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 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 . 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. Chromomelanite 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 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