Temperature- and moisture-dependent powder X-ray diffraction studies of kanemite (NaSi2O4(OH)center dot 3H(2)O)

摘要

The high-temperature- and moisture-dependent behaviour of synthetic kanemite (NaSi2O4(OH)center dot 3H(2)O or SKS-10) has been studied by in situ powder X-ray diffraction. Heating experiments in the range between ambient temperatures and 250 degrees C confirm earlier investigations that the dehydration of kanemite occurs in two steps. According to our results the two different reactions start at similar to 30 and 75 degrees C. The dehydration products have the following compositions: NaSi2O4(OH)center dot H2O (monohydrate) and NaSi2O4(OH), respectively. The crystal structures of both phases have been solved at ambient conditions ab initio from laboratory powder diffraction data using samples that have been carefully dehydrated at 60 and 150 degrees C, respectively, and refined subsequently by the Rietveld method. Basic crystallographic data are as follows: NaSi2O4(OH)center dot H2O: orthorhombic, space group Pna2(1), a = 7.2019(1), b = 15.3252(2), c = 4.8869(1) angstrom, V = 539.37(1) angstrom(3), Z = 4; NaSi2O4(OH): monoclinic, space group P2(1), a = 6.3873(1), b = 4.8876(1), c = 7.1936(1) angstrom, beta = 93.36(1)degrees, V = 224.19(1) angstrom(3), Z = 2. Both compounds belong to the group of single-layer silicates based on Si2O4(OH) sheets. The sodium cations are located between the tetrahedral sheets and are surrounded by oxygen atoms from silicate anions and/or water molecules. Depending on the dehydration step the coordination numbers of the alkali ions vary between six (kanemite) and five (NaSi2O4(OH)). Kanemite and its two dehydration products show structural similarities which are discussed in detail. Moisture-dependent diffraction studies at ambient temperatures indicate that kanemite is stable between 10% and at least 90% relative humidity. Below the lower threshold a transformation to the monohydrate phase was observed. Dehydration and rehydration as a function of humidity is reversible. However, this process is combined with a significant loss of crystallinity of the samples.