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Featured Exhibit: Arizona's Mineral History

Arizona’s Mineral History

The unique geology of Arizona not only gives the state some of the most gorgeous vistas in the nation, it also gave the state its copper riches.

Copper Formation: A Scientific Mystery

Native CopperIn the language of geologists, the “Copper Province” of the American Southwest, centered on Arizona, has few counterparts in the world, and none in North America. These deposits are called Porphyry Copper Deposits because they appear in a type of rock called “porphyries.” More than a century of mining and discovery in this region, and a century of geological study, have yet to provide a conclusive answer about the reasons for this unusual concentration of the red metal. However, geologists continue to grow and refine our understanding of the geology and origin or Arizona’s copper deposits.

UA Expert on the Case

Dr. Spencer Titley, Professor Emeritus in the UA Department of Geosciences, has spent his career trying to answer the geologic mystery of Arizona’s copper deposits. He explains what we know like this: most of the deposits formed in a narrow interval of geologic time, between 60 and 65 million years ago. They formed along a continental margin during a period when, in terms of plate tectonics, oceanic and continental plates were converging rapidly, and they are closely related to ancient volcanoes.

Previously, geologists believed that the copper came from the mantle (a hot layer of rock below the Earth’s surface crust), and mid-century ideas involved rock from the ocean crust. However, evidence from Dr. Titley’s research using isotope analysis now reveals that there is a significant contribution to the deposits from local rocks in the Earth’s crust, and that evidence points to a different, yet still uncertain, origin story.

The final story remains to be discovered. But we know that it took millions of years of geologic transformation to create Arizona’s great copper wealth, a resource that humans have mined for about 140 years.

Mineral Specimens

TurquoiseMany of the same geologic forces that created copper ore deposits created the wondrous mineral specimens that we see in this exhibit. Water, heat, pressure, and minerals combined over millions of years to form incredible colors and crystals when chemical elements met the right conditions. Most of the minerals formed in regions where water and air from the surface could seep in.

In much of North and South America, this surface zone was scraped bare by glaciers during the last ice age, a mere 15,000 years ago, so whatever mineral specimens formed were destroyed, but the glaciers did not reach Arizona. Arizona’s underground copper mines were also crucial to the discovery of many amazing Arizona mineral specimens—if miners had not gone deep underground to dig copper ore, many of these incredible specimens would never have been discovered.

 

Flandrau Science Center, University of Arizona


Prominent Minerals of Arizona

Click through the slideshow below

Wulfenite

Wulfenite. Wulfenite’s vibrant red to orange to yellow crystals are a form of lead molybdate mineral that occurs in several Arizona localities. The colors and crystals are prized among collectors, and specimens from Arizona’s Red Cloud mine are world famous. Photo credit: Ken Don.

Vanadinite

Vanadinite. Vanadinite’s brilliant crystals are relatively rare and thus greatly valued by collectors. The mineral forms in arid climates where other lead based minerals have oxidized. First discovered in Mexico in 1801, many excellent vanadinite specimens have since been discovered in Arizona. Photo credit: Shipherd Reed, UAMM.

Turquoise

Turquoise. This enchanting mineral ranges in color from blue to green, and has been cherished for thousands of years by many cultures. Ancient civilizations in both North and South America traded in turquoise from mines in what is now Arizona. As a hydrous phosphate of copper and aluminum, turquoise often forms in and around copper deposits, and Arizona has produced some of the world’s best. Turquoise is prized for jewelry. Photo credit: Shipherd Reed, UAMM.

Native Silver

Native Silver. The element silver occurs naturally in pure form called “native silver,” taking shapes from crystalline to fine curling filaments. Specimens of aesthetically pleasing native silver are worth much more to collectors than the value of the silver alone. Although most Arizona silver was melted down and utilized as bullion, impressive specimens still exist. Photo credit: Ken Don.

Malachite

Malachite. A copper carbonate mineral, and an ore of copper, malachite is found all over the world and displays a range of rich green colors. Malachite can take many shapes, from crystalline, to fibrous, to globular, to massive. Arizona’s malachite specimens are legendary, and the mineral is often polished and used for ornamentation and jewelry. Photo credit: Sven Bailey, UAMM.

Cuprite

Cuprite. The luscious red color of cuprite, a copper oxide mineral, captivates collectors. Found in the oxidized zones of copper sulfide deposits, exceptional specimens came out of Arizona’s copper mines. Although the mineral often shines with a dazzling luster, it has never become popular as a faceted gem because it is relatively soft. Photo credit: Ken Don.

Native Copper

Native Copper. Copper that occurs naturally in nearly pure form is called “native copper.” In rare cases it appears as crystallized native copper. Copper is one of the few metallic elements that manifests itself in such a nearly pure form, and Arizona localities have produced outstanding specimens. Photo credit: Shipherd Reed, UAMM.

Cerussite

Cerussite. The white crystals of the mineral cerrusite are composed of lead carbonate. Also known as“white lead,” cerrusite ore was mined for centuries as a base for paints and cosmetics, as well as for the lead itself. Collectors prize the elegant pieces found in Arizona. Photo credit: Sven Bailey, UAMM.

Azurite

Azurite. The deep “azure” blue colors and crystalline forms of azurite specimens have captivated collectors for centuries. Both azurite and malachite have been used as pigments for paints since the beginning of history.

Like malachite, azurite is a copper carbonate mineral, and the two minerals frequently occur together. Arizona has produced a bounty of museum-quality azurite specimens. Photo credit: Monica Graeme.