Full and ideal mixing behavior between Zr-Wd (K2ZrSi3O9) and Ti-Wd (K2TiSi3O9): evidences from mineral chemistry, X-ray diffraction pattern and Raman spectrum

摘要

A series of wadeite solid solutions (Wd(ss)) along the join K2ZrSi3O9 (Zr-wadeite, Zr-Wd)-K2TiSi3O9 (Ti-wadeite, Ti-Wd) was synthesized at 2 GPa and 1,200-1,400 A degrees C for 24 h with a piston-cylinder apparatus. The chemical composition data demonstrated that Ti and Zr readily substitute for each other, and a full series of Wd(ss) exists between the Zr-Wd and Ti-Wd end-members at the P-T conditions of our synthesizing experiments. The volume-composition relationship at ambient P-T conditions did not show any significant deviation from the Vegard's law, indicating a nearly ideal mixing between the Zr-Wd and Ti-Wd. All the Raman peaks observed at ambient P-T conditions were linearly correlated with the composition, suggesting an almost full structural relaxation for the Wd(ss). These findings suggest that the complete mixing behavior along the join K2ZrSi3O9-K2TiSi3O9, as demonstrated by our high-P experiments, may extend to a much wider range of P-T conditions. Consequently, the coexistence of the Zr-Wd and Ti-Wd in the lamproites from the Leucite Hills, Wyoming, may be due to a chemical composition change of the magma rather than a crystal structural limit.