In situ synchrotron PXRD investigation of fresh and aged Equisetum ramosissimum Desf. (horsetail grass)

Abstract

AbstractAs one of the oldest species, horsetail grass (Equisetum ramosissimum Desf.) is known as a living fossil plant, dating back to the Mesozoic era. Horsetail grass is also considered one of the most important sources of bio‐silica due to its ability to accumulate high amounts of silica in all parts of the plant; various minerals can also be isolated by heat treatment. Fresh and aged horsetail grass stored for 2 years under ambient conditions was investigated by synchrotron powder X‐ray diffraction (PXRD). Clear crystallites were not observed in a fresh sample stored at room temperature; surprisingly, high amounts of gypsum (CaSO4·2H2O) and syngenite (K2Ca[SO4]2·H2O) were observed in the 2‐day dried and 2‐year aged samples, respectively. However, crystalline silica materials were not observed. In addition, in situ thermal treatment of up to 700°C was applied to investigate the crystals and phase transitions by focusing the X‐ray beam onto a single stem. In situ synchrotron PXRD revealed that dehydration occurred in gypsum in the 2‐day dried sample with an increase in temperature to hemihydrate (CaSO4·xH2O, 0.5 ≤ x ≤ 0.8) and anhydrite (CaSO4). On the other hand, syngenite was transformed to calciolangbeinite (K2Ca2[SO4]3) at high temperatures in 2‐year aged horsetail grass.