measure of mass per unit volume (g/cm3). Density describes "how heavy the mineral feels." Specific gravity is an indirect measure of density; it is the ratio of weight of a substance to the weight of an equal volume of water (specific gravity has no units because it is a ratio).

Hardness is a measure of the mineral's relative resistance to scratching, as listed in the table "Moh's Hardness Scale." Hardness is governed by the strength of bonds between atoms and is very distinctive and useful for mineral identification. A mineral's hardness can be determined by the ease with which one mineral can scratch another. For instance, talc (used for talcum powder) is the softest mineral, whereas diamond is the hardest mineral.

See also crystal, crystal dislocations; Dana, James Dwight; economic geology; geochemistry; petrology and petrography; thermodynamics.

FURTHER READING

Barthelmy, David. "Mineralogy Database." Available online. URL: http://www.webmineral.com/. Last updated August 31, 2008. Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth, an Introduction to Physical Geology. 5th ed. New York: John Wiley and Sons, 2004.

monsoons, trade winds A wind system that changes direction with the seasons is known as a monsoon, after the Arabic term mausim, meaning seasons. The Arabian Sea is characterized by monsoons, with the wind blowing from the northeast for six months, then from the southeast for the other half of the year. Seasonal reversal of winds is probably best known from India and southern Asia, where monsoons bring seasonal rains and floods.

In contrast to monsoons, trade winds are steady winds that blow between 0° and 30° latitude, from the northeast to southwest in the Northern Hemisphere and from southeast to northwest in the Southern Hemisphere. The trade winds are formed as the cool air from Hadley cell circulation returns to the surface at about 15-30° latitude, and then returns to the equatorial region. The Coriolis force deflects the moving air to the right in the Northern Hemisphere, causing the air to flow from northeast to southwest, and to the left in the Southern Hemisphere, causing a southeast to northwest flow. They are named trade winds because sailors used the reliability of the winds to aid their travels from Europe to the Americas. The doldrums, an area characterized by weak stagnant air currents, bound the trade winds, on high latitudes by the horse latitudes, characterized by weak winds, and toward the equator. The origin of the term horse latitudes is uncertain, but legend has it that it comes from ships traveling to the Americas that became stranded by the lack of winds in these regions, and were forced to kill onboard horses to conserve fresh water supplies and to eat their meat.

The Asian and Indian monsoon originates from differential heating of the air over the continent and ocean with the seasons. In the winter monsoon, the air over the continents becomes much cooler than the air over the ocean, and a large, shallow, high-pressure system develops over Siberia. This produces a clockwise rotation of air that rotates over the South China Sea and Indian Ocean, producing northeasterly winds and fair weather with clear skies over eastern and southern Asia. In contrast, in the summer monsoon, the air pattern reverses itself as the air over the continents becomes warmer than the air over the oceans. This produces a shallow, low-pressure system over the Indian subcontinent, within which the air rises. Air from the Indian Ocean and Arabian Sea rotates counterclockwise into the low-pressure area, bringing moisture-laden winds into the subcontinent. As the air rises due to convergence and orographic (mountain) effects, it cools below its saturation point, resulting in heavy rains and thunderstorms that characterize the summer monsoon of India from June through September. Some regions of India, especially the Cherrapunji area in the Khasi Hills of northeastern India, receive more than 40 inches (1,000 cm) of rain during a summer monsoon. A similar pattern develops over southeast Asia. other less intense monsoons are known from Australia, south America, Africa, and parts of the desert southwest, Pacific coast, and Mississippi Valley of the united states.

The strength of the Indian monsoon is related to the El Niño-southern oscillation. During the El Niño events, surface water near the equator in the central and eastern Pacific is warmer than normal, forming excessive rising air, thunderstorms, and rains in this region. This pattern causes air to sink over eastern Asia and India, leading to a summer monsoon with much lower than normal rainfall totals.

See also atmosphere; climate; climate change; El Niño and the Southern Oscillation (ENSO); energy in the Earth system.

FURTHER READING

Ahrens, C. D. Meteorology Today, An Introduction to Weather, Climate, and the Environment. 6th ed. Pacific Grove, Calif.: Brooks/Cole, 2000. Intergovernmental Panel on Climate Change. Climate Change 2007: The Physical Science Basis. Contributions of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by s. solomon, D. Qin, M. Manning, Z. Chen, m. marquis, K. B. Averyt, m. Tignor, and H. L. Miller. Cambridge: Cambridge university Press, 2007.

