All that glitters: discover how rocks buried below the ground turn into the sparkling gold that coats shiny awards.
But before gold can be added to a trophy or medal, mining engineers must dig deep into the earth to bring gold-containing ore to the surface. Take a dazzling tour to find out how gold is extracted from hidden rocks and is then transformed into a gleaming trophy. Along the journey, James Webster, a geologist and curator of the exhibition Gold at the American Museum of Natural History in New York, will give you the scoop on this valuable element. Let's start the tour by traveling deep below ground to see where gold is formed.
A treasure trove of gold is hidden miles below Earth's surface. But the treasure isn't in the form of a giant heap. Instead, tiny specks of gold are scattered within rocks deep underground. Since it's impossible to extract the gold from so far underground, mining engineers have to wait for natural processes to move it toward Earth's surface.
In some areas, the gold-containing rocks are infiltrated by water that has been superheated to a vapor by pockets of magma. The specks of gold dissolve into this sizzling hot water vapor. The vapor, which is less dense than the surrounding rocks, rises--and the gold rides along with it.
Near Earth's surface, gold's free ride comes to an end. "When the hot water that contains the gold comes in contact with cooler rocks, it's going to drop in temperature," explains Webster. Like most solids, gold's solubility (measure of how well a solute can dissolve in a solvent) decreases as the temperature drops. Thus, the once-dissolved gold separates from the liquid, forming a solid gold deposit along shallow cracks in the ground. Called a lode deposit, this gold forms swirls, like the fudge in vanilla-fudge ice cream, throughout the rocks.
Now near Earth's surface, the lode deposits get exposed to the eroding actions of wind and running stream water. These processes can loosen fragments of gold from within cracks. The loosened gold then travels downstream. The gold collects in riverbeds and small depressions in the ground as grains, flakes, and nuggets. These deposits are called placer (PLA-sur) deposits. (See Nuts & Bolts, p. 14)
GO FOR GDLD
These surface deposits created by erosion are scarce. Most gold is found below the surface. To dig up enough gold to make trophies, awards, and even jewelry, engineers create a mine.
Open-pit mines are often used to collect gold from deposits that are spread out over a broad area, Webster says. First, engineers use giant bulldozers and other earth-moving equipment to scoop out loads of dirt and rocks. The resulting pit can span 1,000 meters (3,280 feet) across: That's twice as long as the Empire State Building is tall! Then, miners use explosives to blast through the bowl shaped crater. This round of blasting sends gold-containing rocks, or gold ore, sky-high before they crash to the ground in random piles. Then it's up to huge dump trucks to collect the golden debris.
MAKE IT SHINE
Passersby wouldn't be able to see sparkling nuggets piled up in the dump trucks. That's because gold is rare. The trucks are full of rocks containing just trace amounts of gold scattered throughout. "Once the material's been brought to the surface, then the next thing is to get the gold out of it," Webster explains. To extract the gold, mining experts use a toxic chemical: cyanide. "They slowly pump cyanide solutions through this rock, over and over and over again," says Webster. This chemical mixture of water and cyanide dissolves the gold again, separating it from the other elements in the ore.
Since cyanide can be fatal to humans and wildlife, mines must ensure that none of the noxious mixture seeps into the groundwater. Special pads can be placed under the cyanide and rocks to catch the liquid chemical as it drips out. The chemical is then recycled. However, at times, mines around the world have leaked or spilled the poisonous cyanide. When the chemical enters the environment, it can be devastating to wildlife.
Once the gold is separated from the debris, it is sent to a refinery. There, engineers will use high temperatures and chemicals to remove any remaining impurities, such as other metals, from the gold.
There are a lot of steps involved in making golden awards such as the Academy Awards' Oscar statuette. Months before the awards ceremony, workers at R.S. Owens in Chicago are filling empty molds with a metal that contains silver. Once the molds cool, workers remove the newly formed metal statuettes and sand them.
Now the metal-plating process begins. Each statuette is dipped into a series of tanks, covering it first with copper, then nickel, then silver--and finally with 24-karat gold (see The Making of an Oscar Statuette, p. 16).
Gold was chosen as the final coating for the award partly because of its yellowish luster and because it never tarnishes. So the winner's statuette will stay glowing for years to come.
nuts & bolts
HOW GOLD FORMS
Rainwater seeps into the ground and moves in cracks through pore spaces in the rocks and soil.
