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Chemistry
Turquoise is a hydrated phosphate of aluminum and copper (copper aluminum
phosphate) or CuAl6((OH)2/PO4)4 CuAl6(PO4)4(OH)8 + 4H2O. In the language
of chemists and geologists, turquoise is known as. Turquoise stones
can contain impurities that form veins of sandstone, limonite, psilomelane
or jasper. At temperatures of 500 degrees, blue Turquoise stones will
become greener.
Color
Turquoise is a non-translucent stone whose most valuable specimens are
a robin’s egg blue or deep-blue azure. Most of these fine specimens
come from Iran and indicate the presence of copper within the stone.
Less precious stones come from North America and are greener (from iron)
than the Persian stones. North American specimens also contain impurities
that form matrix streaks within the stones. The veins are inclusions
from nearby rock fragments or oxides and form during the creation of
Turquoise. As mentioned above these veins can contain sandstones, limonite,
malachite, chrysocolla jaspers or psilomelane. The veins in some stones
interlock in patterns to form "spider-web" turquoise.
Crystal Habit
Nearly all Turquoise is cryptocrystalline. In fact, scientists had thought
Turquoise was amorphous (non-crystal forming) until 1911, when crystalline
specimens were found in Virginia.
The Turquoise crystal system is Triclinic, which
is the least symmetrical of all crystal systems with all three axes
of unequal lengths and none intersecting at right angles. The common
shape for this system is the pinacoid.
Geological Environment
The common environment of Turquoise is arid or semiarid zones such as
those that occur in Iran or the Desert Southwest of the United States.
It is found in veins and nodules in weathered rhyolitic igneous rock
where it forms as a secondary mineral of the process known as hydrothermal
replacement depositing that occurs when chemicals leach out of nearby
rock by way of rain or a saturated water table. Copper eroding from
deposits leaches into cracks and combines with phosphates and Turquoise’s
other chemicals.
Enhancements
Turquoise is a controversial stone because most of the stones sold have
received so many treatments that the final product is completely different
from its original form. Enhancements can include, plastic, wax and oils
that change color, durability and polish.
Hardness
Turquoise has a hardness of 5.5 to 6.5 on Mohs Scale. The process by
which Turquoise forms creates a porous stone. The harder, less porous
stones polish better than the pale softer, chalky stones but these can
have waxes or oils pressed into them to help their polish.
Imitators
Because of the rarity of fine specimens of Turquoise, jewelry makers
have been creating imitations of it for centuries. The Egyptians used
a glazed quartz paste as a substitute for the Turquoise for their jewelry
requirements. The most common modern imitators of Turquoise are Howlite,
Magnesite, Turquenite, dyed chalcedony, glasses, ceramics, and plastics.
The minerals most often confused with Turquoise include Amazonite, Prosopite,
Lazulite, Hemamorphite, Chrysocolla, Odontolite, Serpentine, Smithsonite,
Faustite, and Variscite. Bone Turquoise or Vivianite (a hydrous ferrous
phosphate) can leach into fossils and turn them a blue that is close
to Turquoise in color.
Reconstituted Turquoise-The process of reconstituting
Turquoise consists of pulverizing pieces of turquoise that are then
stabilized and hardened with resins to achieve a natural Turquoise appearance.
Resin-reconstituted Turquoise usually has an odor that allows for detection.
Lab-Grown Synthetic Turquoise: Also known as Neo-turquoise,
Hamburger Turquoise or Neolite. Lab-grown Turquoise does not have the
veins of impurities found in most American Turquoise. The refractive
index of natural Turquoise is usually slightly higher than that of lab-grown
stones. Genuine specimens also have homogenous blue matrices that contain
irregular white particles.
Turquoise Care
The most common dangers to Turquoise are scratches, sharp blows, hot
water, and household chemicals. Because it is a hydrous stone, water
or light can change the color of Turquoise stones and its relative softness
can make it vulnerable to scratches. The pores of the stone will easily
absorb body oils or other oils causing the stone to yellow over time.
Do not use an ultrasonic cleaner on Turquoise and avoid chlorine.
Chem: CuAl6(PO4)4(OH)8 * 5H2O Hydrous copper aluminum
phosphate
Color: sky blue, bluish-green, pale green
Refrac. Index: 1.61 - 1.65
Birefraction: 0.04
Hardness: 5 - 6
Spec. Grav.: 2.60 - 2.80
Specific Gravity: 2.6 to 2.8
Cleavage: none
Fracture: conchoidal, uneven
Transparency: opaque
Chemical Composition: CuAl6((OH)2/PO4)4 CuAl6(PO4)4(OH)8 + 4H2O
Luster: waxy, vitreous in macro-crystals
Color of streak: white, usually with brown or black spots, or white
with greenish tint
Class: phosphates
Crystal System: triclinic; bar 1; rarely seen in crystalline form, most
stones are cryptocrystalline
Best Field Indicators: crystal habit, hardness, luster, color and associations
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