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

Abstract

As 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 (CaSO 4 Á2H 2 O) and syngenite (K 2 Ca[SO 4 ] 2 ÁH 2 O) 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 (CaSO 4 ÁxH 2 O, 0.5 ≤ x ≤ 0.8) and anhydrite (CaSO 4 ). On the other hand, syngenite was transformed to calciolangbeinite (K 2 Ca 2 [SO 4 ] 3 ) at high temperatures in 2-year aged horsetail grass.