Deep below Earth's surface, the rainwater combines with water that has escaped from pockets of molten rock called magma. These waters get heated up until they become a vapor.
Gold dissolves into the water vapor, which rises toward the surface.
When this mix of gold and water nears the surface, it cools--causing the gold to become a solid again. The gold forms lode deposits within narrow channels and cracks in the earth.
Over time, erosion from wind and running stream water send the gold from the lode deposits downstream. The gold particles collect in streambeds and pits in the ground. These accumulations form placer deposits.
JAMES WEBSTER, a geologist at the American Museum of Natural History in New York.
NUMBER ONE: Snowboarder Shaun White shows off his gold medal from the Torino 2006 Winter
THE MAKING OF AN OSCAR STATUETTE
1 TAKING SHAPE: A worker prepares the Academy Awards' Oscar molds.
2 MANY METALS: After the statuettes are removed from the molds, they are covered in copper, then nickel, then silver.
3 DOUBLE DIPPING: The statuettes are dipped in gold.
4 BUFFER ZONE: A gold-plated Oscar statuette gets polished.
5 AWARDS TIME: The finished product shines. Reese Witherspoon (right) accepts an Oscar in 2006.
* Each "Academy Award of Merit" statuette, or Oscar, weighs 3.9 kilograms (8.5 pounds) and stands 34 centimeters (13.5 inches) high.
* The Olympic gold medal is actually made of sterling silver covered with a thin coat of pure gold.
* The World Cup Trophy is 32 cm (12.5 in.) high and is made of solid 18-karat gold.
* The purity of gold is measured in karats, with 24 being the highest. So 24-karat gold is pure gold.
* All of the mined gold in the world could easily fit inside your school's gym. It's estimated to be 152,000 metric tons, enough to fill just 60 tractor trailers.
Can you guess what golden award is shown in the photo below? Find the answer at: http://ology.amnh.org/mystery_photo/gold
A gorgeous array of gold and other minerals and gems, such as the Star of india--the largest star sapphire in the world--are on display in the Museum's Morgan Memorial Hall of Gems and Guggenheim Hall of Minerals. These are among the Museum's best-known and beloved halls. The Museum's 200 scientists travel the world on 100 field expeditions each year. They study everything from amber and rubles to tropical birds and the universe. To learn more, ask your teacher, or visit www.amnh.org
Jump-start your lesson with these pre-reading questions:
* Miners have to dig up approximately 30 tons of rock in order to extract one ounce of gold. Where does gold come from? Which nations in the world are the top producers of gold?
* According to the Academy of Motion Pictures Arts and Sciences, it takes 12 people approximately 20 hours to create an Oscar statuette. What is each "Academy Award of Merit" made of?
* Gold is a valuable global commodity. One ounce of gohl costs approximately $600. But some people believe the negative social and environmental factors associated with gold mining make the metal not worth its value. For example: Cyanide, which is used to extract gold from its ore, is a toxic chemical. When it seeps into waterways, it can harm living organisms. (For more information, visit: www.nodirtygold.org.) Weigh the pros and cons of gold mining.
MATH: Gold is rare and soft. Jewelers often combine gold with stronger and less valuable metals such as copper. This alloy is harder and cheaper than pure gold. The purity of gold is measured in a unit called Karat (K). Pure gold is 24K, meaning all 24 out of a total of 24 parts are gold. Gold marked 18K means 18 out of 24 parts are gold, and the remaining 6 parts consist of other metals. How much gold and other metals are in gold items marked 14K and 10K? Answers: 14K (14 parts gold, 10 parts other metals) and 10K (10 parts gold, 14 parts other metals).
* This Web site from the World Gold Council is filled with scientific information, history, and facts about gold: www.gold.org/discover/index.html
* To learn more about the Oscar statuette, visit: www.oscar.com/legacy/statuettel.html
* The companion Web site to the FRONTLINE/World episode "Peru: The Curse of Inca Gold" examines the social and environmental issues linked to gold mining: www.pbs.org/frontlineworld/stories/peru404/
* "Behind Gold's Glitter: Torn Lands and Pointed Questions," by Jane Perlez and Kirk Johnson, The New York Times, October 24, 2005
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|Title Annotation:||EARTH: MINERALS|
|Date:||Jan 15, 2007|
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