Definitions

jadeite

jadeite

[jey-dahyt]
jadeite: see jade.

Uncut (left) and cut jadeite

Gem-quality silicate mineral in the pyroxene family that is one of the two forms of jade. Jadeite (imperial jade), sodium aluminum silicate (NaAlSi2O6), may contain impurities that give it a variety of colours: white, green, red, brown, and blue. The most highly prized variety is emerald green. Jadeite occurs only in metamorphic rocks, most often in those that have been subjected to the high pressures deep below the Earth's surface. The area around the city of Mogaung in northern Myanmar has long been the main source of gem-quality jadeite.

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Jadeite is a pyroxene mineral with composition NaAlSi2O6. It is monoclinic. It has a Mohs hardness of about 6.5 to 7.0 depending on the composition. The mineral is dense, with a specific gravity of about 3.4. Jadeite forms solid solutions with other pyroxene endmembers such as augite and diopside (CaMg-rich endmembers), aegirine (NaFe endmember), and kosmochlor (NaCr endmember). Pyroxenes rich in both the jadeite and augite endmembers are known as omphacite.

Jadeite is formed in metamorphic rocks under high pressure and relatively low temperature conditions. Albite (NaAlSi3O8) is a common mineral of the Earth's crust, and it has 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, as discussed by 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.

Jadeite's color commonly ranges from white through pale apple green to deep jade'' green but can also be blue-green (like the famous and recently rediscovered "Olmec Blue" jade), pink, lavender, and a multitude of other rare colors. Color is largely affected by the presence of trace elements such as chromium and iron. Its translucence can be anywhere from entirely solid through opaque to almost clear. Variations in color and translucence are often found even within a single specimen. Currently, the best known sources of gem quality jadeite are California, Myanmar, New Zealand and more recently Guatemala; other localities of jadeite include Kazakhstan, Russia, British Columbia, Alaska, and Turkestan.

Jadeite is one of the minerals recognized as the gemstone jade. The other is the green amphibole, nephrite. Jadeite from the Motagua Valley, Guatemala is the stone used by the Olmec, Maya peoples, and the indigenous peoples of Costa Rica. Typically, the most highly valued colors of jadeite are the most 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 and is characterized by an emerald green color with a high level of translucence. It the most expensive gem in the world, weight for weight costing more than diamond. Other colors, like "Olmec Blue" jade, which is characterized by its deep blue-green, translucent hue with white flecking, are also becoming more highly valued because of its unique beauty and historical use by the Mesoamerican Olmec and also in Costa Rica ; however, this variety was only recently rediscovered and is only being minimally exploited by native Guatemalans. It is thus difficult to obtain and as yet 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.

References

Sorena Sorensen, George E. Harlow, and Douglas Rumble, The origin of jadeitite-forming subduction-zone fluids: CL-guided SIMS oxygen-isotope and trace-element evidence. American Mineralogist, v. 91, pp. 979-996 (2006).

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