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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