Thermal behavior of afghanite, an ABABACAC member of the cancrinite group

Abstract

Thermal behavior of afghanite, (Na15K5Ca11)Sigma(31)[Si24Al24O96](SO4)(6)Cl-6, P31c, a = 12.7961(7) angstrom, c = 21.4094(13) angstrom, an eight-layer member of the cancrinite group, has been investigated by combined electron microprobe analysis, X-ray single-crystal diffraction, and high-temperature X-ray powder diffraction. Non-ambient X-ray powder diffraction data were collected in the 323-1223 K thermal range on a specimen from Case Collina, Latium, Italy. Structural refinement and site assignment based on the bond-valence analysis, performed on room-temperature single-crystal X-ray diffraction data, provided more accurate site allocation of cations than the available model in the literature. The results show that the cancrinite cages alternating with the liottite cages are more compressed along the c-axis than the remaining ones. As a result the chlorine atom, located at the center of the cages, is driven off-axis to release the steric strain due to the cage compression. Thermal expansion shows a discontinuity at 448 K for both a and c unit-cell parameters, a feature previously reported for other cancrinite-like minerals. Up to 448 K, the c-parameter expands significantly and more than the aparameter. A further discontinuity has been detected at 1073 K for the c-parameter. Mean linear and volume thermal expansion coefficients (x10(-6) K-1) in the 323 < T< 448 K thermal range are (alpha) over bar (a), = 12.9(4), (alpha) over bar (c) = 17.9(9), and (alpha) over bar (v) = 43.7(18). Above this discontinuity temperature, the thermal expansion is reverted becoming greater for the a-parameter. Mean linear and volume thermal expansion coefficients in the 448 < T < 1073 K thermal range are (alpha) over bar (c) = 8.22(3), (alpha) over bar (c) = 3.52(4), and (alpha) over bar (v) = 19.68(8). In the 1073 < T < 1223 K thermal range the values are (alpha) over bar (a) = 6.35(9), (alpha) over bar (c) = 5.02(14), and (alpha) over bar (v) = 17.74(9). Afghanite shows a significant microstrain at RT that increases up to ca. 700 K and subsequently decreases as a function of T. Cooling to RT allows a significantly release of epsilon(0) microstrain, which is coupled with a significant expansion of the c-parameter as compared to the starting RT data. The expansion of the c-parameter has been mainly attributed to the full expansion of the cancrinite cages alternating with the liottite cages. Upon reheating at 1173 K, the microstrain increases back to approximately the same value calculated for the first heating process. Repeated heating-RT-cooling cycles led to the partial afghanite structure disruption and the partial conversion, via an intermediate disordered phase, to hatiyne. As repeated heating/cooling cycles did not modify the epsilon(0) values both at RT and at HT, it can therefore be concluded that the strain release occurs prevalently during the first heating/cooling cycle

A Combined Theoretical and Experimental Study of Solid Octyl and Decylammonium Chlorides and of Their Aqueous Solutions

“…Dependence of cell parameters and volume from temperature of the HT-polymorph was investigated, in the 308 K ≤ T ≤ 473 K thermal range, by LeBail fitting (Figure ). The volume thermal expansivity and the linear thermal expansion coefficients have been calculated applying the approach proposed in reference and following the procedure described in references − , using a T r of 308 K. Results of the fitting procedure are reported in Table .…”

Dehydration dynamics and thermal stability of erionite-K: Experimental evidence of the “internal ionic exchange” mechanism

“…This instrumental set up has been recently proven to provide temperature-resolved data of excellent quality [20][21][22][23][24]. More in detail, the paper is aimed at: a) Defining the thermal stability of erionite-K, including the structure breakdown temperature and its classification on the basis of the categories indicated by Alberti and Vezzalini [17]; b) Detecting and monitoring the eventual occurrence of the socalled ''internal ion exchange'' mechanism; c) Correlating the framework modifications occurring as a function of temperature with both the dehydration and cations migration processes; d) Monitoring the mobility of the various cation species.…”

Thermal behavior of realgar As4S4, and of arsenolite As2O3 and non-stoichiometric As8S8+x crystals produced from As4S4 melt recrystallization

“…The volume thermal expansivity and the linear thermal expansion coefficients have been calculated applying the approach of Fei (1995) as described in Ballirano (2012) and Ballirano and Bosi (2012). Results of the fitting procedure are reported in Table 3.…”