Transformation of Siliceous Spicules from Ephydatia fluviatilis Freshwater Sponge into α-Cristobalite Using a Carbonization Approach

A rare form of SiO2, α-cristobalite, can be obtained through the processing of biosilica. In this study, for the first time, α-cristobalite was obtained by carbonization of siliceous spicules from the freshwater sponge Ephydatia fluviatilis in an oxygen-free atmosphere. The spicules underwent purification to remove organic matter, and were then carbonized in a temperature range of 500–1800 °C. The resulting material preserved the original spicules’ shape and size (20 × 300 µm) below the cristobalite melting point. Crystalline structure formation was observed by means of scanning electron microscopy (SEM) and confirmed by analytical methods, including X-ray diffraction (XRD), infrared (FTIR) spectroscopy, and Raman spectroscopy. The study demonstrates the feasibility of producing α-cristobalite with a size and shape predetermined by the micromorphology of E. fluviatilis sponge spicules.