Solid-state transformations of Zn-clinoptilolite through heating

Zn-clinoptilolite was heated and analyzed ex-situ. Breakdown occurred between 400-700 °C. After 2 h at 850 °C gahnite and Zn-β-quartz nucleated. Exolution of zinc and aluminum from Zn-β-quartz determined Zn-α-quartz formation after 8 h. An increase in temperature and/or heating time determined exolution also in Zn-α-quartz, that progressively turned into cristobalite. Zn-β-quartz decreased over time at 1000 °C and disappeared after 100
h, whereas at 900 °C the quantitative ratio β-quartz/α-quartz asymptotically stabilized at ≈2.5. Rietveld refinements evidenced that at 900 °C the decrease of zinc occupancy followed an almost asymptotic path in both quartz phases. Considering the formula ZnxAl2xSi3-2xO6, if x < 0.173 ± 0.008 Zn-β-quartz is not stable at room temperature. Treatments of 2 h at 1100 and 1200 °C produced the same quantities of gahnite (≈20.6%) and glass (≈11.6%), and slight differences for cristobalite (56 vs. 63%) and quartz (8.4 vs. 1.5%). Basically, gahnite incorporated all Zn2+ contained in clinoptilolite.