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Even the finest of turquoise is fracturable, reaching a maximum
hardness of just under 6, or slightly more than window glass.
Characteristically a cryptocrystalline mineral, turquoise almost
never forms single crystals and all of its properties are highly
variable. Its crystal system is proven to be triclinic via X-ray
diffraction testing. With lower hardness comes lower specific
gravity (high 2.90, low 2.60) and greater porosity: These properties
are dependent on grain size. The lustre of turquoise is typically
waxy to subvitreous, and transparency is usually opaque, but may be
semi translucent in thin sections. Colour is as variable as the
mineral's other properties, ranging from white to a powder blue to a
sky blue, and from a blue-green to a yellowish green. The blue is
attributed to idiochromatic copper while the green may be the result
of either iron impurities (replacing aluminum) or dehydration.
The refractive index (as measured by sodium light, 589.3 nm) of
turquoise is approximately 1.61 or 1.62; this is a mean value seen
as a single reading on a gemological refractometer, owing to the
almost invariably polycrystalline nature of turquoise. A reading of
1.61–1.65 (birefringence 0.040, biaxial positive) has been taken
from rare single crystals. An absorption spectrum may also be
obtained with a hand-held spectroscope, revealing a line at 432
nanometers and a weak band at 460 nanometres (this is best seen with
strong reflected light). Under long wave ultraviolet light,
turquoise may occasionally fluoresce green, yellow or bright blue;
it is inert under shortwave ultraviolet and X-rays.
Turquoise is infusible in all but heated hydrochloric acid. Its
streak is a pale bluish white and its fracture is conchoidal,
leaving a waxy lustre. Despite its low hardness relative to other
gems, turquoise takes a good polish. Turquoise may also be peppered
with flecks of pyrite or interspersed with dark, spidery limonite
veining.
As a secondary mineral, turquoise apparently forms by the action of
percolating acidic aqueous solutions during the weathering and
oxidation of pre-existing minerals. For example, the copper may come
from primary copper sulfides such as chalcopyrite or from the
secondary carbonates malachite or azurite; the aluminum may derive
from feldspar; and the phosphorus from apatite. Climate factors
appear to play an important role as turquoise is typically found in
arid regions, filling or encrusting cavities and fractures in
typically highly altered volcanic rocks, often with associated
limonite and other iron oxides. In the American southwest turquoise
is almost invariably associated with the weathering products of
copper sulfide deposits in or around potassium feldspar bearing
porphyritic intrusives. In some occurrences alunite, potassium
aluminum sulfate, is a prominent secondary mineral. Typically
turquoise mineralization is restricted to a relatively shallow depth
of less than 20 m, although it does occur along deeper fracture
zones where secondary solutions have greater penetration.
Although the features of turquoise occurrences are consistent with a
secondary or supergene origin, some sources refer to a hypogene
origin. The hypogene hypothesis, which holds that the aqueous
solutions originate at significant depth, from hydrothermal
processes. Initially at high temperature, these solutions rise
upward to surface layers, interacting with and leaching essential
elements from pre-existing minerals in the process. As the solutions
cool, turquoise precipitates, lining cavities and fractures within
the surrounding rock. This hypogene process is applicable to the
original copper sulfide deposition; however, it is difficult to
account for the many features of turquoise occurrences by a hypogene
process. That said, there are reports of two phase fluid inclusions
within turquoise grains that give elevated homogenization
temperatures of 90 to 190 oC that require explanation.
Turquoise is nearly always cryptocrystalline and massive and assumes
no definite external shape. Crystals, even at the microscopic scale,
are exceedingly rare. Typically the form is vein or fracture
filling, nodular, or botryoidal in habit. Stalactite forms have been
reported. Turquoise may also pseudomorphously replace feldspar,
apatite, other minerals, or even fossils. Odontolite is fossil bone
or ivory that has been traditionally thought to have been altered by
turquoise or similar phosphate minerals such as the iron phosphate
vivianite. Intergrowth with other secondary copper minerals such as
chrysocolla is also common.
Turquoise was among the first gems to be mined, and while many
historic sites have been depleted, some are still worked to this
day. These are all small-scale, often seasonal operations, owing to
the limited scope and remoteness of the deposits. Most are worked by
hand with little or no mechanization. However, turquoise is often
recovered as a byproduct of large-scale copper mining operations,
especially in the United States.
