Physical Properties of Turquoise for Identification and Classification Purposes.
This site contains information on the Physical Properties and Chemical Composition of Minerals, gemstones, crystals, precious metals, and sedimentary, igneous and metamorphic rocks for crystallography, geology, identification of minerals, Jewelry and mineralogy. This includes cleavage, description of crystal formations, crystal structure, hardness, specific gravity, Mohs’ Scale, crystal features and crystal habits for identification and classification purposes.
Turquoise is one of the most widely used stones in jewelry today. Its popularity has made it among the most valuable, non-translucent mineral used in jewelry. Turquoise is usually cut as a cabochon.
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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