Multinuclear NMR study of Cs-bearing geyserites of the Targejia hot spring cesium deposit in Tibet

The large scale Targejia hot spring type Cs deposit in Tibet is unique and still active today with the distribution of hot springs being controlled by tectonic movements of the Tibetan Plateau. The ore bodies of the Targejia Cs deposit are mainly composed of geyserite, a Cs-containing opal. In this study, a combination of X-ray diffraction, infrared spectroscopy, and ²⁹Si, ¹H, ¹³³Cs magic angle spinning nuclear magnetic resonance (MAS NMR) were used to study the location and mobilization of Cs in geyserites. ²⁹Si NMR spectra indicate that the relative amounts of Q₂, Q₃, and Q₄ species vary in geyserite samples of different mineralization stages. Based on the ¹³³Cs chemical shift as well as the change in ²⁹Si chemical shift ranges (especially for Q ₂), cesium is inferred to associate with Q₃ and Q ₂ silanol groups, where it is coordinated by O2-, OH-, and H2O as a network modifying cation. As Cs-bearing geyserite ages and dehydrates, Q ₂ and Q₃ polymerize giving an increase in Q₄. This is accompanied with a decrease in Cs content indicating that Cs may be leached out from the opals as it loses its original coordination environment in the silicate framework.