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

The paper reports a crystal chemical investigation of a hauyne sample, (SO4)(1.6)(S-3)(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 S-3 (-) 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 (phi(Si)) and AlO4 (phi(Al)) tetrahedra as a function of temperature. The quadric dependence of both phi(Si) and phi(Al) from temperature ends at 973 K, in correspondence of angles of rotation of ca. 3A degrees, 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.