MINERALOGICAL ANALYSIS OF JADE

The name JADE is applied indifferently to two materials that are both characterized by being very strong, compact, and finely granular or fibrous in texture: jadeite and nephrite.

Rigorously speaking, jadeite is a mineral forming part of the group of the pyroxenes and its ideal chemical composition can be written as NaAlSi2O6. Nevertheless, jadeite is often found intimately associated with at least two other very similar pyroxenes: acmite - NaFeSi2O6 - and diopside - NaMgSi2O6 - These three minerals may form solid solutions in any and all proportions and the variations of the properties of jadeite are therefore functions of the proportions in which the three pyroxenes are present. For example, when the composition of the material is intermediate between acmite and jadeite - or between diopside and jadeite - the material will differ in bothappearance and properties from pure jadeite, and is commonly known as chloromelanite. This material has a colour that may range from blackish green to black and the substantial presence Fe2+ tends to increase its specific weight.

The definition of nephrite is even more controversial. In mineralogy text books the material is usually mentioned as a variety of actinolite, a member of the amphibole group. Just like jadeite, however, actinolite - i.e. Ca2(Mg,Fe)5Si8O22(OH,F)2 - is so closely associated with tremolite - Ca2Mg5Si8O22(OH,F)2- that their optical and physical characteristics may become indistinguishable from each other. The Mg2+ content of tremolite is commonly substituted by Fe2+ and each of the two minerals thus tends to gradually change into the other. But the colour of the material can be taken as an indicator of the quantity Fe2+ it contains: actinolite will tend to impart a colour ranging from green to grey, while tremolite, which contains little or no Fe2+, will normally be between white and grey in colour.

The fact that nephrite is really a particular variety of two mineral species has induced the International Mineralogical Association to discourage the use of nephrite as the name of a valid mineralogical variety.

When identifying either jadeite or nephrite, the intimate structure of the material is as important as its mineralogical composition. The material must be very strong, compact and either finely granular (as in the case of jadeite) or made up of interwoven fibres (as in the case of nephrite). When the fibres are no more than parallel (or even subparallel), the material tends to lose the necessary strength and cannot be considered as nephrite.

In commercial practice, therefore, the name JADE is used to indicate indifferently those members of the group of the pyroxenes and the amphiboles that possess such structural characteristics as are necessary to confer a particular strength upon the material, together with values of the refraction index and the specific weight that are commonly accepted for JADEITE and NEPHRITE.

Jadeite is characterised by a great variety of colours: white, pink, lilac, brown, red, orange and, above all, many shades of green. The colour often varies even in one and the same piece of material, and this is as true as regards shade and intensity as it is for distribution: the most precious colour is a warm shade of green similar to that of the finest emeralds.

Nephrite will normally have a lesser variety of colour: green, reddish or yellowish white, grey, chestnut brown, often speckled.