Synthetic olivine (Fe,Ni)$_{2}$SiO$_{4}$ solid solution series annealed at 800 and 900$^{o}$ C was studied by powder X-ray diffraction and Mossbauer methods.
The homogeneous composition of this series was established by the equality of d$_{111}$ profile half-width as the most sensitive to chemical composition. The Fe-Ni distribution in triads of M2-M1-M2 octahedra reflects in concentration dependence of d$_{111}$.
Unit cell parameters are nonlinear against composition and have some inflections. There are positive deviations for V and Vo (molar volume of mixing) from Roul's law, which decrease with annealing temperature reduction.
The Mossbauer investigations indicate the Fe$^{2+}$ preference of M2 position. The degree of long-order increases with annealing temperature decrease and depends significantly on component ratio in Fe-Ni olivines. In low-ferrous olivines (0.0 < XFe <0.3) Fe ions occupy the M2 sites only, at higher iron content (XFe > 0.3), however, Fe$^{2+}$ occupies the M1 sites as well. So, the cation distribution character (long rang order degree) is different for olivines of 0.0 < XFe <0.3; 0.3 < XFe <0.5; 0.5 < XFe <1.0.
The Mossbauer data show nonstatistical Fe$^{2+}$ distribution in M1 sites with preferable asymmetric Fe-Ni pairs (shot range order), which consists with calculations on the basis of quasichemical model for multisite low symmetrical solid solution [1].
We suppose that complicated relations of cell parameters versus M1 and M2 sites occupancy are the result of long and short range degree changes. The a(Å) parameter only have a linear dependence in M1 occupancy for whole series, since M1 octahedron has no M1 as the nearest neighbours along a axis.
The detailed X-ray and Mossbauer investigations carried out make possible a simple definition of chemical composition (graphic determination) and long-range degree by cell parameter (a= 4.728+0.009 M1 XFe with accuracy 0.004).
1. Nikitina L.P. The Thermodynamic of Silicate Solid Solutions - 1986, Leningrad, Nauka, 152 P (in Russian).
This work was supported by Russian Foundation of Theoretical Investigations under No. 95-05-15349a.