A zeolite is any of a group of hydrated alumino-silicate minerals containing loosely held water that can be contiuously expelled on heating. Zeolites are mostly found in volcanic rocks and hydrothermal veins. They are variable in form and light, with an open-framework structure which permits their use as "molecular sieves" and for ion exchange (especially for softening hard water). They undergo reversible dehydration. Zeolites vary in hardness from 3 to 5 and in specific gravity from 2 to 2.4. They include analcime (NaAlSi2O6.H2O), stilbite (NaCa2(Al5Si1336.14H2O), and natroite (Na2Al2Si3O10.2H2O).