The structure of Na2[Zr(SO4)3].3H20 has been determined'~y single-crystal X-ray analysis and refined by least-squares. The crystals, which are orthorhombic with space group P21212t, have the unit-ceU dimensions a=22.16, b=7.73, c=7.08 /~. The structure consists of spirals of composition [Zr(SO4)a(H20)2], extending in the [001] direction which are held together by the sodium atoms and a single lattice water molecule. Two of the sulphate groups form bridges between the zirconium atoms while two more sulphate groups are each doubly bonded to each zirconium atom. All sulphate groups have two terminal oxygen atoms. There are two water molecules coordinated to each zirconium atom. The hydrogen atoms of these water molecules bridge to oxygen atoms of sulphate groups and probably give rigidity to the spiral structure.

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A program is described for the high-speed computation of the absorption correction factor for single-crystal diffraction measurements. The method requires that the sample boundaries be approximated by plane surfaces and that there be no re-entrant angles between these planes.The derivation of structure factors from integrated intensity measurements in X-ray or neutron diffraction usually requires a correction for absorption of radiation in the specimen. Tabulated values of this correction, expressed as the ratio of integrated intensity yielded by an absorbing specimen to that yielded by an hypothetical non-absorbing specimen of equal volume, are available for cylindrical (Bradley, 1935) and spherical (Evans & Ekstein, 1952) sample shapes. For accurate measurement of structure factors, the specimen is often reduced to one of these simple forms.