The crystal structure of hemimorphite, Zn₄Si₂O₇(OH)₂H₂O, has been refined from X-ray intensity data collected at 300°, 600°, and 22°C (after heating to 600°C). The space group throughout the experiment was Imm2, Z=2, and the cell dimensions at each temperature, in the above-mentioned order, are: a=8.337(5), b= 10.724(6), c=5.116(4)Ȧ; a=8.268(5), b=10.784(8), c=5.113(3)Ȧ; and a=8.206(4), b=10.815(6), c=5.089(2)Ȧ. Anisotropic least-squares refinements resulted in unweighted R-factors of 0.046, 0.074, and 0.061, respectively. The structure can be described as a framework consisting of interconnected corrugated sheets composed of 3-membered rings of one SiO₄ and two Zn(OH)O₃ tetrahedra. Heating to 600°C, causing dehydration, along with subsequent cooling to 22°C resulted in the contraction of a and c, expansion of b, and a decrease in the unit cell volume. The unit cell volume decrease is directly proportional to the volume decrease of the cavity initially containing the water molecule. These changes are rationalized in terms of atomic positional shifts, especially those of the hydroxyls which are no longer hydrogen-bonded to a water molecule. The hydroxyls move towards the position occupied by the water molecule prior to dehydration. The effects of dehydration on the crystal structure of hemimorphite were to found to be similar to those found in some zeolites. Although hemimorphite may not qualify for a molecular sieve due to its small aperture size, it may be useful as an isotopic sieve. / Master of Science