Jadeite is a pyroxene mineral with composition NaAlSi2O6. It is hard (Mohs hardness of about 6.5 to 7.0), very tough, and dense, with a specific gravity of about 3.4. It is found in a wide range of colors, but is most often found in shades of green or white. Jadeite is formed only in the subduction zones of continental margins, where rock undergoes metamorphism at high pressure but relatively low temperature. Jadeite is the principal mineral making up the most valuable form of jade, a precious stone particularly prized in China. Most gem-quality jadeite jade comes from northern Myanmar. Jade tools and implements have been found at Stone Age sites, showing that the mineral has been prized by humans since before the beginning of written history.
Name The name jadeite is derived (via French: jade and Latin: ilia) from the Spanish phrase "piedra de ijada" which means "stone of the side". The Latin version of the name, lapis nephriticus, is the origin of the term nephrite, which is a different mineral that also shares the common name jade.
Properties Jadeite is a hard, extremely tough, rare mineral of the clinopyroxene family of minerals. Though highly variable in color, it is typically apple-green to emerald-green, or less commonly white or white with spots of green. Occurrences are typically granular or massive; individual crystals are very rare, occurring only as small prismatic crystals in vugs in massive jadeite. Crystals are four-sided or eight-sided in cross section and show perfect cleavage on [110] at angles of 87 and 93 degrees. There is also a rare parting on [100]. The interlocking crystals of massive jadeite help give it its extreme toughness. Jadeite has a Mohs hardness of 6.5 to 7, slightly less than that of common quartz. Fracture surfaces are sugary in texture, and sparkle from the exposed perfect cleavage on [110]. Jade is relatively dense, with a specific gravity of 3.3 to 3.5 in natural specimens. The specific gravity increases with the iron content, and very pure jadeite has a specific gravity of 3.25. The luster is vitreous, or pearly on exposed cleavage surfaces, and the streak is colorless. Jadeite is translucent to transparent. Jadeite is characterized by its green color and tough aggregates of compact fibrous crystals. It can be distinguished from nephrite by its vitreous luster on polished surfaces (polished nephrite has an oily luster) and by its higher density and refractive index. Serpentine also has a lower density and refractive index than jadeite. Massive jadeite also characteristically shows a more granular texture than nephrite or serpentinite. Jadeite has a fusibility of 2.5 (making it moderately easy to fuse with a propane flame) and gives a yellow flame color. Pure jadeite has the composition NaAlSi2O6 and has the typical clinopyroxene structure. This consists of long chains of silica tetrahedra in which each silicon ion is surrounded by four oxygen ions, with two of the oxygen ions shared with neighboring silica tetraheda. The chains are bonded together by aluminium and sodium ions to form the full three-dimensional structure of a jadeite crystal. The aluminium joins pairs of chains (occupying the so-called M1 site) while sodium joins the paired chains to neighboring paired chains (occupying the so-called M2 site). The resulting crystal structure belongs to the monoclinic system, with space group C2/c.
Chemistry and origin Pure jadeite has the composition NaAlSi2O6. There is no significant replacement of silicon by aluminium in natural jadeite, and only very limited substitution of ferric iron for aluminium. However, calcium substitutes for up to 20% of the sodium, balanced by substitution of magnesium or ferrous iron for aluminium. Omphacite is intermediate in composition between jadeite and diopside. However, there is not a true solid solution series, as omphacite has its own structure that is slightly different from either pure jadeite or pure diopside, so that it is separated from either end member by a miscibility gap. Chloromelanite is a dark green variety of jadeite in which some aluminium is replaced by iron, while imperial jade, the most valuable variety of jade, is colored an intense emerald green by traces of chromium.
Jadeite occurs with albite in metamorphic rock of the low-temperature, high-pressure blueschist facies at destructive plate margins. Although it is intermediate in silica content between albite and nepheline, it is not stable under the conditions in which these two minerals are present. Formation of jadeite requires a pressure of 10 to 25 kbar and a temperature of 600 to 1,000 °C (1,100 to 1,800 °F) via the reaction:
Jadeite can also form at high pressure via the reaction:
At still higher pressure, corresponding to the highest blueschist facies, jadeite reacts with lawsonite to form zoisite and paragonite:
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. Jadeitite is resistant to weathering, and alluvial boulders of jadeitite released from the serpentine-rich environments in which they formed can have weights of up to tons. Raw specimens having Burmese tax stamps or polished slots for evaluating quality are prized by some collectors.
Colors
Jadeite's color commonly ranges from white through pale apple green to deep jade green but can also be blue-green (like the recently rediscovered "Olmec Blue" jade), pink, lavender and a multitude of other rare colors. Chloromelanite is a very dark green to black variety. Color is largely affected by the presence of trace elements such as chromium and iron. Its translucence varies from opaque to almost clear. Variations in color and translucence are often found even within a single specimen.
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