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The crystal structure of kurnakovite, Mg[B3Oa(OH)5] (H20)4.H20, was refined to R=0-051 from 2057 X-ray reflexions (Cu K~ radiation). Crystals are triclinic, space group P], with a= 8.3479, b= 10.6068, c=6.4447/~ (all +_0"0001 /~), ~=98-846, fl= 108.891, 7= 105-581 ° (all +_0-001°); D,,= 1.847 and D~,= 1.855 g cm -3 for Z= 2. The structure contains discrete [B3Oa(OH)5] 2-groups. These share OH groups with Mg(OH)2(HzO)4 octahedra, with which they alternate in the c direction to form chains. The fifth water molecule occurs between these chains; a network of hydrogen bonds provides the inter-chain links, as well as a few intra-chain ones. Hydrogen atoms were located and their parameters refined. The H-bonding system explains the observed cleavages.

The crystal structure of synthetic nasinite, Na2[BsOa(OH)].2H20, has been determined and refined from 1320 single-crystal diffractometer data by means of direct methods and least-squares refinement with anisotropic thermal parameters (isotropic for hydrogen atoms), giving R = 0.027. Hydrogen atoms were located on a AF map. Lattice constants determined on the same diffractometer are: a= 12.015 (2), b=6-518 (I), c= 11"173 (1)/~; space group Pna2x; Z=4. The structure is dominated by sheets in the be plane, resulting from polymerization of [BsOs(OH)] z-polyions. Both Na atoms are eightfold coordinated; one is located on the B-O sheet, the other between the hexagonal rings protruding alternately in the space between the sheets. The Na polyhedra framework is three-dimensional and provides links between B-O sheets. Additional inter-sheet connexions are provided by H bonds from the OH group and from one of the water molecules. The structure closely resembles that of Kz[BsOs(OH)].2H20; comparisons with this compound are made.

The crystal structure of the synthetic compound 3Na20.3B203.2H20, space group Pnma, a= 8.923 (1), b=7.152 (1), c=9-548 (1) A, was determined from single-crystal diffractometer X-ray data. Direct methods were used to solve the structure which was later refined by least-squares calculations to an R of 0.029. The structure determination showed that the compound labelled in the literature as 'l:l:l' has actually a composition corresponding to Na20-B203-H20 molar proportions of 3:3:2. The basic structural unit is the isolated polyion [BaOs(OH)2] 3-formed by corner-sharing among one tetrahedron and two triangles. Of the four attached oxygen atoms only the two linked to the tetrahedral boron exist as hydroxyl groups. Na polyhedra form a tight two-dimensional network by the sharing of corners, edges and faces. All oxygen atoms linked to the Na atoms belong at the same time to the B-O polyions and vice-versa. A high cohesion in all directions is the result of the perpendicular arrangement between B-O rings and Na-O sheets. The crystal chemical formula of the compound investigated is Na3[B3Os(OH)21.