Crystal structures inherited from parent high-temperature disordered microblocks: Ca2SiO4, Na2SO4–K2SO4 sulfates, and related minerals (bubnovaite and dobrovolskyite)

Abstract

Crystal structures of Ca 2 SiO 4 , Na 2 SO 4 -K 2 SO 4 sulfates and related minerals bubnovaite K 2 Na 8 Ca(SO 4 ) 6 and dobrovolskyite Na 4 Ca(SO 4 ) 3 were described as modular ones for the rst time. A microblock, or a module [M(TO 4 ) 6 ] that is consisted of an octahedron interlinked by six vertices with six adjacent tetrahedra was considered as a structural unit inherited upon cooling from a high-temperature disordered parent one. A relationship between the parent and inherited microblocks was established. As a result, fteen possible types of modules maintaining a trigonal symmetry of the microblocks were derived based on this relationship. It was shown, that the minerals and compounds structurally-related with α-Na 2 SO 4 derived superstructures were formed as a result of a cooling of the high-temperature phases containing the disordered parent microblock. Here, a driving force of the inheritance is a tendency of the structure to order upon cooling. The reasons of a formation of a microblock from the parent one are mainly determined by ionic radius and type of a cation occupying the octahedral site. A nding of minerals with the described structural features could be a promising tool for a synthesis of novel compounds with useful properties.

Novel High-Temperature Modification of Belomarinaite KNaSO4: Crystal Structure and Thermal Order–Disorder Phase Transitions

“…The crystal structures of compounds of the Na 2 SO 4 -K 2 SO 4 series were firstly described as consisting of microblocks, including the assumption of the genesis of minerals structurally close to the compounds of this series, as formed from a single type of parent microblock in [40]. The crystal structures of the α-Na 2 SO 4 and α-K 2 SO 4 sulfates contain disordered SO 4 tetrahedron and two symmetrically independent sites for alkali cations.…”

“…The term microblock was proposed in [41]. A microblock [M(TO 4 ) 6 ] that consisted of an octahedron interlinked by six vertices with six adjacent tetrahedra was considered a structural unit inherited upon cooling from a high-temperature disordered parent unit [40].…”

“…The disordered SO 4 tetrahedron can be represented as the five different orientations of SO 4 tetrahedra, and only one is possible at a time [15][16][17][18][19]42] and references terrain (Figure 5). In the trigonal crystal system, 15 types of microblocks were derived that can be obtained by cooling [40] (Figure 5). Some of these microblocks were derived in [41] for heteropolyhedral frameworks, and then, they were described as "elementary bricks" of the mixed octahedral-tetrahedral frameworks [36].…”

“…This allows us to conclude that both M3(2d) and M4(2a) cation sites are involved in the migration path of Na and K ions. All structures in the Na 2 SO 4 -K 2 SO 4 system (excluding Na 2 SO 4 -Fddd) consist of these microblocks with the SO 4 tetrahedra in one of the described orientations [40]. A transition from the parent block to the inherited block occurs upon cooling of the high-temperature modification that is structurally similar to α-Na 2 SO 4 .…”

“…The structures of low-temperature modifications (P3m1 and P3m1) consist of microblocks 11 and 22, and high-temperature modifications (P6 3 /mmc) are formed by parent FBB-the disordered octahedral MO 6 (M = Na, K) site is surrounded by six disordered tetrahedra. Microblock designations are given in accordance with [40].…”