December 2005

Introduction

"Worth its weight in gold," "a pot of gold," "silence is golden." These common expressions reinforce the unique and important role that gold plays in our lives. From the childhood fairy tale of that rascal Rumpelstilskin, who had the power to spin straw into gold, to the adult world of finance and the gold standard, gold is seen as a symbol of great value. So, from where does this King of Metals come?

All of our possessions can be traced back to natural materials found on Earth. Gold is a chemical element (chemical symbol Au for the Latin word for gold, aurum) that occurs widely in nature, but usually only in small amounts in any given area. Sprinkled through rocks and water in tiny quantities, it's very difficult to gather gold in useful amounts. But, there is a natural process involving chlorine that concentrates gold under volcanoes and forms deposits that are worth the expense of mining. The compounds gold chloride (AuCl) and hydrogen gold chloride (HAuCl2) play important natural roles in making gold accessible to us.

Steamed out of Magma.

Volcanoes can be thought of as places on Earth where melted rock from deep underground has squeezed out upon the surface and mounded up, forming volcanic mountains. The Hawaiian Islands and Mount St. Helens are two types of volcanic mountains. Geologists, scientists who study Earth processes, use the term magma for melted rock below the Earth's surface. A large body of underground magma is called a magma chamber. Once magma is squeezed out of a volcano it gets a new name: lava.

Magma chambers are very hot-with temperatures of around 800 degrees C (1,472 degrees F) or greater-and chemically active environments. When a volcano erupts, there is a great release of pressure in the magma chamber, similar to what happens when you open a bottle of soda pop. As soon as the lid comes off the bottle, there is a quick release of carbon dioxide gas. In the case of volcanoes, the gases released include steam, carbon dioxide, sulfur dioxide, hydrogen, hydrochloric acid and hydrogen sulfide. Chlorine and gold from the magma combine to form compounds such as hydrogen gold chloride (HAuCl2). These compounds are drawn out of the magma chamber dissolved in superheated steam

. .And Deposited in Fractures

The superheated steam carrying dissolved gold chloride compounds (and many other types of compounds, for that matter) forces its way out of the magma chamber and then through fractures in surrounding rocks. As it travels away from the magma chamber, the steam cools and eventually a chemical reaction occurs that releases gold from its "chemical partners," chlorine and hydrogen. Here's one chemical reactioni that describes this:

Note that as gold is deposited in rock fractures, hydrochloric acid and oxygen are also produced. These gases chemically wear down the surrounding rock. Geologists call this wearing down process by superheated steam and other gases from magma chambers hydrothermal alteration (hydro = water; thermal = heated).

Steam Rises through Magma and Into Fractures in Surrounding Rock, Depositing Gold

What Is It About Gold?

Gold, along with silver, is one of the best-known precious metals. Precious metals are rare metals of great economic value.

Gold is the most malleable (can be hammered into thin sheets) and ductile (can be drawn into thin threads) of all metals. A single gram of gold can be beaten into a sheet of a square meter.

As a good conductor of electricity, gold is critically important in the current age of electronics. Many medical instruments make use of gold parts, too.

Gold does not tarnish in air the way silver and copper do. For that reason it is well suited for use as jewelry and coins. But as a relatively soft metal, it must be combined with other metals (forming alloys) to increase its strength. 

Follow-up Activities:

  1. What other metals besides gold are pure elements that occur naturally on Earth? For what are they used?

  2. Learn more about volcanoes by visiting the Volcanocam on Mt. St. Helens at: http://www.fs.fed.us/gpnf/volcanocams/msh/

  3. Prove that the chemical reaction for the deposition of gold is balanced, with the same number of atoms of each element on the reactant side as the product side of the reaction, using the table below:

gold_chart

iFrom: Frank, M.R., Candela, P.A., Piccoli, P.M., and Glascock, M.D. (2002) Gold solubility, speciation and partitioning as a function of HCl in the brine-silicate melt-metallic gold system at 800°C and 100 MPa. Geochim. Cosmochim. Acta, 66, 21, 3719-3732.

For a list of previous "Chlorine Compound of the Month" features, click here.

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