CaAl₂Si₂O₇(OH)₂.H₂O

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.

This lawsonite sample is about 12 cm across.