Xonotlite is a mineral with the chemical formula Ca6Si6O17(OH)2. It crystallizes in the monoclinic - prismatic crystal system with typically an acicular crystal form or habit. It can be colorless, gray, light gray, lemon white, or pink. It is transparent with a vitreous to silky luster. It leaves a white streak. Xonotlite is rated 6.5 on the Mohs Scale of hardness. It was first described in 1866 and named for its occurrence in Tetela de Xonotla, Puebla, Mexico. It occurs as veins in serpentinite and contact metamorphism aureoles.