Intergovernmental Panel on Climate Change home page. Available online. uRL: http://www.ipcc.ch/index.htm. Accessed January 29, 2009.

Neogene The Neogene is the second of three periods of the Cenozoic, including the Paleogene, Neogene, and Quaternary, and the second of two subperiods of the Tertiary, younger than Paleogene. Its base is at 23.8 million years ago and its top is at 1.8 million years ago, followed by the Quaternary period. Charles Lyell proposed the subdivision of the Neogene into the Miocene, Pliocene, Pleistocene, and Recent epochs in his book Principles of Geology in 1833. Austrian geologist Moriz Hornes formally proposed the currently accepted division of the Neogene in 1835 and included only the older parts of Lyell's Neogene.

The Atlantic and Indian Oceans were open in the Neogene, and the Earth's plate mosaic resembled the modern configuration. The collision of India with Asia was well under way, and Australia had already rifted and was moving away from Antarctica, isolating Australia and leading to the development of the cold circumpolar current and the Antarctic ice cap. Subduction and accretion events were active along the Cordilleran margins of North and south America. Basin-and-range extension was active, and the Columbia River basalts were erupted in the northwestern United States. The San Andreas fault developed in California during subduction of the East Pacific Rise.

one of the more unusual events to mark the Neogene is the development of up to 1.2 miles (2 km) of salt deposits between 5.5 and 5.3 million years ago in the Mediterranean region. This event, known as the Messinian salt crisis, was caused by the isolation of the Mediterranean Sea by collisional tectonics and falling sea levels that caused the sea to at least partially evaporate several times during the 200,000-year-long crisis. Rising sea levels ended the

Messinian crisis 5.3 million years ago, when waters of the Atlantic rose over the natural dam in the Strait of Gibraltar, probably forming a spectacular waterfall.

A meteorite impact event occurred about 15 million years ago, forming the 15-mile- (24-km-) wide Ries Crater near Nordlingen, Germany. The meteorite that hit the Earth in this event is estimated to have been half a mile (1 km) in diameter, releasing the equivalent of a 100,000 megaton explosion. About 55 cubic feet (155 m3) of material was displaced from the crater, some of which formed fields of tektites, unusually shaped melted rock that flew through the air for up to 248.5 miles (400 km) from the crater.

The Neogene saw the spread of grasses and weedy plants across the continents and the development of modern vertebrates. Snakes, birds, frogs, and rats expanded their niches, whereas the marine invertebrates experienced few changes. Humans evolved from earlier apelike hominids. Continental glaciations in the Northern Hemisphere began in the Neogene, and continue to this day. See also Cenozoic; Tertiary.

FURTHER READING

Prothero, Donald R., and Robert H. Dott. Evolution of the

Earth. 6th ed. Boston: McGraw-Hill, 2002. Stanley, Steven M. Earth and Life through Time. New

York: Freeman, 1986. Walsh, Stephen L. "The Neogene: Origin, Adoption, Evolution, and Controversy." Earth Science Reviews 89 (2008): 42-72.

Neolithic Neolithic is an archaeological term for the last division of the Stone Age, during which time humans developed agriculture and domesticated animals. The transition from hunter-gatherer and nomadic types of existence to the development of farming took place about 10,000-8,000 years ago in the Fertile Crescent, a broad stretch of land that extends from southern Israel through Lebanon, western Syria, Turkey, and through the Tigris-Euphrates Valley of Iraq and Iran. The Neolithic revolution and the development of stable agricultural practices led to an unprecedented explosion of the human population that continues to this day. About a million years ago, an estimated few thousand humans migrated on the Earth, and by about

10 thousand years ago this number had increased only to a mere 5-10 million. When humans began stable agricultural practices and domesticated some species of animals, the population rate increased substantially. The increased standards of living and nutrition caused the population growth to soar to about 20 million by 2,000 years ago, and 100 million by 1,000 years ago. By the 18th century, humans manipulated their environments to a greater degree, began public health services, and recognized and sought treatments for diseases that previously claimed many lives. The average life span soared, and world population surpassed 1 billion in the year

Abu Hureyra

Map of the Fertile Crescent stretching from the Levant (Israel and Lebanon) through rolling hills in parts of Syria, southern Turkey, Iraq, and Iran. Ancient cities and agriculture arose in this area, with many early cities located in the Sumerian region between the Tigris and Euphrates Rivers, in what is now southern Iraq.

