Kosmochlor is a sodium-chromium pyroxene with the idealised formula NaCrSi₂O₆. Structurally it is the chromium analogue of jadeite (NaAlSi₂O₆) and aegirine (NaFeSi₂O₆); the three minerals form a continuous solid solution series, and natural pyroxene crystals from the Burmese jade tract typically sit somewhere between the jadeite and kosmochlor end-members rather than at either extreme. The species name commemorates a meteorite origin: kosmochlor was first described in 1897 in iron meteorites of the Toluca shower, and the name combines the Greek kosmos (heavens) with chloros (green).

Composition and properties

Kosmochlor crystallises in the monoclinic system. Refractive indices run from approximately 1.74 to 1.78 with a birefringence near 0.04, well above the values for end-member jadeite (1.66 to 1.68). Specific gravity ranges from about 3.55 to 3.65, again higher than jadeite owing to the heavier chromium ion in the M1 site. The material is brittle to tough depending on grain interlock, with a Mohs hardness around 6 to 7. Its colour is a saturated chrome green, often very dark, sometimes nearly black, and absorption in the visible spectrum is dominated by chromium with strong bands in the red.

Occurrence

Terrestrial kosmochlor is rare. Apart from the meteoritic type material, gem-relevant kosmochlor occurs in the jadeite-bearing serpentinites of the Tawmaw and Hpakant tracts of Kachin State, Myanmar, where it forms the chromium-rich domains in jade boulders. It has also been reported in similar high-pressure, low-temperature serpentinite environments in Japan (the Itoigawa-Omi area), Cuba, the Polar Urals of Russia, Guatemala (Motagua Valley) and a few other localities, generally as inclusions or thin reaction rims.

Relationship to jade

In Myanmar jade, kosmochlor occurs in three modes. It can dominate small high-grade pockets, producing the very dark, almost black-green material the trade has long called chloromelanite (a name now retained by some authors specifically for kosmochlor-rich jadeitite, although usage is inconsistent). It can form chromite-cored aggregates surrounded by a rim of kosmochlor, then jadeite, set in a serpentine groundmass. Or it can occur as a minor solid-solution component within otherwise typical jadeite, giving the chrome colouring agent for top-grade imperial green.

The trade name kosmochlor jade is sometimes applied to material whose pyroxene fraction is sufficiently kosmochlor-rich to push refractive indices and density above the standard jadeite range. GIA and CIBJO treat kosmochlor as one of the recognised pyroxenes that, together with jadeite and omphacite, can collectively be reported under the heading of jade.

Identification

Kosmochlor cannot be reliably separated from chromium-coloured jadeite on colour alone. Refractometry and density usually show elevated values; Raman spectroscopy gives a characteristic pyroxene pattern with shifts in the Si-O bands relative to jadeite; and ED-XRF or LA-ICP-MS confirms the high chromium content with low aluminium. Polished aggregates may also reveal heterogeneous chromium-rich domains under magnification.

Trade considerations

Pure kosmochlor is rarely available as a discrete gem; almost all rough is part of a polyphase jadeitite. Stones of significant size are unusual, and most of the material reaches the market either inside larger jade carvings or as small cabochons cut to display unusually saturated green. Buyers paying premium prices for top jade should always insist on a laboratory report identifying the dominant pyroxene phase and confirming the absence of polymer impregnation (Type B) or dyeing (Type C).

For the working jeweller and dealer, the practical point is that the term jade on a modern GIA or AGL report can legitimately encompass a range of pyroxene compositions of which jadeite is the most common but not the only acceptable species. Kosmochlor is one of the legitimate constituents, not a treatment or a simulant.