LASER RAMAN SPECTROSCOPIC CHARACTERIZATION OF COMMON ZEOLITES

Zeolites are H₂O-bearing Al-silicates with standard formula of A_m(Si, Al)_xO_2x.nH₂O, where A sites are mainly occupied by Na, Ca, but also by Ba, Sr, K, Mg, and Mn. In zeolite structure, [(Si, Al)O₄] tetrahedra share the corner oxygen atoms to form the framework of these tectosilicates, in which large space and channels are formed to host H₂O and A cation. Because the weak bonding of A cations and H₂O to the silicate framework, the substitution of A cation and the loss/grain of H2O molecules can happen relatively easy under suitable environmental conditions. On Earth, zeolite minerals form normally at hydrothermal or sedimentary settings.

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-O_b-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-O_b-Si, Si-O_b-Al, and Al-O_b-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.