For at least 2,000 years, the region once known as Persia, has
remained the most important source of turquoise, for it is here that
fine material is most consistently recovered. This "perfect color"
deposit is restricted to a mine-riddled, 2,012-metre mountain peak
of Ali-mersai, 25 km from Mashhad, the capital of Khorasan province,
Iran. A weathered and broken trachyte is host to the turquoise,
which is found both in situ between layers of limonite and
sandstone, and amongst the scree at the mountain's base. These
workings, together with those of the Sinai Peninsula, are the oldest
known.
Iranian turquoise is often found replacing feldspar. Although it is
commonly marred by whitish patches, its color and hardness are
considered superior to the production of other localities. Iranian
turquoise has been mined and traded abroad for centuries, and was
probably the source of the first material to reach Europe.
Since at least the First Dynasty (3,000 BCE), and possibly before
then, turquoise was used by the Egyptians and was mined by them in
the Sinai Peninsula, called "Country of Turquoise" by the native
Monitu. There are six mines in the region, all on the southwest
coast of the peninsula, covering an area of some 650 km². The two
most important of these mines, from a historic perspective, are
Serabit el-Khadim and Wadi Maghareh, believed to be among the oldest
of known mines. The former mine is situated about 4 kilometers from
an ancient temple dedicated to Hathor.
The turquoise is found in sandstone that is, or was originally,
overlain by basalt. Copper and iron workings are present in the
area. Large-scale turquoise mining is not profitable today, but the
deposits are sporadically quarried by Bedouin peoples using homemade
gunpowder. In the rainy winter months, miners face a risk from flash
flooding; even in the dry season, death from the collapse of the
haphazardly exploited sandstone mine walls is not unheard of. The
color of Sinai material is typically greener than Iranian material,
but is thought to be stable and fairly durable. Often referred to as
Egyptian turquoise, Sinai material is typically the most
translucent, and under magnification its surface structure is
revealed to be peppered with dark blue discs not seen in material
from other localities.
In proximity to nearby Eilat, Israel, an attractive intergrowth of
turquoise, malachite, and chrysocolla is found. This rock is called
Eilat stone and is often referred to as Israel's national stone: it
is worked by local artisans for sale to tourists.
The Southwest United States is a significant source of turquoise;
Arizona, California (San Bernardino, Imperial, and Inyo counties),
Colorado (Conejos, El Paso, Lake, and Saguache counties), New Mexico
(Eddy, Grant, Otero, and Santa Fe counties) and Nevada are (or were)
especially rich. The deposits of California and New Mexico were
mined by pre-Columbian Native Americans using stone tools, some
local and some from as far away as central Mexico. Cerrillos, New
Mexico is thought to be the location of the oldest mines; prior to
the 1920s, the state was the country's largest producer; it is more
or less exhausted today. Only one mine in California, located at
Apache Canyon, operates at a commercial capacity today.
The turquoise occurs as vein or seam fillings, and as compact
nuggets; these are mostly small in size. While quite fine material—rivalling
Iranian material in both color and durability—is sometimes found,
most American turquoise is of a low grade (called "chalk
turquoise"); high iron levels mean greens and yellows predominate,
and a typically friable consistency precludes use in jewellery in
the turquoise's untreated state. Arizona is currently the most
important producer of turquoise by value, with the vivid Bisbee Blue
being a good example of the state's natural endowment; much of the
Arizona material is recovered as a byproduct of copper mining.
Nevada is the country's other major producer, with an estimated
75–100 mines opened over the state's history. The Nevada material is
noted for its often attractive brown or black limonite veining,
producing what is called "spiderweb matrix".
In 1912, the first deposit of distinct, single-crystal turquoise was
discovered in Lynch Station, Campbell County, Virginia. The
crystals, forming a druse over the mother rock, are very small; 1 mm
(0.04 inches) is considered large. Until the 1980s Virginia was
widely thought to be the only source of distinct crystals; there are
now at least 27 other localities. The specimens are highly valued by
collectors.
In an attempt to recoup profits and meet demand, most American
turquoise is treated or enhanced to a certain degree. These
treatments include innocuous waxing and more controversial
procedures, such as dyeing and impregnation. |
