Molecular sieves are synthetic zeolites (complex alumino-silicates of sodium, potassium or calcium) of various pore sizes, similar to many natural clays and feldspars. When the water of hydration is driven off, the crystal does not collapse or rearrange as do most other hydrated materials. Instead, the physical structure of the crystal remains unchanged which results in a network of empty pores and cavities that comprise about one-half of the total volume of the crystals. Due to their unique crystal structure the cavities and pores are precisely uniform in size; the term "pore size distribution" is not applicable. Molecular Sieves are typically used for separations of the fixed gases; CO, CH4, O2 and Ar are easily separated at room temperature. Molecular sieve 4A has pores of 4 angstrom diameter.