Natrolite: Gemstone Information
Natrolite is Na2[Al2Si3O10]2H2O and a member of the zeolite group in which it occurs as orthorhombic pseudotetragonal short to long striated prismatic crystals, transparent to translucent, usually colourless but sometimes pale shades of grey or yellow to red. There is a perfect cleavage in two directions; hardness is 5 and SG 2.2–2.26. The RI for the alpha, beta and gamma rays is 1.473–1.483, 1.476–1.486, 1.485–1.496,biaxial positive with birefringence 0.012. Natrolite occurs in cavities in amygdaloidal basalts and related igneous rocks in association with other zeolites. Very large crystals are found at the Johnson asbestos mine, Thetford, Quebec, Canada and also from Mt Saint Hilaire in the same province. Many other locations for fine crystals exist. Milky white transparent natrolite is reported from Pakistan.