Haüyne: mutual cations/anionic groups arrangement and thermal expansion mechanism

Abstract

The paper reports a crystal chemical investigation of a haüyne sample, $({\rm Na}_{4.4}{\rm K}_{1.1}{\rm Ca}_{2.1}) _{\Sigma_{7.6}} [{\rm Si}_6{\rm Al}_6{\rm O}_{24}]$(SO₄)_1.6(S₃)_0.3(CO₂)_0.1, from Sacrofano, Italy, conciliating spectroscopic, chemical, and room temperature (RT) X-ray single-crystal diffraction data. The RT structure refinement provides a detailed description of the extra-framework cations and anionic groups mutual arrangement. The occurrence of minor amounts of the S₃ ⁻ poly-sulphur radical, responsible of the light-blue colour of the sample, revealed by Raman spectroscopy, has been shown to play an important role in modifying the cation partition among the three sites: M1, M2, and M3. Moreover, traces of enclathrated CO₂ were revealed by FTIR. Besides, the thermal behaviour has been investigated, up to 1,098 K, by in situ HT-XRPD. Results indicate relevant differences with respect to reference data. The sample at RT shows weak satellite reflections that disappear at 673 K. As expected, the geometry of the rigid TO₄ tetrahedra is independent from temperature, whereas the framework expands by decreasing the angle of rotation of both the SiO₄ (φ_Si) and AlO₄ (φ_Al) tetrahedra as a function of temperature. The quadric dependence of both φ_Si and φ_Al from temperature ends at 973 K, in correspondence of angles of rotation of ca. 3°, indicating the attainment of a nearly full expansion of the framework. At temperatures exceeding such value, only weak tetrahedral expansion acts. As a result of such thermal expansion mechanism, the dependence of the a-parameter from temperature shows a change of regime at 973 K. Such thermal expansion model is similar, albeit non-identical, to that of several cancrinite–sodalite group minerals.