Natrolite's a tectosilicate mineral species belonging to the zeolite group. It's a hydrated sodium and aluminium silicate with the formula . It was named natrolite by Martin Heinrich Klaproth in 1803. Needle stone or needle-zeolite are other names, alluding to the common acicular habit of the crystals, which are often very slender and are aggregated in divergent tufts. Larger crystals have the form of a square prism terminated by a low pyramid, the prism angle being nearly a right angle. The cry… (More on Natrolite)