|Thomas Armbruster's Group
Natural zeolites are framework silicates with large open cavities occupied with cations and water molecules. These minerals are abundant all over the world and applied as ion exchangers when ever large quantities are required (e.g. Cs extraction from nuclear waste material in Chernobyl). We investigate (a) the kinetics of cation exchange (Na, K, Cs, Pb, Ba) in large single crystals of natural zeolites to gain information on diffusion rates and activation energies. Optical methods and electron microprobe analyses are used for the measurement of cation diffusion profiles. (b) We study the extra-framework cation arrangement in cation exchanged natural zeolites by single crystal X-ray methods to understand the bonding between framework and cavity occupants and its influence on framework distortions.
Mn2+- and Mn3+-silicates occur all over the world as reaction product of primary Mn-oxides and SiO2-rich fluids. These minerals can be used as natural sensors for pressure, temperature and oxygen fugacity during their crystallization in rocks. We are interested in the crystal chemistry of these minerals because (a) Mn2+ is mostly too large to be incorporated into oxygen closed packed structures thus new, strongly distorted structure types result; (b) Mn3+ in octahedral coordination shows a characteristic electronic distortion (Jahn-Teller effect). We use single crystal X-ray methods to solve these structure types in order to understand the influence of the condensed silicate anion on the electronic Mn3+ distortion. In addition, hydrothermal experiments are performed to synthesize these Mn-silicates.
Average crystal structures of natural and synthetic minerals are determined by diffraction experiments at various temperatures. These results are compared with spectroscopic findings providing a different time- and space- resolution. This methodic combination leads to a distinction between static and dynamic disorder which is often the key for understanding of 'enigmatic' physical properties of crystals.