Abstract
Natural nepheline, a synthetic Na-rich nepheline, and synthetic kalsilite were ion exchanged in molten MNO3 or MCl (M = Li, Na, K, Ag) at 220–800° C. Crystalline products were characterized by wet chemical and electron microprobe analysis, single crystal and powder X-ray diffraction, and transmission electron microscopy and diffraction. Two new compounds were obtained: Li-exchanged nepheline with a formula near (Li,K0.3,□)Li3[Al3(Al,Si)Si4O16] and a monoclinic unit cell with a = 951.0(6) b = 976.1(6) c = 822.9(5)pm γ = 119.15°, and Ag-exchanged nepheline with a formula near (K,Na,□)Ag3[Al3(Al,Si)Si4O16] and a hexagonal unit cell with a = 1007.4(8) c = 838.2(1.0) pm. Both compounds apparently retain the framework topology of the starting material. Ion exchange isotherms and structural data show that immiscibility between the end members is a general feature in the systems Na-Li, Na-Ag, and Na-K. For the system Na-K, a stepwise exchange is observed with (K,D)Na3[Al3(Al,Si)Si4O16] as an intermediate composition which has the nepheline structure and is miscible with the sodian end member (Na,□)Na3[Al3(Al,Si)Si4O16], but not with the potassian end member (K,□)4[Al3(Al,Si)Si4O16] which shows the kalsilite structure; there was no indication for the formation of trior tetrakalsilite (K/(K + Na)≈0.7) at the temperatures studied (350 and 800° C). The exact amount of vacancies □ on the alkali site depends upon the starting material and was found to be conserved during exchange, with ca 0–0.2 and 0.3–0.4 vacancies per 16 oxygen atoms for the synthetic and natural precursors, respectively. Thermodynamic interpretation of the Na-K exchange isotherms shows, as one important result, that the sodian end member is unstable with respect to the intermediate at K/(K+Na)≈0.25 by an amount of ca 45 kJ/mol Na in the large cavity at 800° C (52 kJ/mol at 350° C).
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Sobrados, I., Gregorkiewitz, M. Ion exchange between tectosilicates with the nepheline-kalsilite framework and molten MNO3 or MO (M = Li, Na, K, Ag). Phys Chem Minerals 20, 433–441 (1993). https://doi.org/10.1007/BF00203114
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DOI: https://doi.org/10.1007/BF00203114