No abstract

The structures of four spinelloid phases (spinel plus phases V, I and II), forming at 1 atm pressure in the NiGa204-Ni2SiO4 system, have been determined from X-ray data collected from single crystals grown from SiO2-rich melts in the temperature range 1798-1923 K. The results show that, under conditions of constant pressure, a strong correlation exists between the spinelloid structures, their atomic distributions and their chemical compositions. In particular, the Si contents of the tetrahedral sites are found to be strongly correlated with the amount of tetrahedral corner sharing: 6-12, 24-35 and 65% Si in the T(1), T(2) and T(3) sites with zero, one and two shared corners respectively. Consequently, the spinelloid structures accommodate increasing amounts of NizSiO4 in the order: spinel (6mo1%), phase V (27mo1%), phase I (33mo1%) and phase II (38 mol%). The crystal chemistry of the nickel gallosilicate spinelloids is discussed in comparison with that of their high-pressure aluminosilicate counterparts.