1810. A mere 100 years later, the world population doubled again to 2 billion, and had reached 4 billion by 1974. World population is now close to 7 billion and climbing more rapidly than at any time in history, doubling every 50 years.

FURTHER READING

Diamond, John. Guns, Germs, and Steel: The Fates of

Human Societies. New York: W. W. Norton, 1999. Leonard, Jonathan N. The First Farmers: The Emergence of Man. New York: Time-Life Books, 1973.

Neptune The eighth and farthest planet from the center of the solar system, the giant Jovian planet Neptune orbits the Sun at a distance of 2.5 billion miles (4.1 billion km, or 30.1 astronomical units), completing each circuit every 165 years. Rotating about its axis every 16 hours, Neptune has a diameter of 31,400 miles (50,530 km) and a mass of more than 17.21 times that of Earth. Its density of 1.7 grams per cubic centimeter shows that the planet has a dense rocky core surrounded by metallic, molecular, and gaseous hydrogen, helium, and methane, giving the planet its blue color.

Neptune is unusual in that it generates its own heat, radiating 2.7 times more heat than it receives from the sun. The source of this heat is uncertain, but it may be heat trapped from the planet's formation that is only slowly being released by the dense atmosphere. The cloud systems that trap this heat are visible from Earth-based telescopes and include some large hurricane-like storms such as the former Great Dark Spot, a storm about the size of the Earth, similar in many ways to the Great Red spot on Jupiter, but that has dissipated.

Neptune has two large moons visible from Earth, Triton and Nereid, and six other smaller moons discovered by the Voyager 2 spacecraft. Triton has a diameter of 1,740 miles (2,800 km) and orbits Neptune at a distance of 220,000 miles (354,000 km) from the planet. It is the only large moon in the solar system that has a retrograde orbit.

See also Earth; Jupiter; Mars; Mercury; Saturn; solar system; Uranus; Venus.

FuRTHER READING

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. Upper Saddle River, N.J.: Addison-Wesley, 2007.

Comins, Neil F. Discovering the Universe. 8th ed. New

York: W. H. Freeman, 2008. National Aeronautic and Space Administration. Solar System Exploration page. Neptune. Available online. URL: http://solarsystem.nasa.gov/planets/profile. cfm?Object=Neptune. Last updated June 25, 2008.

Neptune and swirling clouds. Neptune's blue-green atmosphere is shown in greater detail than ever before by the Voyager 2 spacecraft as the craft rapidly approaches its encounter with the giant planet. This color image, produced from a distance of about 10 million miles (16 million km), shows several complex and puzzling atmospheric features. The Great Dark Spot (GDS) seen at the center is about 8,080 miles (l3,000 km) by 4,100 miles (6,600 km) in size—as large along its longer dimension as the Earth. The bright, wispy "cirrus-type" clouds seen hovering in the vicinity of the GDS are higher in altitude than the dark material of unknown origin that defines its boundaries. A thin veil often fills part of the GDS interior, as seen on the image. The bright cloud at the southern (lower) edge of the GDS measures about 600 miles (l,000 km) in its north-south extent. The small, bright cloud below the GDS, dubbed the "scooter," rotates faster than the GDS, gaining about 30 degrees eastward (toward the right) in longitude every rotation. Bright streaks of cloud at the latitude of the GDS, the small clouds overlying it, and a dimly visible dark protrusion at its western end are examples of dynamic weather patterns on Neptune, which can change significantly on timescales of one rotation (about 16 hours). (NASA Jet Propulsion Laboratory (NASA-JPL)

Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. St. Paul, Minn.: West Publishing Company, 1991.

North American geology The North American continent contains the oldest rocks known on the planet, and its core is made of a complex amalgam of some of the oldest Archean cratons on Earth. These cratons formed in complex accretionary orogenic events and then were brought together to form the cratonic core of North America in a series of col-lisional events in the Proterozoic. Embryonic North America formed an integral part of the superconti-nents—Pangaea, and the progressively older Gond-wana (1.0-0.54 billion years ago), Rodinia (1.6-1.0 billion years ago), Nuna (2.5-1.6 billion years ago), and Kenoraland (Archean)—and is the largest preserved fragment in the world of a continent assembled in the Proterozoic. Following the amalgamation of these continental blocks in the core of the continent, North America continued to evolve and grow by accretion and collision of exotic arc and other terranes in the Appalachian/Caledonian, Cordille-ran, and Franklinian orogens. Understanding how the North American continent formed is instructive to understanding processes in continental formation and growth worldwide.

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