Afghanite

Abstract: Zeolites that belong to the so-called ABC-6 family may be regarded as having been assembled architecturally from individual sheets, each of thickness ca. 2.6 $\overset\circ{\mathrm A}$ ($1 \overset\circ{\mathrm A}$ = 10$^{-10}$ m = 10$^{-1}$ nm). The sheets consist of macro-anions of corner-linked SiO$^{4-}\_4$ and AlO$^{5-}\_4$ tetrahedra (Si:Al ranging from 1-4), charge-compensating cations, typically Na$^+$, Ca$^{2+}$ and K$^+$ and occluded water. Cancrinite (idealized formula Na$\_5$Al$\_6$Si$\_6$O$\_{24}$. H$\_2$O), in which the stacking sequence of the sheets may be symbolized AB, has a 5.1 $\overset\circ{\mathrm A}$ repeat along the direction of stacking; offretite (idealized formula (Na$\_2$Ca)$\_2$Al$\_4$Si$\_14$O$\_{36}$.14H$\_2$O) has AAB stacking and a repeat distance of 7.6 A and sodalite (Na$\_6$Al$\_6$Si$\_6$O$\_24$. mH$\_2$O) has ABC stacking with a repeat distance of 7.7 $\overset\circ{\mathrm A}$. This pattern continues; gmelinite (AABB), losod (ABAC), erionite (AABAAC), chabazite (AABBCC), TMA-E(AB) (ABBACC), liottite (ABABAC), afghanite (ABABACAC), levyne (AABC-CABBC) and franzinite (ABCABCBACB), the repeat distance of the latter being 26.6 $\overset\circ{\mathrm A}$. It has long been suspected, largely on the basis of chemical and diffusive behaviour, that naturally occurring and synthetic analogues of this class of zeolites can, depending upon the nature of their genesis, form intergrowths at the unit-cell level. For example, there is a sliver of erionite or sodalite in offretite. The presence of such features would profoundly affect the catalytic and adsorptive performance of the parent zeolite and for this reason there is a pressing need for a technique to identify such intergrowths. Hitherto no direct method has been available that is capable of probing the nature of these intergrowths and their atomic detail. We show here that high-resolution electron microscopy (h.r.e.m.) combined with computer-generated images can solve this problem, provided that the samples are first \`de-aluminated' under conditions close to those used to activate many of these zeolites before their use as catalysts. The consequence of dealumination, which greatly increases the Si:Al ratio (to more than 10) is to jettison most of the compensating cations originally present and to render the resulting specimens (after annealing to heal the individual sheets) more resistant to electron irradiation. The resulting structure is quite an open one, amenable to imagining by h.r.e.m. and to simple optical simulation. The reliability of the h.r.e.m.-based method of reading off stacking sequences in this family of de-aluminated zeolites was tested by imaging offretite and comparing the result with images produced by rigorous \`multi-slice' calculations and simple optical simulation. From such work it was established that the siting of the six- and eight-membered apertures present in the structures gives unequivocal information regarding the sequencing of the sheets. On the basis of this information, computed images of 29 distinct kinds of stacking sequences are derived, encompassing both the structurally regular (de-aluminated) zeolites cancrinite, offretite, sodalite, gmelinite, erionite, chabazite and TMA-E(AB) and also unit-cell intergrowths (i) within erionite of cancrinite, offretite, sodalite, gmelinite and chabazite; (ii) within offretite of cancrinite, sodalite, erionite, gmelinite and chabazite; (iii) within chabazite of offretite, sodalite, gmelinite and erionite; (iv) within gmelinite of offretite, erionite, chabazite and TMA-E(AB) and (v) within cancrinite of sodalite, offretite and erionite. This compilation can now serve as a reference library that should aid future work in recognizing and characterizing intergrowths recorded by h.r.e.m.