While you are here, also take a peek at the colorful stones on the wall across the street from the cache. If any geologist knows what they are please let me know!
About Copper
In the Roman era, copper was principally mined on Cyprus, hence the origin of the name of the metal as сyprium (metal of Cyprus), later shortened to CUPRUM. Its compounds are commonly encountered as copper salts, which often impart blue or green colors to minerals such as azurite and turquoise and have been widely used historically as pigments.
Azurite, pictured above, is a soft, deep blue copper mineral produced by weathering of copper ore deposits.
Geology
Many of the ordinary elements around us were formed during the evolution of very massive stars. These supergiant stars later exploded as supernovae, catapulting the newly minted copper into space, along with silver and gold and other elements. Four-and-a-half billion years ago, Earth inherited some of this copper. In other words, the copper in your pennies and the gold and silver in your jewelry were forged somewhere in the universe during the death of supergiant stars.
Copper is sometimes found as the free element and is present in many minerals. Because copper reacts readily with other substances, it can be formed in a variety of ways in the Earth's crust. It is often found in deposits with other metals such as sulfur, lead, zinc, gold and silver. By far the largest amounts of copper are found in the earth’s crust in bodies known as PORPHYRY copper deposits. These deposits were once large masses of molten rock that cooled and solidified deep in the Earth's crust. As they cooled, some large crystals grew, which were then surrounded by smaller crystals. Geologists call these rocks porphyries.
At first, the molten rock contains a small amount of copper. As it cools and crystals begin to form the amount of fluid becomes smaller. The copper remains in the fluid, becoming more and more concentrated. When the rock is almost completely solid, it contracts and cracks and the remaining copper-rich fluid is squeezed into the cracks, where it too finally solidifies. Over many millions of years the rocks covering these deposits erode away and the deposits eventually appear at the surface. These deposits can contain 2 billion tons of "gangue" rock which, when processed, can produce 30 million tons of copper metal.
Bisbee Copper: The Lavender Pit Mine
The Lavender Pit Mine, at which you are standing, got its name from its former manager, Harrison Lavender. The mine was active from 1879 to 1975. Production from 1951 (when it was converted to an open pit mine) through 1974 totaled 86 million tons of ore averaging about 0.7% copper in the gangue rock, or about 600,000 tons of copper produced, with turquoise, gold and silver as byproducts. Because of the competent host rock, this pit has much steeper sides than other open pit copper mines in the southwest area. The abandoned pit in front of you is 950 feet deep and an area of 300 acres, is the result of the removal of 351 million tons of material. Large tonnages of dump rock were placed around Bisbee, most notable north of the residential district of Warren. The mine closed in 1975 due to a sharp fall in copper prices. The Lavender Pit is now a tourist stop with rim viewing platforms and guided tours.
Bisbee Blue
Turquoise was another by-product of the Lavender Mine. Bisbee turquoise, also known as Bisbee Blue, is among the finest turquoise found anywhere in the world. Turquoise is an opaque mineral that occurs in beautiful hues of blue, blue-green and yellow-green. It has been treasured as a gemstone for thousands of years, and this is its only important use.
Chemically, turquoise is a hydrous phosphate of copper and aluminum (CuAl6(PO4)4(OH)8•5H2). Turquoise is rarely found in well-formed crystals. Instead it is usually an aggregate of microcrystals. It is formed best in an arid climate, when rainfall infiltrates downward through soil and rock, dissolving small amounts of copper. When this water is later evaporated, the copper combines with aluminum and phosphorus to deposit tiny amounts of turquoise on the walls of subsurface fractures.
Turquoise can also replace the rock in contact with these waters. If the replacement is complete, a solid mass of turquoise will be formed. When the replacement is less complete, the host rock will appear as a "matrix" within the turquoise. The matrix can form a "spider web," "patchy" design, or other pattern within the stone.
To log this as a find, please answer the following questions and email them to the CO through their profile. Short answers are fine:
1. What does the term “gangue” rock refer to?
2. What gives the walls of the mine pit a blue/green tinge in some areas? Why is one side of the mine having a blue tinge, whereas the opposite slope has more of a red hue?
3. Turquoise is a hydrous phosphate of what two elements?
FTF: lafitefl!
Thanks to leeny_c for contributing the following:
Turquios means "Turkish stone" as it came to Europe by way of Turkey. Turquoise is one of the oldest protection amulets; also known as a symbol of wealth in many ancient cultures; was thought to protect the wearer from negative energy and bring good fortune. It is also a symbol of friendship. The best turquoise comes from Iran.
Sources: The Center for Land Use Interpretation: http://clui.org/ludb/site/lavender-pit, http://earthsky.org/earth/the-heavenly-origin-of-copper, http://minerals.usgs.gov/minerals/pubs/commodity/copper/, http://en.wikipedia.org/wiki/Copper, http://www.webelements.com/copper/geology.html, http://www.australianminesatlas.gov.au/education/down_under/copper/formed.html, http://geology.com/minerals/turquoise.shtml