LASER RAMAN SPECTROSCOPIC CHARACTERIZATION OF COMMON ZEOLITES
We have analyzed 13 common zeolites using laser Raman spectroscopy with a 532 nm excitation. Similar to other tectosilicates, the strongest Raman peaks of zeolites are contributed by breathing vibrational mode of T-Ob-T (where T= [Si, Al]O4 tetrahedron) that occurs below 600 cm-1. Many zeolites have multiple spectral components in 400-600 cm-1 spectral range, corresponding to varieties of Si-Ob-Si, Si-Ob-Al, and Al-Ob-Al bonds in tetrahedra. Many of them have fine spectral peaks below 400 cm-1 contributed by lattice vibrational modes. In addition, we have noticed that although all zeolites have structural H2O, while the Raman spectral peak of H2O does not appear in all spectra, in 4000-3200 cm-1 spectral range where H2O/OH vibration modes normally occur. Similar phenomena occur in many zeolite Raman spectra collected in Rruff database. We believe it is caused by the weak bonding between structural H2O and silicate framework in some zeolites, where the loss/gain of H2O can be induced by minor changes of environmental conditions. Similar spectral features in H2O/OH spectral range were observed from some phyllosilicates. Further in depth investigations are needed in order to understand the chemical and structural causes of the Raman spectral variations of zeolites.
This preliminary study shows that common zeolites do have finger-print Raman spectral characters, that are different from other tectosilicates, can be used for phase identification and characterization during robotic surfaces explorations on other planets, especially on Mars where the evidences of hydrothermal processes were found by the past rover exploration, e.g., the Spirit rover at Gusev crater.