A Thermal Study of the New Mineral Belomarinaite KNaSO4

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

“…New data on the belomarinaite crystal structure obtained in this work at 300 • C indicate that with increasing temperature, the belomarinaite crystal structure changes slightly and translationsgleiche phase transition occurs. The belomarinaite crystallizes in the P3m1 space group at 300 • C. The presence of this transition is confirmed by the DSC data of synthetic KNaSO 4 , e.g., [10,30,31], and those of belomarinaite [5]; there are three thermal effects at about 123, 446 and 840 • C. It was suggested in [30] for synthetic KNaSO 4 that the first broad endothermic peak corresponds to a second-order phase transition. Thus, according to the high-temperature single crystal X-ray diffraction data, the first peak can be attributed to a P3m1↔ P-3m1 phase transition.…”

“…In the 𝑃3 𝑚1-polymorph, there is only an M1(2d) site coordinated by ten oxygen atoms with <M1-O> bond lengths equal to 2.937 Å (Figure 2). Na and K atoms are completely disordered in the M1(2d) site-50%/50%, due to its mobility, in contrast to room temperature, where the M1(1c) site is occupied by 78% Na and 22% K, while the M2(1b) site is The temperature dependencies of the unit cell parameters for belomarinaite in the temperature range of 25-800 • C were investigated by our group in [5]. The K and Na atoms are ordered in M1O 10 and M2O 10 polyhedra in the belomarinaite crystal structure at room temperature.…”

“…It is worth noting that the coordinates of oxygen sites differ at temperatures of 500 and 750 °C, which further indicates dynamic disorder. The temperature dependencies of the unit cell parameters for belomarinaite in the temperature range of 25-800 °C were investigated by our group in [5]. The K and Na atoms are ordered in M1O10 and M2O10 polyhedra in the belomarinaite crystal structure at room temperature.…”

“…The thermal behavior of belomarinaite was studied using high temperature X-ray powder diffraction over the temperature range of 30-800 • C and using DSC/TG over the temperature range of 30-1000 • C in [5]. The existence of a high-temperature modification (P6 3 /mmc) of this mineral above 446 • C has been demonstrated.…”

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

“…According to (Eysel 1973;Belousova et al 2021), the K 3 Na(SO 4 ) 2 and KNaSO 4 compounds upon cooling from the isotypical Na 2 SO 4 (P6 3 /mmc) high-temperature modi cation transform into the P-3m1 and P3m1 ones at 400 and 480°C, respectively. Probably a similar situation occurs during a cooling of the high-temperature Na 3 K(SO 4 ) 2 phase (P3m1), but this phase exists only as a form of an impurity-stabilized (Cu) modi cation.…”