The refined oxygen parameter, x, is 0.242 ± 0.003, if a center of symmetry is chosen as the origin of coordinates. The cation-oxygen bond distance is 1.632 Å in tetrahedral sites (92% Si, 8% Ni), and 2.076 Å in octahedral sites (4% Si, 96% Ni).

The structure refinement shows that 8% ± 6% of the total silicon atoms in Ni 2 SiO 4 spinel are octahedrally coordinated. In the unit cell of AB 2 O 4 spinel (space group Fd 3 m, Z = 8), tetrahedrally-coordinated cations occupy equipoint 8 a and octahedrally-coordinated cations occupy equipoint 16 d oxygen atoms occupy equipoint 32 e with coordinates x, x, x.

Using the observed structure factors of these 21 reflections and the least-squares structure refinement computer program of F inger (1969), refinements were performed on the cation occupancy, the oxygen parameter, three temperature factors and the scale factor. Integrated x-ray intensities have been measured for 21 reflections in the 2 θ range up to 150° of polycrystalline Ni 2 SiO 4 spinel (a quench product from 40 kbar, 900☌). This is obviously necessary to compensate the differences between the cell bases of both phases. The rest of the fibers is randomly rotated around c. This is in agreement with the structural relations. In addition, most fibers also have parallel a axes, i.e. Starting from aragonite single crystals, it is shown that the apatite formed is not a single crystal but consists of fibers with the fiber axis parallel to c. The two-dimensional relationship is the basis of a topotactic reaction aragonite→apatite if aragonite crystals are treated hydrothermally with phosphate solutions. the CO 3 triangles of aragonite correspond to the PO 4 tetrahedra and OH ions in apatite, and two Ca sites of the apatite structure are not occupied in aragonite. In addition, there exists a rough two-dimensional structural relation which is schematically represented by the formula aragonite Ca 8 □ 2 (CO 3 ) 6 (CO 3 ) 2 = 8 CaCO 3 apatite Ca 10 (PO 4 ) 6 (OH) 2 i.e.

The orthorhombic aragonite structure can be described by a pseudohexagonal cell with a = 9.38, c = 5.74 Å and γ = 117°, the base of which resembles that of apatite ( a = 9.43, c = 6.88, γ = 120°) closely.

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