Jadeite

Jadeite
Chinese jadeite buttons.jpg
A selection of antique, hand-crafted Chinese jadeite buttons
General
Category mineral variety of pyroxene
Chemical formula NaAlSi2O6[1]
Identification
Color Semitransparent to opaque and often mottled, white, green, yellow to reddish orange, brown, gray, black, light purple or lavender.[1]
Crystal habit massive[1]
Crystal system monoclinic[1]
Fracture granular to splintery[1]
Mohs Scale hardness 6.5 - 7[1]
Luster dull[1]
Refractive index 1.666 - 1.680 (+/- .008); spot reading is 1.66[1]
Optical Properties Double refractive with anomalous aggregate reaction [1]
Birefringence usually not detectable[1]
Pleochroism none[1]
Specific gravity 3.34 (+.06, -.09)

Jadeite is one of the minerals recognized as the gemstone jade.[2] Its color commonly ranges from white through pale apple green to deep green, but it can also be blue-green (like the famous, rediscovered "Olmec blue" variety), pink, lavender, and a host of other rare colors.

Contents

Formation

Jadeite is a pyroxene mineral with composition NaAlSi2O6. It is formed in metamorphic rocks under high pressure and relatively low temperature conditions. Albite (NaAlSi3O8) is a common mineral in the Earth's crust, with a specific gravity of about 2.6, much less than that of jadeite. With increasing pressure, albite breaks down to form the high-pressure assemblage of jadeite plus quartz. Minerals associated with jadeite include glaucophane, lawsonite, muscovite, aragonite, serpentine, and quartz.

Rocks that consist almost entirely of jadeite are called jadeitite. In all well-documented occurrences, jadeitite appears to have formed from subduction zone fluids in association with serpentinite (Sorensen et al. 2006). Jadeitite is resistant to weathering, and boulders of jadeitite released from the serpentine-rich environments in which they formed are found in a variety of environments.

Currently, the best-known sources of gem-quality jadeite are California, Myanmar (Burma), New Zealand, and (more recently) Guatemala. Other localities of jadeite include Kazakhstan, Russia, British Columbia, Alaska, and Turkestan.

Characteristics

Jadeite is composed of monoclinic crystals. Their colors are largely affected by the presence of trace elements, such as chromium and iron. The crystals can be anywhere from entirely solid through opaque to almost clear. Variations in color and translucence are often found even within a single specimen.

This mineral has a Mohs hardness of about 6.5 to 7.0 depending on its composition. It is dense, with a specific gravity of about 3.4. Jadeitite forms solid solutions with other pyroxene endmembers such as augite and diopside (calcium and magnesium-rich endmembers), aegirine (sodium and iron-rich endmember), and kosmochlor (sodium and chromium-rich endmember). Pyroxenes rich in both the jadeite and augite endmembers are known as omphacite.

Varieties and value

Jadeite from the Motagua Valley, Guatemala, is the stone used by the Olmec, Maya peoples, and indigenous peoples of Costa Rica. Typically, the colors of jadeite most valued are the intensely green, translucent varieties, though traditionally white has been considered the most valuable of the jades by the Chinese, known for their carefully crafted jade pieces.

Currently, the most highly valued variety of jadeite is known as "Imperial Green" jade, characterized by an emerald green color and a high level of translucence. It the most expensive gem in the world, carat-by-carat, costing more than diamonds. Olmec blue jade, characterized by its deep blue-green, translucent hue with white flecking, is also becoming more highly valued because of its unique beauty and historical use by the Mesoamerican Olmec and the peoples of Costa Rica.[3] The Olmec blue variety, however, was only recently rediscovered and is being minimally exploited by native Guatemalans. It is thus difficult to obtain and too rare and little-known to have attained great value as a gemstone.

When purchasing jade, quality is determined by the degree of translucence, cleanness of color, and purity of color. Occasionally, other minerals like serpentine or quartz are sold as jade but the difference can be determined by cleavage and hardness.

See also

Notes

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Gemological Institute of America, Gem Reference Guide (Carlsbad, CA: Gemological Institute of America, 1988, ISBN 0873110196).
  2. The other mineral recognized as "jade" is nephrite, a green amphibole.
  3. Elizabeth Kennedy Easby, Pre-Columbian Jade from Costa Rica (New York: André Emmerich, 1968).

References

  • Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412
  • Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science, 23rd ed. New York: John Wiley. ISBN 0471721573
  • Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060
  • Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Revised ed. New York: St. Martin's Press. ISBN 1582381321
  • Sorensen, Sorena, George E. Harlow, and Douglas Rumble. 2006. “The origin of jadeitite-forming subduction-zone fluids: CL-guided SIMS oxygen-isotope and trace-element evidence.” American Mineralogist 91: 979-996.

External links

All links retrieved April 26, 2014.

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