serpentine 

The hydrous magnesium SILICATE MINERAL serpentine forms from the 
alteration of olivine and pyroxene in the presence of abundant water. 
ASBESTOS, its most fibrous form, is used as an insulating material. 
Serpentinite, the massive form of serpentine, may be cut and polished 
for ornamental stone. Serpentinite rocks are found associated with 
alpine PERIDOTITES and ophiolite sequences and are thought to be 
segments of altered mantle material that have been thrust upward into 
the Earth's crust. JAMES A. WHITNEY