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In this chapter, we illustrate and discuss two distinct groups of microporous phases: the cancrinite group and the C-S-H compounds of the tobermorite and gyrolite families. The compounds in the first group present a three-dimensional purely tetrahedral framework with, apart from a single exception, Si:Al ratio equal to 1; in the mineralogical classifications they are included among feldspathoids and are generally “regarded …… distinct from zeolites, in part, at least, because of the presence of large volatile anions” (Coombs et al. 1998). The members of the second group are characterized by mixed frameworks built up by silicon (and aluminum) tetrahedra and calcium polyhedra. A common feature of both groups is the modular character of their frameworks, which are built up through various stacking ways of a single module (as in the minerals of the cancrinite-davyne family) and two or more modules as in the case of the C-S-H phases.
The minerals belonging to the cancrinite group (Merlino 1984; Deer et al. 2004) are feldspathoids with a Si:Al ratio equal to 1, with the only exception of cancrisilite, which has Si:Al = 7:5 (Khomyakov et al. 1991a,b). The available structural data for the phases with Si: Al = 1 indicate that silicon and aluminum regularly alternate in the tetrahedral sites, in accordance with the Loewenstein rule. The structural cavities host alkaline and earth-alkaline cations, and a wide variety of extra-framework anions, as well as H2O molecules. Their framework is characterized by layers containing six-membered rings of tetrahedra (Fig. 1⇓). Every ring is linked to three similar rings in the preceding layer and to three rings in the succeeding one. If the position of the rings in the first layer is called “A,” and the two possible alternative positions in the adjacent layers are “B” and …