The Origin and Occurrence of Turquoise

An article by Lee Anderson

Turquoise consists of the chemical elements copper (Cu), aluminum (Al), phosphorus (PO4), and water (H2O). It is described as a “hydrous basic aluminum phosphate of copper” or a “hydrous aluminum phosphate colored by copper salts.” Its chemical formula is:

CuAl6 (PO44)8 4H2O,

although this varies widely. This molecular structure permits the inclusion of other elements, principally iron (Fe), calcium (Ca), magnesium (Mg), manganese (Mn), silicon (Si), and zinc (Zn). These additional elements, when incorporated in the molecular structure of turquoise, influence its color and hardness.

Turquoise is formed when the proper minerals, present in the proper proportions, are subjected to certain physical and chemical processes. These minerals are broken down, or “weathered,” from nearby “source” rocks and then dissolved, transported, and deposited in cracks, openings, and hollows in “host” rocks that lie beneath the surface. This mineral “solution” must remain in these host rocks for millions of years, at just the right pressures and temperatures, to form turquoise. (Keep in mind that over these enormous periods of time, mountains rise and wear away, and seas advance and recede.) It’s remarkable that a specific grouping of minerals could be subjected to the forces of pressure and temperature for such long periods, eventually forming something as beautiful as turquoise!

Turquoise usually forms in areas with some volcanic or thermal history. Most is found in volcanic rocks such as phyolite and trachyte; a lesser, but still significant amount, is found in intrusive (granite-like) rocks. Metamorphic and sedimentary rocks, on the other hand, are least likely to contain turquoise, although the turquoise found in the Sinai and in Australia occurs in sandstone and shale.

Most turquoise is found in “alteration zones” — areas where the native rocks have been altered by heat from magma or other thermal influences. This “hydrothermal” alteration is created when magma solutions from deep within the earth flow to the surface through fractures or pores, eventually changing the original rocks because of the intense heat and chemical exchange with the new rock (magma). This activity, coupled with the long weathering of the surface rocks through wind and water and their resulting chemical breakdown, creates the environment necessary for turquoise to form.

Another key geological process is “silicification.” It, too, is involves hydrothermal and intrusive alteration. Silica, a common associate of turquoise, is introduced into the turquoise deposit. This process, in addition to periods of intense heat, is responsible for the hardness of the turquoise and frequently the matrix as well.

In order for turquoise to form, several conditions must be met. First, there must be a source of copper, a relatively rare element. Collocated with this copper must be a source of phosphorus — usually the mineral apatite, which in turn is restricted to certain rocks (which are not all associated with copper). Phosphorus is typically leached from the apatite in the form of phosphoric acid. There must also be feldspar for the aluminum and deep hydrothermal alteration to break down these feldspars and free this aluminum. The copper is usually introduced into the “host” rocks by the rising hot magma. It readily oxidizes near the surface and, when in solution, reacts freely with the aluminum and phosphoric acid to form turquoise. At this time, other minerals may enter into the turquoise structure, creating color variations.

Other factors affect the creation of turquoise. For example, the best, hardest turquoise is usually found within 100 feet of the earth’s surface. Why? Well, turquoise sitting in a pocket waiting for someone to mine is subject to the elements. If it’s near the surface, it “dries out.” As it “dries,” it hardens; deeper formations are generally softer. Shallow deposits have less contact with the acids created by water percolating through the earth, so they are less likely to “soften” or become more porous. Some cases appear to contradict this observation, such as the Lone Mountain Mine in Nevada. In this area, tunneling along the vein has been very productive even below 100 feet. That’s because the rocks have been faulted to the side —the turquoise actually formed near the surface before faulting.

Undoubtedly, many similar formations have been lost forever as a result of the earth’s convolutions, which have sent the deposits deep into the crust.

References / Recommended Readings

Joseph E. Poque, Ph.D, The Turquoise, A report to the National Academy of Science, vol. XII, Second and Third Memoir, 1915. Reprinted in 1974 by Rio Grande Press, Inc., Glorieta, NM. (This reprint includes a foreword and details on Southwestern turquoise mines by Rex Arrowsmith and an excellent reference list. )

The International Turquoise Annuals, vol. I and II, 1975 and 1976 (only two published) Impart Pub, Reno, NV. Note in vol. I the article on pages 31–55 by D. Allen Penick, “Turquoise, the Mineral that’s an Accident.

Stuart A. Northrop, Turquoise and Spanish Mines in New Mexico, University of New Mexico, Press, 1975.

M.G. Brown, Blue Gold, The Turquoise Story, Main Street Press, Anaheim, CA, 1975

Stuart A. Northrop, David L. Newman, David H. Snow, Turquoise, reprinted by General Printing and Paper Co., Topeka, KS. A reprint from El Palacio, vol. 79, No. 1, 1973, Museum of New Mexico.

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