Fig. P1-2 Ball-and-stick models of beryl (Fig. P1) and pezzottaite (Fig. P2). A projection of the crystal structure in the direction of the c-axis is shown in order to explain some details of the ring systems in these two minerals. The size and position of the unit-cell (as seen perpendicular to the c-axis) is also shown. Green balls = aluminium cations (Al), blue = beryllium (Be), yellow = lithium (Li), red = caesium (Cs), grey = oxygen (O), black = silicon (Si). The structure of beryl is characterized by six-membered rings of SiO4 tetrahedra alternating along the c-axis with twelve-membered rings composed of alternating BeO4 and AlO6 polyhedra. In the mineral pezzottaite, an arrangement of beryl-like rings is found, but there are two different types of twelve-membered rings: (1) Those identical to the corresponding rings in beryl, and (2) those assembled of LiO4, BeO4 and AlO6 polyhedra. To allow for the ordered (Be, Li) distribution in the structure of pezzottaite, the unit-cell had to be enlarged relative to beryl (compare P1 and P2). Both models are represented in hexagonal settings for better comparison. It must be remembered, however, that pezzottaite has a rhombohedral crystal lattice and beryl has a hexagonal lattice. In trigonal/rhombohedral symmetry, the highest symmetry element is a three-fold rotation axis, whereas in hexagonal symmetry a six-fold rotation axis is required.