TRANSFORMATIONS OF CUBIC LAZURITE WITH INCOMMENSURATE-MODULATED STRUCTURE. V.L.Tauson, A.N.Sapozhnikov, K.E.Kuznetzov, and V.V.Akimov, Institute of Geochemistry, Irkutsk, 664033, Russia

The sodalite-like minerals are the convenient object for studying different types of structure modulations, especially that ones which can be considered as the result of forced equilibria attainment (Tauson & Akimov, 1993;1994). The set of experiments is performed using lazurite sample of composition (Na,Ca)7-8Si6Al6O24(SO4,S,Cl)2 from the Baikal region deposit. The sample is characterized with the three-dimensional incommensurate modulation of structure with the modulation parameter n=0.217. When annealed in air, the sample changes the initial modulation to the commensurate one with n=0.168 via an intermediate state without superstructure. The diagrams of lazurite stability on coordinates temperature-gas fugacity (T-fO2,T-fS2,T-fSO2) are obtained. Lazurite is unstable under fO2>10-14-10-12 bars and decomposes with the formation of complex sulphates of Ca, Na and Al which are qualitatively similar to the clusters observed in lazurite and hauyne structures. However, the actual reason of lazurite unstability lies in the high fSO2, not high fO2 value. Under a sufficiently low fSO2, lazurite undergoes the transformation to the sulfide sodalite because the SO4 - ion loses its stability. This is accompanied by a strong contraction of the lazurite lattice. Under very low fSO2 (<10-14 bar) at 550oC, a new phase with incommensurate modulation n=0.136 is observed in addition to S-sodalite. For moderate fSO2 values, the ordering and condensation of clusters occure before lazurite decomposition. As a result, a persistent density modulation is formed which originates from the balance of cluster ordering energy and Na-Ca mixing energy because the formation of cluster condensates needs a local change in solid solution composition for its accomodation to the clusters stoichiometry. This state is recognized as metastable forced equilibrium of type II .

References

Tauson V.L. & Akimov V.V. Geologia i Geofizika 34,103 (1993), 35,57 (1994).