What makes serpentinite look and feel the way it does?
Serpentinite rocks are almost exclusively made of serpentine minerals. The most common serpentine mineral in Franciscan rocks is antigorite. This mineral gives the serpentinite its characteristic light to dark green color. Serpentine minerals are made of tiny sheets of silica tetrahedrons that are loosely held together. The weak bonds between these sheets gives serpentine its greasy or scaly look, and slippery feel (like a snake skin). Serpentinite often contains many veins, some of which may be filled with the fibrous mineral chrysotile (a form of asbestos). Chrysotile is a serpentine mineral in which the silica sheets are rolled into tiny tubes to form hollow fibers. Loose asbestos fibers cause lung disease if you inhale them. Pay attention to serpentinite – if you see fibrous asbestos, don’t handle the rock.
