The crystal structure of apophyllite, with a = 8.963(2), c = 15.804(2), P4/mnc, Z = 2, has been refined (R = 0.042) by least squares with 546 independent intensities collected with a diffractometer. Apophyllite is a sheet structure with the bridging Si-0 bond lengths 1.619(3), 1.631(3), and 1.626(3)Å, while the non-bridging bond length Si-O(3) is 1.589(3)Å. The two independent Si-O-Si angles are 139.9(2) and 140.6(2) degrees. The mean Si-O bond length is 1.616Å. As it was not possible to locate unequivocally the hydrogen ions with the X-ray data, the interpretation of the role of the water in the structure was based on the charge balance approach of Donnay and Allman (1970). Assuming that the fluorine ion is bonded to the calcium ion, the water oxygen, O(4), is hydrogen bonded to O(3), while the second proton appears to form a bifurcated hydrogen bond with O(2) and O(3). The D.T.A. and T.G.A. curves show that the water loss is discontinuous and results in a weight loss of 16 percent. The infrared absorption spectra show O-H stretching vibration bands at 3560 cm–1 and 3070 cm–1; and single crystal infrared data show strong absorption bands using a cleavage flake with (001) oriented 70 and 90 degrees from the infrared beam.