X-ray data indicate that at ordinary temperatures the two minerals argentite and acanthite, both with the composition Ag2S and supposedly dimorphous, have identical structures,1 which are probably orthorhombic. Ag2S has an inversion point at about 180°C., and a recent experiment2 indicates that above this temperature the substance gives a cubic diffraction pattern. The described pattern appears to agree with a body-centered cubic structure, and it is assumed, therefore, that the heavier Ag atoms occupy positions at the corners and center of the unit cube. Because of their relatively slight difiraction efiects the positions of the lighter S atoms could not be determined. The length of the unit cube is given as 4.84 A. U., and the calculated density is 7.2 (Dana 7.2-7.3)

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