On the dimorphy between the variscite and clinovariscite group: refined finestructural relationship of strengite and clinostrengite, Fe (PO 4 The isodimorphy of the ortho-and clino-minerals in the variscite group has been suggested by Strunz & von Sztrókay [6], but, so far the common stereo-chemical patterns within the monoclinic and orthorhombic modifications had not been recognized. Structure refinements of the hydrated phosphate minerals strengite and clinostrengite, which are re-presentative members of the variscite and clinovariscite group (Strunz [7]), are presented, and by appropiate axis transformations their structural relationship is illustrated.

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The structure of solvent-free single crystals of sublimed c6, has been analysed at 300 K by X-ray diffraction. A unique solution of atom coordinates gave a satisfactory agreement with the intensity data. This indicates that the c 6 0 balls exhibit a time-averaged preferred position in the crystals which are, however, heavily affected by rotational disorder. The structure reveals the truncated icosahedral molecular shape with two sets of carboncarbon bond distances. Contour plots indicate the anisotropic distribution of electron density within the almost perfectly spherical shell of the molecule. The analytically pure crystals contained an impurity of molecular oxygen located statistically over some hexagons of each 'buckyball ' resulting in a limiting stoichiometry of c60 O2.

Sodium dihydrogen phosphate dihydrate (NaH2PO4.2H20) is orthorhombic, space group P212121, with a=7.275 (9), b=11-384 (6) and c=6-606 (4) A; Z=4. The structure was solved from X-ray measurements by direct methods and refined anisotropically with X-ray (R = 0.032 over 898 reflexions) and neutron data (R=0.059 over 751 reflexions); both sets of intensities were collected on singlecrystal diffractometers (Mo Ks radiation and 2= 1-035 A respectively). A correction for secondary extinction was applied to the neutron data. While the two acidic hydrogens form strong hydrogen bonds (2.577 and 2.536 ,&), one of those of the water molecules is involved in a very weak O-H...O contact of 3-106 ,&. The P tetrahedra are linked by hydrogen bonds in a distorted diamond-type framework. Six systems of pseudo-hexagonal channels are present; the largest of them, along [001], accommodates a chain of Na octahedra. The configuration of the hydrogen-bonded groups is described. Bond-strength/bond-length correlations are discussed and their weakness for the case of strongly hydrogen-bonded atoms is pointed out. A comparison of X-ray and neutron results by Z 2 tests and half-normal probability plots shows significant differences for the thermal parameters only.

The new powder diffractometer installed on the beamline B2 at HASYLAB/DESY is described. It features a new design with a blocked double circle plus a third single circle which can be translated towards one another. The wide space between the two circle units conveniently holds complicated and spacious attachments and makes the instrument especially suitable for measurements under controlled environment conditions.