Sue McGuire, age 13, of Billings, Mont., for her question:
HOW IS GOLD FORMED?
Gold has been valuable throughout the ages chiefly because it is scarce. This metallic element was one of the first known of all metals. Scientists believe that gold is formed and deposited from gases and liquids rising from beneath the earth's surface.
These gases and liquids travel toward the surface of the earth through faults or cracks in the crust. Gold is found in lode, or vein deposits, in placer deposits, as a minor element in porphyry copper and in seawater.
Placer deposits are large particles, called nuggets, and grains of gold in the beds of streams. These particles have been washed and carried away from a lode, or vein, by surface water, usually floodwater.
Placer deposits are of two types: eluvial and alluvial. Eluvial deposits are found close to the vein of gold. Alluvial deposits are farther away, usually in stream beds.
Ail seawater yields about one grain of gold per ton. Scientists are seeking ways to mine this gold profitably.
Gold is seldom found in a pure state. It is usually combined with silver in a natural alloy called an electrum. But it is usually associated with quartz, calcite, lead, tellurium, zinc or copper and is usually mined as a by product of these metals.
Gold in its pure form is very heavy. A quart of gold would weigh about 65 pounds.
Native gold is found mostly in quartz veins or in masses of iron pyrites, which are both minerals. Where the quartz or pyrites have been worn away by water and wind, the gold has weathered out of the rocks.
The main gold producing countries of the world, in the order of their importance, are South Africa, Russia, Canada, the United States, Australia and Ghana. These countries produce more than 85 percent of the total gold in the world.
Pure gold is too soft for ordinary uses. Therefore, it is nearly always mixed or alloyed with copper or silver, or both, to harden it. The addition of these other metals changes the color of the gold. Copper makes it redder than pure gold and silver makes it whiter. The different colors of gold are used to make attractive jewelry and ornaments.
The amount of gold in an alloy is expressed or shown in two ways. One way is based on a scale of 24 and the purity is expressed in carats. Pure gold is called 24 carat gold.
The other way is based on a scale of 1,000 and the purity is expressed in fineness, pure gold being 1,000 parts fine.
An alloy containing 18 parts gold and six parts of copper or silver is 18 carat gold or 750 parts fine. U.S. gold coins contained 9 parts of gold and one part of copper, so they were 900 parts fine.
The main uses of gold are coinage, jewelry and dental alloys. The alloys used in jewelry are usually of 10 to 14 carat quality, while those used in dental work are 20 to 22 carat. The alloys used for jewelry need more of the hardening metals to make them hard enough to stand wear. The dental alloys must have a higher purity to prevent staining.