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

The fracture surfaces of analcimes reported by Sugano and Kyono (Phys Chem Miner, 2018) were re-investigated using field-emission scanning electron microscopy. The results of scanning electron microscopic observation show the fractures can be characterized by smoothly curved surfaces, called sub-conchoidal fracture, and neither the lamellar twin nor the domain walls of twin were found on the surfaces of the analcimes. In addition, the lamellar twin is generally formed by transformation from the high-temperature phase or from pseudomorphic replacement under strong alkaline conditions. Actually, the symmetry lowering of analcime reported by Sugano and Kyono ( 2018 ) occurs at 200 °C. The temperature is much lower than the transformation from the high-temperature phase. In the hydrothermal experiment (Sugano and Kyono 2018 ), moreover, the analcimes were synthesized under acidic condition and reheated in pure water. No twin domain is likely to be formed under the hydrothermal condition. These facts strongly deny the possibility that the twin domains cause the pseudo-symmetry of tetragonal analcime. Consequently, it can be concluded that the observed forbidden reflections for the cubic Ia 3 d symmetry are not due to the presence of twin domains, but due to the symmetry lowering of analcime from cubic Ia 3 d to orthorhombic Ibca .