An experimental study of symmetry lowering of analcime

Abstract

Single crystals of analcime were hydrothermally synthesized from a gel of analcime composition at 200 o C for 24 h. They were grown up to 100 µm in size with typical deltoidal icositetrahedron habit. The chemical composition determined by EPMA and TG analyses was Na0.84(Al0.89Si2.12)O6•1.04H2O. The single-crystal X-ray diffraction method was used to determine the symmetry and crystal structure of analcime. The analcime grown from a gel crystallized in cubic space group Ia3d with lattice parameter a = 13.713( 3) Å. In the cubic analcime, Si and Al cations were totally disordered over the framework T sites with site occupancy of Si:Al = 0.6871:0.3129(14). The single crystals of analcime with cubic symmetry were hydrothermally reheated at 200 o C in ultrapure water. After the hydrothermal treatment for 24 h, forbidden reflections for the cubic Ia3d symmetry were observed. The reflection conditions led to an orthorhombic space group Ibca with lattice parameters a = 13.727(2) Å, b = 13.707(2) Å, and c = 13.707(2) Å. The unit-cell exhibits a slight distortion with (a + b)/2 > c, yielding a flattened cell along c. In the orthorhombic analcime, Al exhibited a site preference for T11 site, which shows that the Si/Al ordering over the framework T sites lowers the symmetry from cubic Ia3d to orthorhombic Ibca. After the hydrothermal treatment for 48 h, reflections corresponding to orthorhombic space group Ibca were observed as well. The lattice parameters were a = 13.705(2) Å, b = 13.717(2) Å, and c = 13.706(2) Å, retaining the flattened cell shape with (a + b)/2 > c. The Si and Al cations were further ordered among the framework T sites than the case of the hydrothermal treatment for 24h. As a consequence, the Si/Al ordering was slightly but significantly accelerated with increasing the hydrothermal treatment time. During the hydrothermal reaction, however, chemical compositions were almost unchanged. The site occupancies of Na over the extra-framework sites remained unaffected with the heating time; thus, the hydrothermal heating influences the degree of ordering of Si and Al over the framework T sites rather than that of Na among the extra-framework sites.

The Tricks of the Chameleon. Unexpected Symmetry of the Diffraction Pattern

“…A textbook example was reported recently on analcime, a mineral belonging to the family of feldspathoids which crystallizes in a cubic space group of type or in its translationengleiche subgroup of type I 4 1 / acd . Sugano & Kyono investigated two tetragonal synthetic samples that had been reheated for 24 and 48 h, respectively. As reminded in Section 1, the choice of labeling c the basis vector along the unique axis of a tetragonal crystal is a matter of convention and not imposed by any symmetry consideration.…”

“…As reminded in Section 1, the choice of labeling c the basis vector along the unique axis of a tetragonal crystal is a matter of convention and not imposed by any symmetry consideration. For reasons that are unclear (no CIF file was provided), the diffraction data were indexed in one case in an a ‐unique setting, and in the other case in a b ‐unique setting. These unusual settings misled authors to think their samples were orthorhombic because no systematic absences from the fourfold screw axis were observed along the 00 l row (they were along the h 00 and 0 k 0 rows, respectively) and to draw conclusions about cation ordering that turned out to be contradictory with respect to the experimental data, once the latter were re‐indexing in the conventional c ‐unique setting .…”

A reply to comment on “An experimental study of symmetry lowering of analcime”

“…It has long been recognized that the symmetry lowering of analcime from cubic Ia3d to orthorhombic Ibca is caused by the ordering of Si and Al cations in the framework T sites (Mazzi and Galli 1978). Recently, we reported that the hydrothermal heating of analcime influences the degree of ordering of Si and Al over the framework T sites, which lowers its symmetry from cubic Ia3d to orthorhombic Ibca (Sugano and Kyono 2018). In the comment on "An experimental study of symmetry lowering of analcime" (Nespolo 2018), however, the author points out that the reheated analcimes are not orthorhombic Ibca but tetragonal I41/acd.…”

“…In addition, the fine lamellar is considered due to transformation from the high-temperature phase (Coombs 1955;Liou 1970;Takéuchi et al 1979). These authors described that the transformation takes place above at least temperatures exceeding 300 o C. The symmetry lowering of analcime (Sugano and Kyono 2018), however, occurs at 200 o C. The temperature is a much lower than the transformation from the high-temperature phase. Although the fine lamellar texture of analcime occurs at 210 o C by hydrothermal treatment (Xia et al 2009), it exhibits a strong pH-dependent.…”

“…Although the fine lamellar texture of analcime occurs at 210 o C by hydrothermal treatment (Xia et al 2009), it exhibits a strong pH-dependent. The formation of twin domains favor the strong alkaline conditions, whereas in the hydrothermal experiment (Sugano and Kyono 2018), the analcimes were synthesized under acidic condition and reheated in pure water. These facts also deny the possibility that the twin domains causes the pseudosymmetry of tetragonal analcime.…”