Jadeite Deposits of the Clear Creek Area , New Idria District

Tectonic inclusions within the New Idria serpentine body contain jadeite in two distinct assemblages: (1) Lenslike inclusions containing a monomineralic green jadeite core surrounded by a calc-silicate rim. (2) Jadeite veins cross-cutting albite-crossite schist inclusions. In these veins jadeite coexists with low albite; green jadeite (Jd 75 Ac+Di+He 25) coexists with low albite within the host schist. Albite schists are related to the pre-metamorphic keratophyres and it can be shown that desilication of such rocks produces a bulk composition similar to that of jadeite. Metamorphism of keratophyric tectonic inclusions within serpentine produced jadeite-albite schists along margins of the inclusions. Jadeite-albite veins formed from mobilized liquids rich in the jadeite molecule and deficient in water. Geological evidence such as would indicate extreme pressures or temperatures during jadeite formation is lacking; rather, a low silica and dry system allowed its formation at reduced temperatures and pressures.