These samples of lawsonite are displayed in the Smithsonian Museum of Natural History. Lawsonite is a silicate mineral of calcium and aluminum with the composition CaAl2Si2O7(OH)2.H2O. The sample at left is about 8 cm across and is from Franciscan Complex, California. This mineral forms only at the high pressures within subduction zones. The blocky, bluish-white crystals are lawsonite. The fibrous, dark blue crystals are glaucophane.