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!
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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.
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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.
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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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Related Indian Jewelry Articles by Lee and Eric Anderson
• “The
History of American Indian Jewelry”
• “Turquoise
in Indian Jewelry”
• “How
the Quality of Turquoise Affects Its Use in Jewelry”
• “Stones
Used In Indian Jewelry”
• “The
History of Turquoise”
• “The
Origin and Occurrence of Turquoise”
• “The
Physical Properties of Turquoise”
• “Natural,
Stabilized, Treated, Fake, and Synthetic Turquoise”
• “Turquoise
Quality”
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