Abstract
Delhayelite, ideally K4Na2Ca2[AlSi7O19]F2Cl, is a mineral showing structural features of both phyllosilicates and silicates with heteropolyhedral frameworks. Its crystal structure is based upon the two-layer tetrahedral blocks, or double layers, [(Al,Si)4Si12O38], linked by the columns of edge-shared Ca-centered octahedra (Cannillo et al. 1969; Pekov et al. 2009). As it is shown in (Pekov et al. 2011), these two main building units form a heteropolyhedral quasi-framework with zeolite-like channels of two types: (1) inside the tetrahedral blocks, hosting K+ and Cl-; (2) in the interlayer space, between Ca columns, hosting Na+ and F-. Such ordering of three large cations and two halogen anions is a feature of delhayelite making it unique among minerals.
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